Weather Modification by Dept of Commerce – Congress Confessions

US Congress
Thur, Oct 29, 2015
Subject; Congress Confessions on Weather Modification

95 2d affi" } COMMITTEE PRINT


Prepared at the Keqtiest of
Hon. Howard W. Cannon, Chairman

Printed for the use of the
Committee on Commerce, Science, and Transportation



2d Session J



Prepared at the Request of

Hox. Howard W. Cannon, Chairman


MAY 1978

Printed for the use of the
Committee on Commerce, Science, and Transportation

U.S. government printing office

34-857 WASHINGTON : 1978


HOWARD W. CANNON, Nevada, Chairman

RUSSELL B. LONG, Louisiana
ERNEST F. HOLLINGS, South Carolina
JOHN A. DURKIN, New Hampshire
DONALD W. RIEGLE, Jr., Michigan

Aubrey L. Sarvis, Staff Director and Chief Counsel

Edwin K. Hall, General Counsel
Malcolm M. B. Sterrett, Minority Staff Director



U.S. Senate,

Committee on Commerce, Science, and Transportation,

November 15, 1978.
To the members of the Committee on Commerce. Science, and
Transportation, U.S. Senate:

I am pleased to transmit herewith for your information and use the
following report on "Weather Modification: Programs, Problems,
Policy, and Potential."

The report was prepared at my request by the Congressional Re-
search Service under the direction of Dr. Robert Morrison, Specialist
in Earth Sciences, Science Policy Research Division. We thank Dr.
Morrison and the others involved in the study for their extremely
thorough and scholarly report. Substantial material on almost all
areas of weather modification are included and the report will provide
the committee with an excellent reference source for future delibera-
tions on the subject.

The completion of the report is particularly timely due to the up-
coming recommendations expected from the Weather Modification
Advisory Board and the Department of Commerce (as directed by
Public Law 94-490) on the future Federal role in weather

James B. Pearson,
Ranking minority member.



U.S. Senate,

Committee on Commerce, Science, and Transportation,

Washington, D.C., July 30, 1976.

Dr. Norman A. Beckman,

Acting Director, Congressional Research Service,
Library of Congress, W ashington, D.C.

Dear Dr. Beckman: Weather modification, although a relatively
young science, has over the years stimulated great interest within the
scientific, commercial, governmental, and agricultural communities.
Such responses are readily understandable. Weather-related disasters
and hazards affect virtually all Americans and annually cause untold
human suffering and loss of life and result in billions of dollars of eco-
nomic loss to crops and other property. While weather modification
projects have been operational for nearly 25 years and have been
shown to have significant potential for preventing, diverting, moderat-
ing, or ameliorating the adverse effects of such weather related disas-
ters and hazards, I am greatly concerned regarding the lack of a
coordinated Federal weather modification policy and a coordinated
and comprehensive program for weather modification research and
development. This fact is all the more disturbing in view of the mani-
fest needs, and benefits, social and economic, that can be associated with
weather modification activities. These deficiencies in our Federal orga-
nizational structure have resulted in a less than optimal return on our
investments in weather modification activities and a failure, with few
exceptions, to recognize that much additional research and develop-
ment needs to be carried out before weather modification becomes a
truly operational tool.

Reports and studies conducted by such diverse organizations as the
National Academy of Sciences, the National Advisory Committee on
Oceans and Atmosphere, the General Accounting Office, and the
Domestic Council have highlighted the lack of a comprehensive Federal
weather modification policy and research and development program.
Hearings that I chaired in February of this year reinforced my con-
cerns regarding the wisdom of our continued failure to implement a
national policy on this very important issue.

I am therefore requesting the Congressional Research Service to
prepare a comprehensive report on weather modification. This report
should include a review of the history and existing status of weather
modification knowledge and technology; the legislative history of
existing and proposed domestic legislation concerning weather mod-
ification; socio-economic and legal problems presented by weather
modification activities; a review and analysis of the existing local,
State, Federal, and international weather modification organizational



structure: international implications of weather modification activi-
ties: and a review and discussion of alternative U.S. and international
weather modification policies and research and development programs.

If you have any questions with respect to this request, please contact
Mr. Gerry J. Kovach, Minority Staff Counsel of the Senate Commerce
Committee. He has discussed this study with Mr. Robert E. Morrison
and Mr. John Justus of the Science Policy Division, Congressional
Research Service.

Very truly yours,

James B. Pearsox,

U.S. Senator.


The Library of Congress,
congressional research service,

Washington, D.C., June 19, 1978.

Hon. James B. Pearson,

Committee on Commerce, Science, and Transportation,
U.S. Senate, Washington, D.C.

Dear Senator Pearson: The enclosed report, entitled "Weather
Modification: Programs, Problems, Policy, and Potential," has been
prepared by the Congressional Research Service in response to your

The study reviews the history, technology, activities, and a number
of special aspects of the field of weather modification. Activities
discussed are those of the Federal, State, and local governments, of
private organizations, and of foreign nations. Consideration is given
to international, legal, economic, and ecological aspects. There are
also an introductory chapter which includes a summary of issues, a
chapter discussing inadvertent weather and climate modification, and
a chapter summarizing recommendations from major Federal policy

The study has been coordinated by Dr. Robert E. Morrison, Special-
ist in Earth Sciences, Science Policy Research Division, who also
prepared chapters 1, 2, 3, 5, 7, 8, and 9 as well as the Summary and
Conclusions. Mr. John R. Justus, Analyst in Earth Sciences, and
Dr. James E. Mielke, Analyst in Marine and Earth Sciences, both
of the Science Policy Research Division, contributed chapters 4 and
6, respectively. Chapter 10 was prepared by Mrs. Lois B. McHugh,
Foreign Affairs Analyst, Foreign Affairs and National Defense Di-
vision. Chapter 11 was written jointly by Mrs. Nancy Lee Jones,
Legislative Attorney, and Mr. Daniel Hill Zaf ren, Specialist in Ameri-
can Public Law, both of the American Law Division. Dr. Warren
Viessman, Jr., Senior Specialist in Engineering and Public Works,
contributed chapter 12; and Mr. William C. JolW, Analyst in En-
vironmental Policy, Environment and Natural Resources Division,
was responsible for chapter 13. In addition, appendixes C, F, Q, and R
were assembled by Mrs. McHugh ; appendixes D and S were prepared
by Mrs. Jones; and information in the remaining appendixes was
collected by Dr. Morrison.

I trust that this report will serve the needs of the Committee on
Commerce, Science, and Transportation as well as those of other
committees and individual Members of Congress who are concerned
with weather modification. On behalf of the Congressional Research
Service, I wish to express my appreciation for the opportunity to
undertake this timely and worthwhile assignment.

Gilbert Gtjde,



Digitized by the Internet Archive
in 2013



Letter of transmittal in

Letter requesting study v

Letter of submittal vn

Summary and conclusions xix

Chapter 1

Introduction and summary of issues 1

Perspective 1

Situation 1

Advantages 3

Timeliness 5

Definitions and scope of report 7

Summary of issues in planned weather modification 9

Technological problems and issues 9

Governmental issues 12

The role of the Federal Government 12

Roles of State and local governments 14

Legal issues 15

Private rights in the clouds 15

Liability for weather modification 16

Interstate legal issues 17

International legal issues 17

Economic issues 18

Issues complicating economic analyses of weather modifica-
tion 18

Weather modification and conflicting interests 19

Social issues 19

Social factors 20

Need for public education on weather modification 21

Decisionmaking 22

International issues 23

Ecological issues 24

Chapter 2

History of weather modification 25

Introduction 25

History of weather modification prior to 1946 26

Prescientific period 26

Early scientific period 27

Development of scientific fundamentals 32

Early cloud-seeding experiments 34

Weather modification since 1946 35

Chronology 35

Langmuir, Schaefer, and Vonnegut 37

Research projects since 1947 39

Project Cirrus 39

The Weather Bureau cloud phvsics project 41

The U.S. experiments of 1953-54 42

Arizona Mountain cumulus experiments 44

Project Whitetop 44

Climax experiments 45

Lightning suppression experiments 46

Fog dispersal research 46

Hurricane modification. 46

Hail suppression 46

Foreign weather modification research 47

Commercial operations 48

History of Federal activities, committees, policy studies, and

reports 53



Chapter 3


Technology of planned weather modification 55

Introduction 55

Assessment of the status of weather modification technology 56

Classification of weather modification technologies 61

Principles and status of weather modification technologies 62

Precipitation augmentation 64

Cumulus clouds 66

Cumulus modification experiments 67

Effectiveness of precipitation enhancement research and

operations 69

Results achieved through cumulus modification 70

Recent advances in cumulus cloud modification 71

Orographic clouds and precipitation 71

Orographic precipitation modification 75

Orographic seeding experiments and seedability criteria 77

Operational orographic seeding projects 81

Results achieved through orographic precipitation modifi-
cation 82

Hail suppression 84

The hail problem 84

Modification of hail 86

Hail seeding technologies 87

Evaluation of hail suppression technology 88

Surveys of hail suppression effectiveness 89

Conclusions from the TASH study 91

Dissipation of fog and stratus clouds 92

Cold fog modification 93

Warm fog modification 93

Lightning suppression 96

Lightning modification 98

Evaluation of lightning suppression technology 99

Modification of severe storms 101

Hurricanes 101

Generation and characteristics of hurricanes 104

Modification of hurricanes 108

Tornadoes 112

Modification of tornadoes 113

Technical problem areas in planned weather modification 115

Seeding technology 115

Evaluation of weather modification projects 118

Extended area effects of weather modification 124

Approaches to weather modification other than seeding 129

Research needs for the development of planned weather modification- 131

General considerations 131

Recommendations from the 1973 National Academv of Sciences

study i 134

Recommendations of the Advanced Planning Group of NOAA__. 136

Summary of Federal research needs expressed by State officials. 138
Research recommendations of the AMS Committee on Weather

Modification 139

Research recommendations related to extended area and time

effects 143

Chapter 4

Inadvertent weather and climate modification 145

Introduction 145

Terminology 145

Climate 145

Climatic fluctuation and climatic change 146

Weather 146

Weather modification 146

Climate modification 146

Planned climate modification 147

Inadvertent climate modification 148



Background 149

Historical perspective 149

Understanding the causes of climatic change and variability 151

The concept of climatic change and variability 152

When and how do climatic changes occur 154

The facts about inadvertent weather and climate modification 156

Airborne particulate matter and atmospheric turbidity 156

Do more particles mean a warming or cooling? 157

Sources of atmospheric particulates: Natural vs. manmade.. 158

Atmospheric processes affected by particulates 159

The La Porte weather anomaly: Urban climate modification. 162

Carbon dioxide and water vapor 164

Increases in atmospheric carbon dioxide concentration:

What the record indicates 164

Predicting future atmospheric carbon dioxide levels 166

Sources and sinks for carbon dioxide 168

Atmospheric effects of increased carbon dioxide levels 169

Implications of increasing atmospheric carbon dioxide con-
centrations 169

Implications of a climatic warming 170

Carbon dioxide and future climate: The real climate vs.

"model climate" 171

Ozone depletion 172

Concerns regarding ozone destruction 172

Action by the Government on the regulation of fluorocar-

bons 175

Climatic effects of ozone depletion 176

Waste heat 177

The urban "Heat Island" 177

Albedo 179

Large-scale irrigation 180

Recapitulation 181

Issues in inadvertent weather and climate modification 184

Climatic barriers to long-term energy growth 184

Thoughts and reflections — Can we contemplate a fossil-fuel-free

world? 185

Research needs and deficiencies 186

Chapter 5

Federal activities in weather modification 193

Overview of Federal activities..– '— — 193

Legislative and congressional activities 194

Federal legislation on weather modification 194

Summary 194

The Advisory Committee on Weather Control 195

Direction to the National Science Foundation 196

Reporting of weather modification activities to the Federal

Government 197

The National Weather Modification Policy Act of 1976 198

Congressional direction to the Bureau of Reclamation 201

Proposed Federal legislation on weather modification 203

Summary 203

Legislation proposed in the 94th Congress and the 95th

Congress, 1st sessions 205

Other congressional activities 207

Resolutions on weather modification 207

Hearings 208

Studies and reports by congressional support agencies 209

Activities of the executive branch 209

Introduction 209

Institutional structure of the Federal weather modification

program 210

Current status of Federal organization for weather modifica-
tion 210



Federal structure; 1946-57 214

Federal structure; 1958-68 215

Federal structure; 1968-77 216

Future Federal organization for weather modification 216

Coordination and advisory mechanisms for Federal weather

modification programs 221

Introduction 221

The Interdepartmental Committee for Atmospheric Sciences

(ICAS) 222

The National Academv of Sciences/Committee on At-
mospheric Sciences (N AS/CAS) 226

The National Advisory Committee on Oceans and Atmos-
phere (NACOA) 227

Other coordination and advisory mechanisms 228

Weather Modification Advisory Board 231

Weather modification activities reporting program 232

Background and regulations 232

Reporting of Federal activities 233

Summary reports on U.S. weather modification activities 233

Federal studies and reports on weather modification 234

Introduction 234

Studies of the early 1950's 235

Advisory Committee on Weather Control 236

National Academy of Sciences studies 237

Studies bv the Interdepartmental Committee for Atmos-
pheric Sciences (ICAS) 238

Domestic Council study 239

Policy and planning reports produced by Federal agencies 239

Federal programs in weather modification 241

Introduction and funding summaries 241

Department of the Interior 246

Introduction 246

Project Skywater; general discussion 247

The Colorado River Basin Pilot Project (CRBPP) 254

The High Plains Cooperative Program (HIPLFX) 258

The Sierra Cooperative Pilot Project (SCPP) 263

Drought mitigation assistance 266

National Science Foundation 267

Introduction and general 267

Weather hazard mitigation 274

Weather modification technology development 282

Inadvertent weather modification 283

Societal utilization activities 287

Agricultural weather modification 288

Department of Commerce 290

Introduction and general discussion 290

The Florida Area Cumulus Experiment (FACE) 292

Project Stormfurv 296

Research Facilities Center (RFC) 300

Global Monitoring for Climatic Change (GMCC) 301

Lightning suppression 302

Modification of extratropical severe storms 302

Department of Defense 303

Introduction 303

Air Force fog dispersal operations 303

Army research and development 304

Navy research and development 304

Air Force research and development 305

Overseas operations 307

Department of Transportation 308

Department of Agriculture 309

Department of Energy 310


Chapter 6

Review of recommendations for a national program in weather modifica- Page

tion 313

Introduction ^Jy

Summaries of major weather modification reports 314

Final report of the Advisory Committee on Weather Control — 314
Weather and climate modification: Report of the Special Com-
mission on Weather Modification 315

Weather and climate modification: Problems and prospects 317

A recommended national program in weather modification 318

A national program for accelerating progress in weather modifica-
tion 320

Weather and climate modification: Problems and progress 321

Annual reports to the President and Congress by NACOA 323

Need for a national weather modification research program 324

The Federal role in weather modification 325

Trends and analysis 326

Chapter 7

State and local activities in weather modification 331

Overview of State weather modification activities 331

Introduction 331

North American Interstate Weather Modification Council 333

Survey and summary of State interests and activities in weather

modification 340

State contacts for information on weather modification activities. 343

Non-Federal U.S. weather modification activities 343

Analysis of calendar year 1975 projects 344

Preliminary analysis of projects for calendar years 1976-77_ 347
General discussion of local and regional weather modification policy

activities „ 348

Weather modification activities within particular States 351

California 352

State weather modification law and regulations 352

Weather modification projects 353

State-sponsored emergency projects 356

Illinois 358

Illinois weather modification law and its administration 358

Operational projects 359

Research activities 360

Kansas 361

Kansas Weather Modification Act 361

Research activities 362

Operational activities 364

Emergenc}- Drought Act of 1977 364

North Dakota 365

Weather modification law and administration of regulations- 365

Authority and organization for local projects 370

North Dakota operational projects in 1975 and 1976 371

South Dakota 376

Utah 381

Washington 382

Chapter 8

Private activities in weather modification 385

Introduction 385

Commercial weather modifiers 386

Scope and significance of contract activities 386

Summary of contract services 386

Evaluation and research by commercial firms 388

Participation in Federal research programs 389

Weather modification organizations 389

Professional organizations 389

Weather Modification Association 390

American Meteorological Society 395



Opposition to weather modification 399

General discussion 399

Opposition to the seeding project above Hungry Horse Dam. 399

Tri-State Natural Weather Association 400

Citizens for the Preservation of Natural Resources 402

Chapter 9

Foreign'activities in weather modification 405

Introduction 405

World Meteorological Organization register of weather modification

projects 408

Description of weather modification activities in some foreign nations. 412

The Union of Soviet Socialist Republics 412

Overview of projects in the U.S.S.R 412

Summary of weather modification and related atmospheric

research in the U.S.S.R 413

Israel 415

Australia 416

Canada 418

Mexico 419

People's Republic of China 420

Kenya 421

Republic of South Africa 422

Rhodesia 423

India 423

The Swiss hail experiment 424

Chapter 10

International aspects of weather modification 427

Introduction 427

Convention on the prohibition of military or any other hostile use of

environmental modification techniques 429

Development of the treaty 429

Criticism of the convention 431

Activities since the United Nations approval of the convention.. 432
Activities of the World Meteorological Organization in weather

modification 433

Precipitation enhancement program (PEP) 434

Other WMO activities in weather modification 436

Registration and reporting of weather modification projects. 436

WMO conferences on weather modification 436

Typhoon and serious storm modification 437

Global atmospheric research programme 437

Legal aspects of weather modification 437

United Nations Conference on the Human Environment 438

Declaration of the United Nations Conference on the Human

Environment 438

Action Plan for the Human Environment 438

Earthwatch Program 439

Study of Man's Impact on Climate 439

Other international activities 440

United States/Canadian agreement 440

North American Interstate Weather Modification Council 440

Congressional activities 441

Weather modification as a weapon of war 441

Senate Resolution 71, prohibiting environmental modification

as a weapon of war 441

Congressional activities related to hostile use of weather

modification, 1974-76 442

Other Congressional actions relating to weather modification 443

Senate Concurrent Resolution 67 — U.S. participation in the

world weather program 443

National Weather Modification Policy Act of 1976 444

Senate Resolution 49 444



U.S. foreign policy 444

Various executive branch proposals 445

National Advisory Committee on Oceans and Atmosphere 447

Activities in 1977 448

Chapter 11

Legal aspects of weather modification 449

Domestic 449

Private rights in the clouds 449

Liability for weather modification 453

Defenses which may be raised against claims of liability 456

Interstate allocation of atmospheric water 457

Methods of controlling weather modification 459

Congressional authority under the Constitution to regulate or

license weather modification activities 461

Federalism 461

The commerce clause 461

The commerce clause generally 462

The commerce clause and the regulation of navigable

waters 463

Limitations on the commerce power 464

Fiscal powers 465

War powers 466

Property power 466

Treaty power 467

Conclusion 467

International 468

Certain hostile uses of weather modification are prohibited 471

Nations are responsible for environmental conduct which causes

injury or damage in or to other nations 471

Nations are liable for injuries sustained by aliens within their
territory caused by tortuous conduct in violation of inter-
national law 472

Nations or their citizens may be liable for injury and damage
they caused to citizens of another nation occurring in that

nation 472

Chapter 12

Economic aspects of weather modification 475

Introduction 475

Economic setting 476

Economic aspects of weather modification procedures 477

Fog dispersal 477

Precipitation augmentation 478

Orographic cloud seeding 478

Convective cloud seeding 478

Precipitation augmentation and energy considerations 479

Hail suppression 480

Lightning suppression and reduction in storm damage 480

Analytic methods for economic analysis 481

Case studies of the economics of weather modification 482

Hungry Horse Area, Montana 482

Connecticut River basin 483

State of Illinois 483

Nine-county Southeastern Crop Reporting District, South Dakota, 483

Colorado River 484

Conclusions 486

Chapter 13

Ecological effects of weather modification 487

Introduction 487

Modification of weather and climate 487

Ecology and ecological systems — 487

Knowledge of ecological implications of applied weather modifi-
cation technologies 488



Important variables 490

Temporal considerations 491

Season of modification effort 491

Duration of effort: Short- v. long-term 491

Regularity of modification effort 491

Ecosystem type 492

Aquatic v. terrestrial systems 492

Cultivated v. natural systems 492

Arid v. humid systems 492

Cumulative and synergistic effects 492

Effects of silver iodide* 493

Deliberate weather modification 496

Precipitation enhancement 496

Increased rainfall 496

Snowpack augmentation 497

Severe storm abatement 498

Fog dispersal 499

Hail suppression 499

Alteration or arrest of lightning discharges 499

Inadvertent weather modification 499

Extra-area effects 499

Long-term, climatic, and global implications 500

Summary and conclusions 501


A. Statement on weather modification in Congressional Record of

June 17, 1975, by Congressman Gilbert Gude, containing White

House statement on Federal weather modification policy 503

B. Department of Defense statement on position on weather modification. 509

C. Text of United Nations Convention on the prohibition of military

or any other hostile use of environmental modification techniques 510

D. State statutes concerning weather modification 514

Arizona 515

California 516

Colorado 520

Connecticut 528

Florida 529

Hawaii 531

Idaho 531

Illinois 533

Iowa 541

Kansas 543

Louisiana 549

Minnesota 550

Montana 554

Nebraska 557

Nevada 565

New Hampshire 571

New Mexico 571

New York 573

North Dakota 573

Oklahoma 584

Oregon 59 1

Pennsylvania 599

South* Dakota 604

Texas 600

Utah 612

Washington 613

West Virginia 618

Wisconsin 622

Wyoming 622

E. List of State contacts for further information on weather modification

activities within the States 625

F. Agreement on exchange of information on weather modification

between the United States of America and Canada 627


G. Weather modification activities in the United States during calendar Pa?e

year 1975 630

H. Selected bibliography of publications in weather modification 641

I. Public laws dealing specifically with weather modification 640

J. Summary of language in congressional documents supporting public

works appropriations for the Bureau of Reclamation's atmospheric

water resources program 655

K. Membership and charter of the U.S. Department of Commerce

Weather Modification Advisory Board 660

L. Rules and regulations and required forms for submitting information
on weather modification activities to the National Oceanic and
Atmospheric Administration, U.S. Department of Commerce, in

accordance with requirements of Public Law 92-205 662

M. Selected State rules and regulations for the administration of State

weather modification statutes 676

Illinois 676

Kansas 6 S3

North Dakota 691

Utah 707

Washington 712

N. Documents of the Weather Modification Association 717

O. Policy statement of the American Meteorological Society on purposeful

and inadvertent modification of weather and climate 722

P. Reporting agencies of member countries and questionnaire circulated
to receive weather modification information from members of the

World Meteorological Organization 724

Q. Report of the World Meteorological Organization/ United Nations
Environment programme informal meeting on legal aspects of

weather modification 727

R. Text of Senate Resolution 71; considered, amended, and agreed to

July 11, 1973 734

S. Reported cases on weather modification 740

T. Glossary of selected terms in weather modification 741

34-857—79 2


Weather modification is generally considered to be the deliberate
effort to improve atmospheric conditions for beneficial human pur-
poses — to augment water supplies through enhanced precipitation or
to reduce economic losses, property damages, and deaths through
mitigation of adverse effects of hail, lightning, fog, and severe storms.
Not all weather modification activities, however, have been or can be
designed to benefit everyone, and some intentional operations have
been used, or are perceived to have been used, as a weapon of war
to impede the mobility or tactical readiness of an enemy. Further-
more, environmental change is also effected unintentionally and with-
out any purpose at all, as man inadvertently modifies the weather and
climate, whether for better or worse scientists are not certain, through
activities such as clearing large tracts of land, building urban areas,
and combustion of fossil fuels.

Historically, there have been attempts, often nonscientific or pseudo-
scientific at best, to change the weather for man's benefit. Until the
20th century, however, the scientific basis for such activities was
meager, with most of our current understanding of cloud physics and
precipitation processes beginning to unfold during the 1930's. The
modern period in weather modification is about three decades old, dat-
ing from events in 1946, when Schaefer and Langmuir of the General
Electric Co. demonstrated that a cloud of supercooled water droplets
could be transformed into ice crystals when seeded with dry ice. Soon
afterward it was discovered that fine particles of pure silver iodide,
with crystal structure similar to that of ice, were effective artificial
ice nuclei, and that seeding clouds with such particles could produce
ice crystals at temperatures just below freezing. Silver iodide remains
the most often used material in modern "cloud seeding."

By the 1950's, many experimental and operational weather modifi-
cation projects were underway; however, these early attempts to
augment precipitation or to alter severe storm effects were often in-
conclusive or ineffective, owing to improper experimental design, lack
of evaluation schemes, and the primitive state of the technology.
Through research programs over the past two decades, including
laboratory studies and field experiments, understanding of atmos-
pheric processes essential to improved weather modification tech-
nology has been advanced. Sophisticated evaluation schemes have been
developed, using elaborate statistical tools; there has also been im-
provement in measuring instruments and weather radar systems ; and
simulation of weather processes using numerical models and high
speed computers has provided further insights. Meanwhile, commer-
cial weather modifiers, whose number decreased dramatically along
with the total area of the United States covered by their operations
after the initial surge of the 1950 era, have grown in respectability and
competence, and their operations have incorporated improvements as
they benefited from their accumulated experience and from the re-



suits of research projects. Since such operations are designed for prac-
tical results, such as increased precipitation or reduced hail, however,
the sophisticated evaluation procedures now used in most research
projects are most often not used, so that the effectiveness of the opera-
tions is frequently difficult to assess.

Weather modification is at best an emerging technology. Progress in
development of the technology over the past 30 years has been slow,
although there has been an increased awareness of the complex nature
of atmospheric processes and a steady improvement in basic under-
standing of those processes which underlie attempts at deliberate modi-
fication of weather phenomena. Though most cloud-seeding practices
are based on a common theory and form the basis for a number of seed-
ing objectives, there are really a series of weather modification
technologies, each tailored to altering a particular atmospheric pheno-
menon and each having reached a different state of development and
operational usefulness. For example, cold fog clearing is now consid-
ered to be operational, while, at the other extreme, the abatement of
severe storms such as hurricanes remains in the initial research phase.
Development progress for each of these technologies appears to be
much less a function of research effort expended than a dependence on
the fundamental atmospheric processes and the ease by which they can
be altered. There continues to be obvious need for further research and
development to refine those few techniques for which there has been
some success and to advance technology where progress has been slow
or at a virtual standstill.

The following summary provides a reasonably accurate assessment
of the current status of weather modification technology :

1. The only routine operational projects are for clearing cold fog.
Research on warm fog has yielded some useful knowledge and good
models, but the resulting technologies are so costly that they are usable
mainly for military purposes and very busy airports.

2. Several longrunning efforts to increase winter snowpack by seed-
ing clouds in the mountains suggest that precipitation can be increased
by some 15 percent over what would have happened "naturally."

3. A decade and a half of experience with seeding winter clouds on
the U.S. west coast and in Israel, and summer clouds in Florida, also
suggest a 10- to 15-percent increase over "natural" rainfall. Hypotheses
and techniques from the work in one area are not directly transferable
to other areas, but will be helpful in designing comparable experiments
with broadly similar cloud systems.

4. Numerous efforts to increase rain by seeding summer clouds in the
central and western parts of the United States have left many questions
unanswered. A major experiment to try to answer them — for the High
Plains area — is now in its early stages.

5. It is scientifically possible to open holes in wintertime cloud layers
by seeding them. Increasing sunshine and decreasing energy consmp-
tion may be especially relevant in the northeastern quadrant of the
United States.

0. Some $10 million is spent by private and local public sponsors for
cloud-seeding efforts, but these projects arc not designed as scientific
experiments and it is difficult to say for sure that operational cloud
seeding causes the claimed results.


7. Knowledge about hurricanes is improving with good models of
their behavior. But the experience in modifying that behavior is primi-
tive so far. It is inherently difficult to find enough test cases, especially
since experimentation on typhoons in the Western Pacific has been
blocked for the time being by international political objections.

8. Although the Soviets and some U.S. private operators claim some
success in suppressing hail by seeding clouds, our understanding of the
physical processes that create hail is still weak. The one major U.S.
held experiment increased our understanding of severe storms, but
otherwise proved mostly the dimensions of what we do not yet know.

9. There have been many efforts to suppress lightning by seeding
thunderstorms. Our knowledge of the processes involved is fair, but the
technology is still far from demonstrated, and the U.S. Forest Service
has recently abandoned further lightning experiments. 1

Modification processes may also be initiated or triggered inadvert-
ently rather than purposefully, and the possibility exists that society
may be changing the climate through its own actions by pushing on
ceitain leverage points. Inadvertently, man is already causing measur-
able variations on the local scale. Artificial climatic effects have been
observed and documented on local and regional scales, particularly in
and downwind of heavily populated industrial areas where waste heat,
particulate pollution and altered ground surface characteristics are
primarily responsible for the perceived climate modification. The cli-
mate in and near large cities, for example, is warmer, the daily range
of temperature is less, and annual precipitation is greater than if the
cities had neA^er been built. Although not verifiable at present, the time
may not be far off when human activities will result in measurable
large-scale changes in weather and climate of more than passing sig-
nificance. It is important to appreciate the fact that the role of man at
this global level is still controversial, and existing models of the gen-
eral circulation are not yet capable of testing the effects in a conclusive

Nevertheless, a growing fraction of current evidence does point to
the possibility of unprecedented impact on the global climate by hu-
man activities, albeit the effects may be occurring below the threshold
where they could be statistically detected relative to the record of nat-
ural fluctuations and. therefore, could be almost imperceptible amid
the ubiquitous variability of climate. But while the degree of influence
on world climate may as yet be too small to detect against the back-
ground of natural variations and although mathematical models of
climatic change are still imperfect, significant global effects in the
future are inferred if the rates of growth of industry and population

For over 30 years both legislative and executive branches of the
Federal Government have been involved in a number of aspects of
weather modification. Since 1947 about 110 weather modification bills
pertaining to research support, operations, grants, policy studies, regu-
lations, liabilities, activity reporting, establishment of panels and com-
mittees, and international concerns have been introduced in the Con-

1 Weather Modification Advisory Board. "A U.S. Policy to Enhance the Atmospheric
Environment," Oct. 21, 1977. In testimony by Harlan Cleveland. Weather modification.
Hearing before the Subcommittee on the Environment and the Atmosphere, Committee on
Science and Technology. U.S. House of Representatives. 93th Cong., 1st sess., Oct. 26,
1977, Washington, U.S. Government Printing Office, 1977. pp. 28-30.


gress. Resolutions, mostly concerned with using weather modification
ns a weapon and promotion of a United Nations treaty banning such
activities, have also been introduced in both houses of the Congress ;
one such resolution was passed by the Senate.

Six public laws specifically dealing with weather modification have
been enacted since 1953, and others have included provisions which are
in some way relevant to weather modification. Federal weather modi-
fication legislation has dealt primarily with three aspects — research
program authorization and direction, collection and reporting of in-
formation on weather modification activities, and the commissioning
of major policy studies. In addition to direction through authorizing
legislation, the Congress initiated one major Federal research pro-
gram through a write-in to an appropriations bill; this program
regularly receives support through additional appropriations beyond
recommended OMB funding levels.

There are two Federal laws currently in effect which are specifically
concerned with weather modification. Public Law 92-205, of Decem-
ber 18, 1971, and its amendments requires the reporting of all non-
Federal activities to the Secretary of Commerce and publication "from
time to time" of summaries of such activities by the Secretary of
Commerce. 2 The National Weather Modification Policy Act of 1976
(Public Law 94-490), enacted October 13, 1976, directed the Secretary
of Commerce to conduct a major study on weather modification and to
submit a report containing a recommended Federal policy and Fed-
eral research program on w T eather modification. The Secretary ap-
pointed a non-Government Weather Modification Advisory Board to
conduct the mandated study, the report on which is to be submitted
to the Secretary for her review and comment and subsequent trans-
mittal to the President and the Congress during 1978. It is expected
that, following receipt of the aforementioned report, the Congress will
consider legislation on Federal weather modification policy, presuma-
bly during the 96th Congress.

Congressional interest in weather modification has also been mani-
fested in a number of hearings on various bills, in oversight hearings
on pertinent ongoing Federal agency programs, in consideration of
some 22 resolutions having to do with weather modification, and in
commissioning studies on the subject by congressional support

The principal involvement in weather modification of the Federal
Government has been through the research and development programs
of the several Federal departments and agencies. Although Federal
research programs can be traced from at least the period of World
War II, the programs of most agencies other than the Defense Depart-
ment were not begun until the 1950's and 1960's. These research and
development programs have been sponsored at various times by at
least eight departments and independent agencies — including the De-
partments of Agriculture, Commerce, Defense, Energy, Interior, and
Transportation, the National Aeronautics and Space Administration
(NASA), and the National Science Foundation (NSF). In fiscal year

2 Although Federal agencies were excluded from the requirements of this not. upon agreement, the agencies also submit information on their weather mollification
projects to tlie Secretary of Commerce, so that there is a single repository for information
on nil weather modification activities conducted within the United States.


1978 six agency programs were reported, those of Transportation and
NASA having been phased out, while that of Agriculture was severely

Total funding for Federal weather modification research in fiscal
year 1978 is estimated at about $17 million, a decline from the highest
funding level of $20 million reached in fiscal year 1976. The largest
programs are those of the Departments of Interior and Commerce and
of the NSF. The NSF has supported weather modification research
over a broad spectrum for two decades, although its fiscal year 1978
funding was reduced by more than 50 percent, and it is not clear that
more than the very basic atmospheric science supportive of weather
modification will be sponsored hereafter by the Foundation.

The present structure of Federal organization for weather modifi-
cation research activities is characterized essentially by the mission-
oriented approach, whereby each of the agencies conducts its own
program in accordance with broad agency goals or under specific direc-
tions from the Congress or the Executive. Programs have been loosely
coordinated through various independent arrangements and/or advi-
sory panels and particularly through the Interdepartmental Commit-
tee for Atmospheric Sciences (ICAS). The ICAS, established in 1959
by the former Federal Council for Science and Technology, provides
advice on matters related to atmospheric science in general and has
also been the principal coordinating mechanism for Federal research
in weather modification.

In 1958 the National Science Foundation was designated lead agency
for Federal weather modification research by Public Law 85-510, a
role which it maintained until 1968, when Public Law 90-407 removed
this responsibility from NSF. No further action was taken to name a
lead agency, although there have been numerous recommendations to
designate such a lead agency, and several bills introduced in the Con-
gress would have named either the Department of the Interior or the
Department of Commerce in that role. During the 10-year period from
1958 to 1968 the NSF promoted a vigorous research program through
grants to various research organizations, established an Advisory
Panel for Weather Modification, and published a series of 10 annual
reports on weather modification activities in the United States. Since
1968 there has been a lapse in Federal weather modification policy and
in the Federal structure for research programs, although, after a
hiatus of over 3 years, the responsibility for collecting and disseminat-
ing information on weather modification activities was assigned to the
Commerce Department in 1971. An important consideration of any
future weather modification legislation will probably be the organiza-
tional structure of the Federal research program and that for admin-
istration of other related functions which may be the responsibility of
the Federal Government. Options include a continuation of the present
mission-oriented approach with coordination through the ICAS or a
similar interagency body, redesignation of a lead agency with some
autonomy remaining with the several agencies, or creation of a single
agency with control of all funding and all research responsibilities.
The latter could be an independent agency or part of a larger depart-
ment ; it would presumably also administer other aspects of Federal
weather modification responsibilities, such as reporting of activities,


regulation and licensing, and monitoring and evaluation of operations,
if a n}' or all of these functions should become or continue to be services
performed at the Federal level.

In addition to specific research programs sponsored bv Federal agen-
cies, there are other functions related to weather modification which
are performed in several places in the executive branch. Various Fed-
eral advisory panels and committees and their staffs — established to
conduct in-depth studies and prepare comprehensive reports, to pro-
vide advice and recommendations, or to coordinate Federal weather
modification programs — have been housed and supported within exec-
utive departments, agencies, or offices. The program whereby Federal
and non-Federal U.S. weather modification activities are reported to
the Government is administered by the National Oceanic and Atmos-
pheric Administration (NOAA) within the Commerce Department.
The State Department negotiates agreements with other nations which
might be affected by U.S. experiments and has arranged for Federal
agencies and other U.S. investigators to participate in international
meteorological projects, including those in weather modification. In
the United Nations, the United States has been active in promoting the
adoption of a treaty banning weather modification as a military

In accordance with the mandates of several public laws or self-ini-
tiated bv the agencies or interagency committees, the executive branch
of the Federal Government has undertaken a number of major weather
modification policy studies over the past 25 years. Each of the com-
pleted major studies was followed by a report which included findings
and recommendations. The most recent study is the one noted earlier
that is being conducted by the Weather Modification Advisory Board
on behalf of the Secretarv of Commerce, pursuant to requirements of
the National Weather Modification Policy Act of 1976. Nearly all
previous studies emphasized the needs for designation of a lead agency,
increased basic meteorological research, increased funding, improve-
ment of support and cooperation from agencies, and consideration of
legal, socioeconomic, environmental, and international aspects. Other
recommendations have included improvement of program evaluation,
studv of inadvertent effects, increased regulation of activities, and a
number of specific research projects. Although some of the recom-
mended activities have been undertaken, many have not resulted in
specific actions to date. Almost invariably it was pointed out in the
studies that considerable progress would result from increased fund-
ing. Although funding for weather modification research has increased
over t he past 20 years, most funding recommendations have been for
considerably higher levels than those provided. Since fiscal year 1976,
the total Federal research funding for weather modification research
hn=. in fact, decreased.

Most States in the Nation have some official interest in weather
modification ; 29 of them have some form of law which relates to such
activities, usually concerned with various facets of regulation or con-
trol of operations within the Slate and sometimes pertaining to au-
thorization for funding research and/or operations at the State or
local level. A State's weather modification law usually reflects its gen-
eral policy toward weather modification; some State laws tend to en-


courage development and use of the technology, while others dis-
courage such activities.

The current legal regime regulating weather modification has been
developed by the States rather than the Federal Government, except
in the areas of research support, commissioning studies, and requiring
reporting of activities. The various regulatory and management func-
tions which the States perform include: (1) issuance, renewal, sus-
pension, and revocation of licenses and permits; (2) monitoring and
collecting of information on activities through requirements to main-
tain records, submission of periodic activity reports, and inspection
of premises and equipment; (3) funding and managing of State or
locally organized operational and/or research programs ; (4) evalua-
tion and advisory services to locally organized public and private op-
erational programs within the State; and (5) miscellaneous admin-
istrative activities, including the organization and operation of State
agencies and boards which are charged with carrying out statutory
responsibilities. Administration of the regulatory and managerial re-
sponsibilities pertaining to weather modification within the States is
accomplished through an assortment of institutional structures, in-
cluding departments of water or natural resources, commissions, and
special governing or advisory groups. Often there is a combination of
two or more of these agencies or groups in a State, separating func-
tions of pure administration from those of appeals, permitting, or ad-
visory services.

Involvement in weather modification operational and research pro-
grams varies from State to State. Some support research only, while
others fund and operate both research and operational programs. In
some cases funding only is provided to localities, usually at the county
level, where operational programs have been established. The recent
1976-77 drought led some Western States to initiate emergency cloud-
seeding programs as one means of augmenting diminishing water sup-
plies. Research conducted by atmospheric and other scientists at State
universities or other research agencies may be supported in part with
State funds but is often funded by one of the major Federal weather
modification programs, such as that of the Bureau of Reclamation or
the National Science Foundation. In a few cases. States contribute
funds to a Federal research project which is conducted jointly with
the States and partly within their borders.

In 1975, 1976, and 1977, respectively, there were 58, 61, and 88 non-
federally supported weather modification projects, nearly all opera-
tional, conducted throughout the United States. These projects were
sponsored by community associations, airlines, utilities, private in-
terests, municipal districts, cities, and States. Eighty-five percent of
all projects in the United States during 1975 were carried out west of
Kansas City, with the largest number in California. In that State
there were 11 proipets in each of the vears 1975 and 1976, and 20
projects during 1977. The majority of these operational projects were
designed to increase precipitation; others were intended for sup-
pression of hail or dispersal of fogs, the latter principally at airports.

In most instances, the principal beneficiaries of weather modification
are the local or regional users, who include farmers and ranchers,
weather-related industries, municipalities, airports, and utilities —


those individuals and groups whose economic well-being and whose
lives and property are directly subject to adverse consequences of
drought or other severe weather. It is at the local level where the need
to engage in weather modification is most keenly perceived and also
where possible negative effects from such activities are most apparent
to some sectors of the population. It follows that both the greatest sup-
port and the strongest opposition to weather modification projects are
focussed at the local level. The popularity of a particular project and
the degree of controversy surrounding it are frequently determined by
the extent to which local citizens and local organizations have had a
voice in the control or funding of the project. At the local level, deci-
sions to implement or to withdraw from a project can most often be
made with minimum social stress. Indeed, studies have shown that most
people are of the opinion that local residents or local government offi-
cials should make decisions on whether or not to use weather modifica-
tion technology in a given situation.

Many of the operational weather modification services provided for
private groups and governmental bodies within the States are carried
out under contract by commercial firms who have developed expertise
in a broad range of capabilities or who specialize in particular services
essential to both operational or research projects. Contracts may cover
only one season of the year, but a number of them are renewed an-
nually, with target areas ranging from a few hundred to a few thou-
sand square miles. In 197G, 6 of the 10 major companies having
substantial numbers of contracts received about $2.7 million for op-
erations in the United States, and a few of these companies also had
contracts overseas. Owing to increased demand for emergency pro-
grams during the recent drought, it is estimated that 1977 contracts
totaled about $3.5 million.

The initial role of the private weather modification operators was to
sustain activities during the early years, when there was often heated
scientific controversy with other meteorologists over the efficacy of
cloud seeding. Later, their operations provided a valuable data base
which permitted the early evaluation of seeding efforts and estimates
of potential prospects for the technology, meanwhile growing in com-
petence and public respect. Today, more often than not, they work
hand in hand with researchers and, in fact, they often participate in
research projects, contributing much of their knowhow acquired
through their unique experiences.

Important among private institutions concerned with weather modi-
fication are the professional organizations of which research and op-
erational weather modifiers and other interested meteorologists are
members. These include the American Meteorological Society, the
Weather Modifical ion Association, and the Irrigation and Drainage
Division of the American Society of Civil Engineers. Through the
meetings and publications of these organizations the scientific, tech-
nical, and legal problems and findings on weather modification are
aired and discussed. These groups also address other matters such as
statements of weather modification policy, opinions on pending legis-
lation, social implieations. and professional standards and certifica-
tion. Tn addition, the North American Interstate Weather Modifica-
tion Council is an organizai ion whose membership consists of govern-


ments of U.S. States and Canadian Provinces and the Government of
Mexico, which serves as a forum for interstate coordination and ex-
change of information on weather modification.

Weather modification is often controversial, and both formal and
informal opposition groups have been organized in various sections
of the country. Reasons for such opposition are varied and are based
on both real and perceived adverse consequences from weather modifi-
cation. Sometimes with little or no rational basis there are charges
by these groups that otherwise unexplained and usually unpleasant
weather- related events are linked to cloud seeding. There are also cases
where some farmers are economically disadvantaged through receiving
more, or less than optimum rainfall for their particular crops, when
artificial inducement of such conditions may have indeed been planned
to benefit those growing different crops with different moisture re-
quirements. Opposition groups are often formed to protect the legiti-
mate rights of farmers under such circumstances.

While the United States is the apparent leader in weather modifi-
cation research and operations, other countries have also been active.
Information on foreign weather modification activities is not uni-
formly documented and is not always available. In an attempt to
assemble uniform weather modification activities information of its
member nations, the World Meteorological Organization (WMO) in
1975 instigated a system of reporting and of maintaining a register on
such activities. Under this arrangement 25 nations reported weather
modification projects during 1976, and 16 countries provided similar
information in 1975. The largest weather modification effort outside
the United States is in the Soviet Union, where there are both a con-
tinuing research program and an expanding operational program. The
latter is primarily a program designed to reduce crop damage from
hail, the largest such effort in the world, covering about 5 million
hectares (15 million acres) in 1976. Other countries with weather modi-
fication programs of some note include Canada, Israel, Mexico, and
the People's Republic of China. Projects in Rhodesia and the Republic
of South Africa are not reported through the WMO register since
these countries are not WMO member nations.

Recent years have seen increased international awareness of the
potential benefits and possible risks of weather modification technology
and increased international efforts to control such activities. The major
efforts of the international community in this area are to encourage
and maintain the high level of cooperation which currently exists in
weather prediction and research and to insure that man's new abilities
will be used for peaceful purposes. There has been exchange of ideas
on weather modification through international conferences and
through more informal exchanges of scientists and research documents.
As with many scientific disciplines, however, the problems arising
from use of and experiments with weather modification are not just
scientific in nature, but are political problems as well.

In addition to the problems of potential damage to countries through
commercial or experimental weather modification activities, another
growing area of concern is that weather modification will be used for
hostile purposes and that the future will bring weather warfare be-
tween nations. The United States has already been involved in one


such instance during the Vietnam war when attempts were made to
impede traffic by increasing rainfall during the monsoon season. In the
future, even the perception that weather modification techniques are
available or in use could lead to an increase in international tensions.
Natural drought in a region, or any other natural disaster will be
suspect or blamed on an enemy.

In light of these problems the international community has made
scattered attempts both to further the study of weather and its modifi-
cation and to insure the peaceful use of this new technology. One such
attempt was the development of the Convention on the Prohibition
of Military or Any Other Hostile Use of Environmental Modification
Techniques, which was adopted by the General Assembly of the United
Nations and opened for signature on May 18. 19TT, at which time it was
signed by the United States and 33 other nations (though it has not
yet been submitted to the U.S. Senate for ratification) . Another exam-
ple of promotion of peaceful use of weather modification is the Pre-
cipitation Enhancement Program, sponsored by the WMQ, whose aim
is to plan, set up, and carry out an international, scientifically con-
trolled precipitation experiment in a semiarid region of the world
under conditions where the chances are optimal for increasing pre-
cipitation in sufficient amounts to produce economic benefits.

The United Nations Conference on the Human Environment, held
in June 1972 in Stockholm, has been the pivotal point in much recent
international environmental activity. It too has been an important
catalyst in international activities relating to weather modification
through portions of its "Declaration," its "Action Plan for the Human
Environment," its "Earthwatch Program," and its "Study of Man's
Impact on Climate."

Legal issues in weather modification are complex and unsettled.
They can be considered in at least four broad categories : private rights
in the clouds, liability for weather modification, interstate legal issues,
and international legal issues. Since the body of law on weather modi-
fication is slight, existing case law offers few guidelines to determine
these issues. Regarding the issue of private rights in the clouds, there
is no general statutory determination of ownership of atmospheric
water, so it is often necessary to use analogies to some general common
law doctrines pertaining to water distribution, although each such
doctrine has its own disadvantages when applied to weather modifica-
tion. Some State laws reserve ownership or right to use atmospheric
water to the State.

Issues of liability for damage may arise when drought, flooding,
or other severe weal her phenomena occur following attempts to modify
the weather. Such issues include causation, nuisance, strict liability,
trespass, negligence, and charges of pollution of the air and water
through introduction of artificial nucleants. Statutes of 10 States dis-
cuss weather modification liability: however, there is much variation
among the specific provisions of the laws in those States. Before a
case can be made for liability based on causation, it must be pro\en
that the adverse weather conditions were indeed induced by the wen: r
modifier; but, in fact, no one lias ever been able to establish causation
of damages through such activities in view of the scientific uncer-
tainties of weather modification.


Significant issues may arise when weather modification activities
conducted in one State affect another State as well. There may be, for
example, the claim that seeding in one State has removed from the
clouds water that should have fallen in an adjacent State or that
excessive flooding resulted from cloud seeding in a State upwind.
Operation of cloud-seeding equipment near the border of one State
may also violate local or State regulations or prohibitions of such
operations in that State. There have been some attempts to resolve these
and other issues through specific legislation in some States and through
informal bilateral agreements. While no formal compacts currently
exist, some compacts allocating waters in interstate streams may be

Because atmospheric processes operate independent of national
borders, weather modification is inherently of international concern,
and. international legal issues have similarities to domestic interstate
activities and dangers. Whereas domestic weather modification law is
confused and unsettled, international law in this area is barely in the
formative stage. In time, ramifications of weather modification may
lead to major international controversy.

Whereas the potential for long-term economic gains through weather
modification cannot be denied, current economic analyses are tenuous in
view of present uncertainty of the technology and the complex nature
of attendant legal and economic problems. Economic evaluation of
weather modification activities has therefore been limited to special,
localized cases, such as the dispersal of cold fog at airports, where
benefit-cost ratios greater than 5 to 1 have been realized through sav-
ings in delayed or diverted traffic. It has also been estimated, on the
basis of a 15-percent increase in snowpack through seeding orographic
clouds, that about 2 million additional acre-feet of water per year
could be produced in the Colorado River Basin, at a cost of about
$1.50 per acre-foot.

Costs of most weather modification operations are generally small
in relation to other costs in agriculture, for example, and are normally
l>elieved to be only a fraction of the benefits which could be achieved
from successful operations. However, if all the benefits and all the costs
are considered, benefit-cost ratios may be diminished. While direct co«ts
and benefits from weather modification are reasonably apparent, in-
direct costs and benefits are elusive and require further study of
sociological, legal, and ecological implications.

There are numerous cases of both real and perceived economic losses
which one or more sectors of the public may suffer while another
group is seeking economic advantage through some form of weather
modification. Overall benefits from weather modification are accord-
ingly reduced when net gains are determined from such instances of
mixed economic advantages and disadvantages. In fact, when mecha-
nisms are established for compensating those who have suffered losses
resultinof from weather modification, benefits to those groups seeking
economic gain through such projects will probably be accordingly

Economically significant weather modification activities will have
an eventual ecological effect, though appearance of that effect may be
hidden or delayed by system resilience and/or confused by system


complexity. Prediction of ecological effects may never be possible with
any precision; however, the greater the precision with which the
weather modifier can predict results of his activities, the more pre-
cisely can the ecologist predict ecological effects. Such effects will
rarely be sudden or catastrophic, but will result from moderate
weather-related shifts in rates of reproduction, growth, and mortality
of plants and animals. Adjustments of plant and animal communities
will thus occur more slowly in regions of highly variable weather than
in those with more uniform conditions which are slowly changing with
some regularity over time. Deliberate weather modification, such as
precipitation augmentation, is likely to have a greater ecological im-
pact in semi-arid regions than in humid ones.

Widespread cloud seeding, using silver iodide, could result in esti-
mated local, temporary increases in silver concentrations in precipita-
tion approaching those in natural waters, but exchange rates would be
an order of magnitude lower than the natural exchange rates. Ex-
change rates will likely be many orders of magnitude less than those
rates at which plants and soils are adversely affected.


1. Weather modification is an emerging technology ; there is a wide
spectrum of capabilities to modify various weather phenomena, rang-
ing from the operational readiness of cold fog dispersal to little prog-
ress beyond initial research in the case of modifying severe storms
such as hurricanes.

2. Along with cold fog dispersal, the only other weather modifica-
tion capability showing near readiness for application is the aug-
mentation of winter snowpack through seeding mountain cloud sys-
tems. A probable increase of about 15 percent is indicated by a number
of experiments and longrunning operational seeding projects in the
western United States.

3. Most scientists and weather modification operators agree that
there is continued need for a wide range of research and development
activity both to refine weather modification techniques where there
has been some success and to advance capabilities in modifying other
weather phenomena where there has been much less or little progress.

4. Current Federal policy for weather modification research and
development follows the mission-oriented approach, where each agency
charged with responsibility for dealing with a particular national
problem is given latitude to seek the best approach or solution to the
problem; this approach or solution may involve weather modification.

5. The structure of Federal organization for weather modification
reflects the mission-oriented approach which is characteristic of the
current Federal policy, the programs loosely coordinated through ad-
visory groups and the Interdepartmental Committee for Atmospheric

0. The interest of the Congress in weather modification has been
shown by the introduction of 110 bills related to the subject since
1017 — of which have become public law — and the consideration of 22
resolutions on weather modification, one of which was passed by the

7. A number of major weather modification policy studies have been
directed by public law or initiated within the executive branch over


the past 25 years ; most of these studies recommended designation of
a lead agency, increased basic meteorological research, increased fund-
ing, improvement of support and cooperation from agencies, and con-
sideration of legal, socioeconomic, environmental, and international
aspects. Although some recommended actions have been undertaken,
others have not seen specific action to date.

8. While major policy studies have recommended increased funding
for Federal weather modification, research and development and fund-
ing has generally increased over the past 20 years, recommended levels
have been consistently higher than those provided, and funding has
actually decreased since fiscal year 1976.

9. With enactment of the National Weather Modification Policy
Act of 1976 and completion of the major policy study mandated by
that act, there is a fresh opportunity for the Congress to assess the
potential usefulness and problems in application of weather modifica-
tion technology and to establish a new Federal policy for weather
modification research and operations.

10. The principal role in regulating weather modification and in
supporting operational programs has been taken by the States, while
the role of the Federal Government has been support of research and
development programs.

11. The majority of the States (29) have some form of law which
relates to weather modification, and the general policy of a State
toward weather modification is usually reflected in the weather modi-
fication law of that State ; laws of some States tend to encourage devel-
opment and use of the technology, while others discourage such

12. The majority of operational weather modification projects in the
United States (58 of a total of 72, or 80 percent in calendar year 1975)
are conducted west of Kansas City, and the largest number of projects
has been in California (20 during 1977) ; most operational projects
are intended to increase precipitation, while others are designed to
suppress hail or disperse fog.

13. Both the greatest support and the strongest opposition to weather
modification projects are focused at the local level, where the economic
and personal interests of local organizations and individuals are most
directly affected; it follows that there is also the least social stress
when decisions to apply or withhold weather modification are made
at the local level.

14. Commercial weather modification operators have substained ac-
tivities since the early days, after which some operations fell into
disrepute, providing a valuable data base for evaluation of long-term
projects and developing expertise over a broad range of capabilities:
most have incorporated improvements into their technology as they
have benefited from accumulated experience and from research results.

15. While the United States is the apparent leader in overall research
and operational weather modification activities, there have been ap-
proximately 20 foreign countries in which activities are conducted an-
nually (25 countries reported such projects for 1976 through the
register of the World Meteorological Organization) ; the largest for-
eign program is that of the Soviet Union, whose operational hail
suppression program covered about 15 million acres in 1976, the largest
such effort in the world.


16. The international community has attempted to further the study
o f weather modification and insure its peaceful use through the recent
development of a Convention on the Prohibition of Military or Any
Other Hostile Use of Environmental Techniques (adopted by the
U.N. General Assembly and opened for signature in May 1977) and
through sponsorship by the World Meteorological Organization of
an international precipitation enhancement program.

17. Legal issues in weather modification are complex and unsettled;
they include resolution of problems of ownership of atmospheric water,
issues of liability, conflicting statutes and regulations of respective

e laws, and the need to develop a regime of relevant international


18. Although the long-term potential for economic gains through
weather modification cannot be denied, attempts to quantify benefits
mnd costs from such activities will in most cases be difficult to undertake
on a practical basis until the technology is more highly developed and
control systems are perfected to permit reliable predictions of

19. Economically significant w r eather modification will always have
an eventual ecological effect, though appearance of the effect may be
delayed or hidden by system resilience and/or confounded by system
complexity ; the more precisely the weather modifier can specify effects
lie will produce, the more precise can be the ecologist's prediction of
likely ecological effects.

20. Modification processes may also be initiated or triggered inad-
vertently rather than purposefully ; man is already causing measurable
variations unintentionally on the local scale, and artificial climate
effects have been observed on local and regional scales. Although not
veri fiable at present, the time may not be remote when human activities
will result in measurable large-scale changes in weather and climate
of more than passing significance.



(I?y Robert E. Morrison, Specialist in Earth Sciences, Science Policy Research
Division, Congressional Research Service)


u It is entirely possible, were he wise enough, that man could produce
favorable effects, perhaps of enonnous practical significance, trans-
forming his environment to render it more salutary for his purposes.
This is certainly a matter which should be studied assiduously and
explored vigorously. The first steps are clear. In order to control
meteorological matters at all we nee d to understand them better than
we now do. When we understand fully ice can at least predict weather
with assurance for reasonable intervals in the future.

''With modem analytical devices, with a team of sound background
and high skills, it is possible today to do a piece of work in this field
which will render immediate benefits, and carry us for toward a more
thorough understanding of ultimate possibilities. By all means let us
get at it."

— Vanne var Bush 1


Two decades after completion of a major study and report on
weather modification by the Advisory Committee on Weather Control
and after the assertions quoted above, many would agree that some
of the more fundamental questions about understanding and using
weather modification remain unsolved. There is a great difference of
opinion, however, on the state of technology in this field. According
to Grant, "Some believe that weather modification is now ready for
widespread application. In strong contrast, others hold that applica-
tion of the technology may never be possible or practical on any
substantial scale." 2 It has been demonstrated that at least some atmos-
pheric phenomena can be modified with some degree of predictable
success, as a consequence of seeding supercooled clouds with artificial
ice nuclei, and there is some promise that the present technology will
be expanded to include a greater scope of weather modification capa-
bilities. Nevertheless, a systematic approach and reasonable progress
in development of weather modification technology have been impeded
by a number of problems.

Changnon asserts that a continuing and overriding problem restrict-
ing progress has been the attempt to apply an ill-defined technology
to increase rain or suppress hail without an adequate scientific under-

1 From statement of Dec. 2, 1957, quoted in final report of the Advisory Committee on
Weather Control, Washington, D.C., U.S. Government Printing Office. 1958. vol. I. p. 1.

2 Grant, Lewis O., "Scientific and Other Uncertainties of Weather Modification. In
William A. Thomas (editor), Legal and Scientific Uncertainties of Weather Modification.
Proceedings of a symposium convened at Duke University. Mar. 11-3 2. 1976, by the
National Conference of Lawyers and Scientists, Durham, N.C., Duke University Press,
1977, p. 7.


34-857—79 3


standing and predictable outcome. 3 Experimentation has been poorly
conducted, intermittent, or too short ; and "results have not been inte-
grated with those of other projects so as to develop a continuing thread
of improving knowledge." 4

In response to the query as to why progress in weather modification
lias been so slow, Fleagle identifies three broad, general impediments.
"First, the physical processes associated with clouds have turned out to
be especially complex and difficult * * *. A second possibility may be
that the atmosphere is inherently stable, so that within broad limits, no
matter what we do to increase precipitation, the results are likely to be
small and roughly the same * * *. A third reason * * * is that progress
has been hamstrung by fragmentation of resources, by submarginal
funding, ineffective planning and coordination, and a general lack of
administrative toughness and fiscal stability." 5

Droessler points out the need to "formulate a comprehensive national
weather modification policy which has the broad support of the scien-
tific community, the general public, private industry, and the Govern-
ment," contending that "the greatest deterrent in getting on with the
task of preparing a satisfactory national policy is the lack of a con-
sensus about the national goals for weather modification." 6

Although operational readiness varies from one form of weather
modification to another, as a result of the degree of understanding and
the complexity of decisionmaking in given situations, the prospects for
successful weather modification are sufficiently promising that at-
tempts to develop effective applications will continue. This was one of
the major areas of co?isensus at a recent symposium on the uncertainties
of weather modification :

There will be increased attempts to modify weather, both because people tend
to do what is technically possible and because the anticipated benefits of precipi-
tation augmentation, hail or lightning suppression, hurricane diversion, and other
activities often exceed the associated costs. 7

With the inevitable increases in weather modification capabilities
and the increasing application of these capabilities, the development of
a technology that is socially useful must be insured through a careful
analysis of attendant benefits and disbenefits. According to Fleagle.
et al.. deliberate efforts to modify the weather have thus far had only
marginal societal impacts; however, as future activities expand, "they
will probably be accompanied by secondary effects which in many
instances cannot be anticipated in detail * * *." Consequently, "rational
policy decisions are urgently needed to insure that activities are di-
rected toward socially useful goals." 8

The lack of a capability to deal with impending societal problems

8 Changnori, Stanley A.. Jr.. "The Federal Role In Weather Modification." bgckgrbund
paper prepared for use by the U.S. Department of Commerce Weather Modification Advi-
sory Board. Mar. !). 3 077, p. 5.

' Ibid., pp. ">-G.

s Fleagle. Robert O.. "An Analysis of Federal Policies in Weather Modification.'' back-
ground paper prepared for use by the U.S. Department of Commerce Weather Modification
Adv:s< rv Hoard. Mar. 1<»77. pp. 17-18. « Droessler, Farl (».. "Weather Modification" (Federal Policies. Funding From AIT Sources Interagency Coordination), background paper prepared for use of the U.S. Depart- ment of Commerce Weather Modification Advisory Board, Mar. l. l!>77. p. 10

7 Thomas. William A. (editor). "Legal and Scientific Uncertainties of Weather Modifie-i-
tion," proceedings of a Symposium convened at Duke University. Mar 11-12. 1970, by the
Vf»'onal Conference of Lawyers and Scientists. Durham, N.C., Dnke Universitv Pres.,
1077, p. vl.

Flt*agie. Robert r > • -lames A. Crutchfteld, Ralph W. Johnson, and Mohamed F. AbdO,
"Weather Modification in the PUbllC Interest." Seattle, American Meteorological Society

and the University of Washington Press, i<>73. p. 3, 31-32.


and emerging management issues in weather modification has been
aphoristically summed up in the following statement by Crutchfield:

Weather modification is in the throes of a serious schizoid process The slow
and sober business of piecing together the scientific knowledge of weather proc-
esses developing the capacity to model the complex systems involved, and assess-
ing systematically the results of modification efforts has led to responsible opti-
mism about the future of these new technologies. On the other hand, the social
technology" of evaluation, choice, and execution has lagged badly. Ihe present de-
cisionmaking apparatus appears woefully inadequate to the extraordinarily ^diffi-
cult task of fitting weather modification into man s pattern of life m optimal
fashion There are' too many game plans, too many coaches, and a disconcerting
proclivity for running hard before deciding which goal line to aim for— or, indeed,
which field to play on. ,J . . . _ .

Mounting evidence indicates that weather modification of several types is,
or may soon become technically feasible. That some groups will derive economic
or other social benefits from such technology is a spur to action. But a whole
thunderhead of critical questions looms on the horizon waiting to be resolved
before any valid decisions can be made about the scale, composition, location,
and management of possible operations. 9


In a study for the Interdepartmental Committee for Atmospheric
Sciences, Homer E. Newell highlighted the potential benefits of inten-
tional weather modification :

The Earth's weather has a profound influence on agriculture, forestry, water
resources, industry, commerce, transportation, construction, field operations,
commercial fishing, and many other human activities. Adverse effects of weather
on man's activities and the Earth's resources are extremely costly, amounting
to billions of dollars per year, sometimes causing irreparable damage as when
human lives are lost in severe storms. There is, therefore, great motivation
to develop effective countermeasures against the destructive effects of weather,
and, conversely, to enhance the beneficial aspects. The financial and other ben-
efits to human welfare of being able to modify weather to augment water
supplies, reduce lightning, suppress hail, mitigate tornadoes, and inhibit the full
development of hurricanes would be very great. 10

More recently. Louis J. Battan gave the following two reasons, with
graphic examples, for wanting to change the weather :

First, violent weather kills a great many people and does enormous property
damage. A single hurricane that struck East Pakistan in Novemlier 1970 killed
more than 250,000 people in a single day. Hurricane Camille hit the United States
in 1969 and did approximately $1.5 billion worth of damage. An outbreak of
tornadoes in the Chicago area on Palm Sunday of 1965 killed about 250 people,
and the tornadoes of April 1974 did likewise. Storms kill people and damage
property, and it is reasonable to ask whether it is necessary for us to accept
this type of geophysical destruction. I say, "No, it is not — it should be possible
to do something."

Second, weather modification involves, and in some respects might control,
the production of those elements we need to survive. Water and food are cur-
rently in short supply in many areas, and these shortages almost certainly will
be more severe in the future. We can develop new strains of wheat and rye and
corn and soybeans and rice, but all is for naught if the weather fails to coop-
erate. If the monsoons do not deliver on schedule in India, residents of that
country starve in large numbers. And if the drought that people have been
predicting for the last several years does spread over the Great Plains, there
will be starvation around the world on a scale never before experienced.

Weather is the one uncontrollable factor in the whole business of agriculture.
Hail, strong winds, and floods are the scourges of agriculture, and we should
not have to continue to remain helpless in the face of them. It may be impossible

9 Crntehfielri. James A.. "Social CVoice and Weather Modification : Concepts and Measure-
ment of Impact." In W. R. Derrick Sewell (editor). Modifying the Weather: a Social
Assessment, Victoria, British Columbia. University of Victoria. 1978. p. 1S7.

10 Newell. Homer E., "A Recommended National Program in Weather Modification." Fed-
eral Council for Science and Technology, Interdepartmental Committee for Atmospheric
Sciences, ICAS report No. 10a, Washington, D.C., November 1966, p. 1.


for us to develop the kind of technology we would like to have for modification
of weather, but to assume failure in such an important endeavor is a course
not to be followed by wise men. 11

Specific statistics on annual losses of life and economic losses from
property damages resulting from weather-related disasters in the
United States are shown in table 1, which w r as developed in a recent
study by the Domestic Council. 12 In the table, for comparison, are
the fiscal year 1975 expenditures by the Federal Government in
weather modification research, according to the several categories of
weather phenomena to be modified. Although it is clear that weather
disasters can be mitigated only partially through weather modifica-
tion, even if the technology were fully developed, the potential value,
economic and otherwise, should be obvious. The following quotation
from a Federal report written over a decade ago summarizes the full
potential of benefits to mankind which might be realized through use
of this technology :

With advances in his civilization, man has learned how to increase the fruit
of the natural environment to insure a livelihood. * * * it is fortunate that
growing knowledge of the natural world has given him an increasing awareness
of the changes that are occurring in his environment and a' so hopefully some
means for deliberate modification of these trends. An appraisal of the prospects
for deliberate weather and climate modification can be directed toward the
ultimate goal of bringing use of the environment into closer harmony with its
capacities and with the purposes of man — whether this be for food production,
relief from floods, assuring the continuance of biologic species, stopping pollu-
tion, or for purely esthetic reasons. 13


Property Modification
damage 1 research

Weather hazard Loss of life 1 (billions) (millions)

Hurricanes 2 30 2 $rj. 8 3 $o. 8

Tornadoes . 2140 2.4 4 1.0

Hail 5.8 3.9

Lightning « 110 .1 .4

Fog M.000 7.5 1.3

Floods 6 240 8 2.3

Frost (agriculture) 7 1. 1

Drought 7 .7 93.4

Total 1,520 6.7 10.8

1 Sources: "Assessment of Research on Natural Hazards," Gilbert F. White and J. Eugene Haas, the MIT Press, Cam-
bridge, Mass., 1975, pp 68, 286, 305, 374; "The Federal Plan for Meteorological Services and Supporting Research, Fiscal
Year 1976," U.S. Department of Commerce, National Oceanic and Atmospheiic Administration (NOAA), Washington, D.C.,
April 1975, p 9; "Weatheiwise," February 1971, 1972, 1973, 1974, 1975, American Meteorological Society, Boston, Mass.;
"Summary Report on Weather Modification, Fiscal Years 1969, 1970, 1971," U.S. Department of Commerce, NOAA, Wash-
ington, D.C., May 1973, pp 72, 81; "Estimating Crop Losses Due to Hail — Wot king Data for County Estimates," U.S. De-
partment of Agriculture, Economic Research Service, September 1974; "Natural Disasters: Some Empirical and Economic
Considerations," G. Thomas Sav, National Bureau of Standards, Washington, D.C., February 1974, p 19; Traffic Safety
magazine, National Safety Council, February 1974.

2 1970-74 average.

3 These funds do not include capital investment in research aircraft and instrumentation primarily for hurricane modi-
fication, which in fiscal year 1975 amounted to $9,200,000.

4 These funds support theoretical research on modification of extratropical cloud systems and their attendant severe
storms such as thunderstorms and tornadoes.

5 1973.

« 1950-72 average.

7 Average.

1 1965-69 average.

9 These funds support precipitation augmentation research, much of which may not have direct application to drought

11 Battan, Louis J.. "The Scientific Uncertainties: a Scientisl Responds." in William A.
Thomas (editor), "Legal and Scientific Uncertainties of Weather Modification." proceed-
ings of a symposium Convened at Duke University, .Mar. 11-12, 197©, by C e National Con-
ference of Lawyers and Scientists. Durham. N.C., Duke University Press. 1!)77. p. 20.

12 U.S Domestic Council. Environmental Resources Committee, Subcommittee on Climate
Change. "The Federal Rofe in Weather Modification," December i ( ->~r», p. 2.

u» Special Commission on Weather Modification. "Weather and Climate Modification,"
National Science Foundation. NSF 6G-3, Washington, D.C., Dec. 20, 1965, p. 7.



The modern period in weather modification is about three decades
old, dating from events in 1946, when Schaefer and Langmuir demon-
strated that a cloud of supercooled water droplets could be transformed
into ice crystals when seeded with dry ice. Activities and interests
among scientists, the commercial cloud seeders, and Government spon-
sors and policymakers have exhibited a nearly 10-year cyclic behavior
over the ensuing years. Each of the three decades since the late 1940's
has seen an initial burst of enthusiasm and activity in weather modi-
fication experiments and/or operations; a midcourse period of con-
troversy, reservations, and retrenchment; and a final period of
capability assessment and policy examination, with the issuance of
major Federal reports with comprehensive recommendations on a
future course.

The first such period ended with the publication of the final report
of the Advisory Committee on Weather Control in 1957. 14 In 1959,
Dr. Robert Brode, then Associate Director of the National Science
Foundation, summarized the significance of that study in a 1959
congressional hearing :

For 4 years the Advisory Committee studied and evaluated public and private
cloud-seeding experiments and encouraged programs aimed at developing both
physical and statistical evaluation methods. The final report of the com-
mittee * * * for the first time placed before the American public a body of
available facts and a variety of views on the status of the science of cloud
physics and the techniques and practices of cloud seeding and weather modifica-
tion. 15

The year 1966 was replete with Government weather modification
studies, major ones conducted by the National Academy of Sciences,
the Special Commission on Weather Modification of the National
Science Foundation, the Interdepartmental Committee for Atmos-
pheric Sciences, and the Legislative Reference Service of the Library
of Congress. During that year, or thereabouts, planning reports were
also produced by most of the Federal agencies with major weather
modification programs. The significance of that year of reevaluatiori
and the timeliness for congressional policy action were expressed by
Hartman in his report to the Congress :

It is especially important that a comprehensive review of weather modification
be undertaken by the Congress at this time, for a combination of circumstances
prevails that may not be duplicated for many years. For the first time since
1957 there now exists, in two reports prepared concurrently by the National
Academy of Sciences and a Special Commission on Weather Modification, created
by the National Science Foundation, a definitive appraisal of the entire scope
of this subject, the broad sweep of unsolved problems that are included, and
critical areas of public policy that require attention. There are currently before
the Congress several bills which address, for the first time since enactment of
Public Law 85-510. the question of the formal assignment of Federal authority
to undertake weather modification programs. And there is increasing demand
throughout the country for the benefits that weather modification may bring. 16

14 F^tablishment of the Advisory Committee on Weather Control by the Congress and its
actJ^ties are discussed in following chapters on the history of weather modification and
on Federal activities, chs. 2 and 5, respectively. Recommendations of the final report are
summarized in ch. 6. Other renorts mentioned in the following paragraphs in this section
are also discussed and referenced in chs. 5 and 6. ■ \ – ..

15 U.S. Congress. House of Representatives. Committee on Science and Astronautics.
"Weather Modification." Hearing. Sfith Cong.. 1st sess., Feb. 16, 1959. Washington, JJ.L.,
U.S. Government Printing OfhYp 19^9. p 3. . t _ _

16 Hartman, Lawton M. "Weather Modification and Control.' Library of Comrress,
Legislative Reference Service. Apr. 27. 1966. Issued as a committee print by the Senate
Committee on Commerce. 89th Cone.. 2d sess., Senate Rept. No. 1139, Washington,

U.S. Government Printing Office, 1966, p. 1.


Toward the close of the third decade, a number of policy studies and
reports appeared, starting in 1973 with a second major study by the
National Academy of Sciences, and including others by the U.S. Gen-
eral Accounting Office and by the U.S. Domestic Council. The major
study of this period was commissioned by the Congress when it enacted
Public Law 94-490, the National Weather Modification Policy Act of
1976, in October of 1976. By that law the Secretary of Commerce was
directed to conduct a study and to recommend the Federal policy and a
Federal research program in weather modification. That study was
conducted on behalf of the Secretary of Commerce by a Weather Modi-
fication Advisory Board, appointed by the Secretary, and the required
report will be transmitted to the Congress during 1978. The importance
of that act and its mandated study was assessed by Dr. Robert M.
White, former Administrator of the National Oceanic and Atmos-
pheric Administration (NOAA), the Commerce Department agency
with administrative responsibilities and research programs in weather
modification :

The National Weather Modification Policy Act of 197C> * * * will influence
X( )AA to some degree during the next year, and its effect may have a large impact
on the agency and the Nation in future years. The comprehensive study of and
report on weather modification that will result from our implementation of this
act will provide guidance and recommendations to the President and the Congress
in the areas of policy, research, and utilization of this technology. We look to this
study and report as an opportunity to help set the future course of a controversial
science and technology with enormous potential for henefit to the Nation. 17

Thus, conditions once more are ripe and the stage has been set, as in
1957 and again in 1966, for the Congress to act in establishing a defini-
tive Federal weather modification policy, one appropriate at least for
the next decade and perhaps even longer. Among other considerations,
such a policy would define the total role of the Federal Government,
including its management structure, its responsibilities for research
and development and for support operations, its authorities for regu-
lation and licensing, its obligation to develop international cooperation
in research and peaceful applications, and its function in the general
promotion of purposeful weather modification as an economically vi-
able and socially accepted technology. On the other hand, other factors,
such as constraints arising from public concern over spending, may
inhibit the development of such policy.

While some would argue that there exists no Federal policy, at least
one White House official, in response to a letter to the President, made
a statement of weather modification policy in 1975:

A considerable amount of careful thought and study has been devoted to the
subject of weather modification and what the Federal role and. in particular, the
role of various agencies should he in (his area. As a result of this study, we have
developed a general strategy for addressing weather modification efforts which
we believe provides for an appropriate level of coordination.

We believe that the agency which is charged with the responsibility for dealing
with a particular national problem should Ite given the latitude to seek the best
approach or solution to the problem. In some instances this may involve a form
of weather modification, while in other instances other approaches may be more

While we would certainly agree that some level of coordination of weather
modification research efforts is logical, we do not believe that a program under

w CJ.S. Congress, Souse of Representatives, Committee on Science and Technology. Sub*

committi d the EBaTlronmeal snd the Atmosphere. "Briefing «"i the National Oceanic and

Atmospheric Administration." Hearings. 9.1th Cong., 1st sess., May 17. 18, 1977. Washing-
Jon. I'.S. Government Printing Ollice, 1977. i». 4-i5.


the direction of any one single agency's leadership is either necessary or desirable.
We have found from our study that the types of scientific research conducted by
agencies are substantially different in approach, techniques, and type of equip-
ment employed, depending on the particular weather phenomena being addressed.
Each type of weather modification requires a different form of program manage-
ment and there are few common threads which run along all programs. 13

Presumably, there will be a resurgence of congressional interest in
weather modification policy during the first session of the 96th Con-
gress, when the aforementioned report from the Secretary of
Commerce has been reviewed and considered. In view of the recom-
mendations in numerous recent studies and the opinions of the Weather
Modification Advisory Board (the group of experts preparing the re-
port for the Secretary of Commerce) , it seems unlikely that any action
by the Congress would perpetuate the policy expounded in the White
House letter quoted above.

It is expected that this present report, intended as an overall review
of the subject of weather modification, will be valuable and timely dur-
ing the anticipated congressional deliberations.


In the broadest sense, weather modification refers to changes in
weather phenomena brought on purposefully or accidentally through
human activity. Weather effects stimulated unintentionally — such as
urban influences on rainfall or fogs produced by industrial com-
plexes — constitute what is usually termed inadvertent weather modifi-
cation. On the other hand, alterations to the weather which are
induced consciously or intentionally are called planned or advertent
weather modification. Such activities are intended to influence single
weather events and to occur over relatively short time spans, ranging
from a few hours in the case of clearing airport fog or seeding a
thunderstorm to perhaps a few days when attempts are made to re-
duce the severity of hurricane winds. Weather modification experi-
ments or operations can be initiated or stopped rather promptly, and
changes resulting from such activities are transient and generally
reversible within a matter of hours.

Climate modification, by contrast, encompasses changes of long-time
climatic variables, usually affecting larger areas and with some degree
of permanence, at least in the short term. Climatic changes are also
brought about by human intervention, and they might result from
either unintentional or planned activities. There are numerous ex-
amples of possible inadvertent climate modification; however, at-
tempts to alter climate purposefully are only speculative. The con-
cepts of inadvertent weather and climate modification are defined
more extensively and discussed fully in chapter 4 of this report.

The primary emphasis of this report is on intentional or planned
modification of weather events in the short term for the general bene-
fit of people, usually in a restricted locality and for a specific time.
Such benefit may accrue through increased agricultural productiv-

18 Ross, Norman E., Jr., letter of June 5, 1975. to Congressman Gilbert Gude. This letter
was the official White House response to a letter of April 25. 1975. from Congressmen
Giule and Donald M. Fraser and Senator Claiborne Pell, addressed to the President, urging
that a coordinated Federal program be initiated in the peaceful uses of weather modifica-
tion. The letter to the President, the replv from Mr. Ross, and comments by Congressman
Gude appeared in the Congressional Record for June 17. 1975, pp. 19201-19203. (This
statement from the Congressional Record appears in app. A.)


ity or other advantages accompanying augmentation of precipitation
or they may result from mitigation of effects of severe weather with
attendant decreases in losses of life or property. There are broader
implications as well, such as the general improvement of weather for
the betterment of man's physical environment for aesthetic and cul-
tural reasons as well as economic ones. The following recent definition
sums up succinctly all of these purposes :

Weather modification is the deliherate and mindful effort by men and women
to enhance the atmospheric environment, to aim the weather at human purposes. 1 "

The specific kinds of planned weather modification usually consid-
ered, and those which are discussed, in turn, in some detail in chapter
3, are the following:

Precipitation enhancement.

Hail suppression.

Fog dissipation.

Lightning suppression.

Mitigation of effects of severe storms.
Planned weather modification is usually considered in the context
of its net benefits to society at large. Nevertheless, it should be recog-
nized that, in particular instances, benefits to some segment of the
population may be accompanied by unintended injuries and costs,
which may be real or perceived, to other segments. There is yet an-
other aspect of advertent weather modification, which has engendered
much controversy, both in the United States and internationally, not
designed for the benefit of those directly affected — the use of weather
modification for hostile purposes such as a weapon of war. This aspect
is not a major consideration in this report, although there is some
discussion in chapters 5 and 10 of congressional concern about such use
of the technology, and in chapter 10 there is also a review of recent
efforts by the United Nations to develop a treaty barring hostile use
of weather modification. 20

Following this introductory chapter, witli its summary of issues,
the second chapter sets the historical perspective for weather modi-
fication, concentrating primarily on activities in the United States to
about the year 1970, The third chapter attempts to review the scien-
tific background, the status of technology, and selected technical prob-
lems areas in planned weather modification; while chapter 4 contains
a discussion of weather and climate changes induced inadvertently by
man's activities or by natural phenomena.

The weather modification activities of the Federal Government —
those of the Congress and the administrative and program activities
of the executive branch agencies — are encompassed in chapter 5 ; and
the findings and recommendations of major policy studies, conducted
by or on behalf of the Federal Government, are summarized in chap-
ter 6. The seventh, eighth, and ninth chapters are concerned with
weather modification activities at the level of State and local govern-
ments, by private organizations, and in foreign countries, respectively.

111 :'77. A discussion paper, included with testimony of Harlan Cleve-
land, Chairman of the Advisory Hoard, in a congressional hearing: U.S. Congress. House
of Representatives. Committee on Science and Technology. Subcommittee on the Environ-
ment and the Atmosphere. Weather Modification. !).".th Cong., 1st sess., Oct. 2(5, 1J>77,
Washington, D.C., U.S. Government Printing Office, H»77. p. 25.

211 Copies of the current official position of the I'.S. Department of Defense on weather
modification and of the draft T T .\ convention prohibiting hostile use of environmental
modification, respectively, are found in apps. B and C.


The increasingly important international problems related to weath-
er modification are addressed in chapter 10, while both domestic and
international legal aspects are discussed in chapter 11. Chapters 12
and 13, respectively, contain discussions on economic and ecological
aspects of this emerging technology.

The 20 appendixes to the report provide materials that are both sup-
plementary to textual discussions in the 13 chapters and intended
to be valuable sources of reference data. In particular, attention is
called to appendix D, which contains excerpts dealing with weather
modification from the statutes of the 29 States in which such activities
are in some way addressed by State law, and to appendix E, which
provides the names and affiliations of individuals within the 50 States
who are cognizant of weather modification activities and interests with-
in the respective States. The reader is referred to the table of contents
for the subjects of the remaining appendixes.

Summary or Issues in Planned Weather Modification

"The issues we now face in weather modification have roots in the
science and technology of the subject, but no less importantly in the
politics of Government agencies and congressional committees and in
public attitudes which grow out of a variety of historical, economic,
and sociological factors." 21 In this section there will be an identifica-
tion of critical issues which have limited development of weather
modification and which influence the ability to direct weather modifi-
cation in a socially responsible manner. The categories of issues do
not necessarily correspond with the subjects of succeeding chapters
dealing with various aspects of weather modification ; rather, they are
organized to focus on those specific areas of the subject where there
has been and there are likely to be problems and controversies which
impede the development and application of this technology.

The following sections examine technological, governmental, legal,
economic, social, international, and ecological issues. Since the primary
concern of this report is with the intentional, planned use of weather
modification for beneficial purposes, the issues summarized are those
involved with the development and use of this advertent technology.
Issues and recommendations for further research in the area of inad-
vertent weather modification are included in chapter 4, in which that
general subject is fully discussed.


In a recent discussion paper, the Weather Modification Advisory
Board summarized the state of weather modification by concluding
that "no one knows how to modify the weather very well, or on a very
large scale, or in many atmospheric conditions at all. The first require-
ment of a national policv is to learn more about the atmosphere it-
self." 22 Representative of the state of weather modification science

21 Fleagle. Crutchfield, Johnson, and Abdo, "Weather Modification in the Public Inter-
est," 1973, p. 15. . . . .

22 Weather Modification Advisory Board. "A U.S. Policy To Enhance the Atmospheric
Environment." Oct. 21, 1977. This discussion paper was included with the testimony ot
Mr. Harlan Cleveland, Chairman of the Advisory Board, in a recent congressional hearing :
U.S. Congress, House of Representatives, Committee on Science and Technology, Subcom-
mittee on the Environment and the Atmosphere. "Weather Modification. 9oth Cong., 1st
sess. Oct. 26, 1977, Washington, D.C., U.S. Govt. Print. Off., 1977, p. 25.


and technology is the following commentary on the state of under-
standing in the case of precipitation enhancement, or rainmaking as it
is popularly called :

Today, despite the fact that modern techniques aimed at artificial stimulation
of rain rest upon sound physical principles, progress is still fairly slow. The
application of these principles is complicated by the overwhelming complexity
of atmosheric phenomena. It is the same dilemna that meteorologists face when
they attempt to predict weather. In both cases, predicting the evolution of
atmospheric processes is limited by insufficient knowledge of the effects produced
by the fairly well-known interactive mechanisms governing atmospheric phenom-
ena. Moreover, the temporal and spatial variability of atmospheric phenomena
presents an additional difficulty. Since any effects that are produced by artificial
intervention are always imposed upon already active natural processes, assess-
ment of the consequences becomes even more difficult. 23

Grant recognizes the current progress and the magnitude of remain-
ing problems when he says that :

Important^and steady advances have been made in developing technology
for applied weather modification, but complexity of the problems and lack of
adequate research resources and commitment retard progress. Advances have
been made in training the needed specialists, in describing the natural and
treated cloud systems, and in developing methodology and tools for the necessary
research. Nevertheless, further efforts are required. 24

Though it can be argued that progress in the development of weather
modification has been retarded by lack of commitment, ineffective
planning, and inadequate funding, there are specific scientific and tech-
nical problems and issues needing resolution which can be identified
beyond these management problems and the basic scientific problem
quoted above with respect to working with the atmosphere. Particular
technical problems and issues at various levels which continue to affect
both research and operational activities are listed below :

1. There is substantial diversity of opinion, even among informed
scientists, on the present state of technology for specific types of
weather modification and their readiness for application and with
regard to weather modification in general.- 5

% 2. There are many who view weather modification only as a drought-
relief measure, expecting water deficits to be quickly replenished
through its emergency use; however, during such periods weather
modification is limited by less frequent opportunities ; it should, in-
stead, be developed and promoted for its year-round use along with
other water management tools.-

3. The design and analysis of weather modification experiments is
intimately related to the meteorological prediction problem, which
needs further research, since the evaluation of any attempt to modify
the atmosphere depends on a comparison between some weather pa-
rameter and an estimate of what would have happened naturally.

4. Many of the problems which restrict Understanding and predic-
tion of weather modification phenomena stem from imprecise knowl-
edge of fundamental cloud processes; the level of research in funda-

2:1 Dennis, Arnett S., and A. Ge^in. "Recommendations for Future Research in Weatlier
Modification," U.S. Department < »i" Commerce, National Oceanic and Atmospheric Admin- istration, Environmental Research Laboratories. Boulder, Colo.. November 1077. p. VI. -"Grant. "Scientific and Other Uncertainties of Weather .Modification," 1977. p. 17. 88 Sec table 2, ch. D. ">!>.

-• Silverman. Bernard A., "What Do We Need In Weather Modification?" In preprints
of the Sixth Conference on Planned and Inadvertent Weather .Modification, Oct. lO-l.'i,
1077, Champaign, 111., Boston, American Meteorological Society, 1977, p. 308.


mental cloud physics and cloud modeling has not kept pace with
weather modification activity. 27

5. Progress in the area of weather modification evaluation meth-
odology has been slow, owing to the complexity of verification prob-
lems and to inadequate understanding of cloud physics and dynamics.

6. Most operational weather modification projects, usually for the
sake of economy or in the anticipation of achieving results faster and
in greater abundance, fail to include a satisfactory means for project

7. There are difficulties inherent in the design and evaluation of any
experiment or operation which is established to test the efficacy of
any weather modification technique, and such design requires the
inclusion of proper statistical methods.

8. In view of the highly varying background of natural weather
phenomena, statistical evaluation of seeding requires a sufficiently
long experimental period: many research projects just barely fail
to achieve significance and credibility because of early termination;
thus, there is a need for longer commitment for such projects, perhaps
5 to 10 years, to insure that meaningful results can be obtained. 2S

9. There is a need to develop an ability to predict possible adverse
weather effects which might accompany modification of specific
weather phenomena : for example, the extent to which hail suppression
or diminishing hurricane winds might also reduce beneficial precipi-
tation, or the possibility of increasing hailfall or incidence of light-
ning from efforts to stimulate rainfall from cumulus clouds. 29

10. The translation of cloud-seeding technologies demonstrated in
one area to another geographical area has been less than satisfactory;
this has been especially so in the case of convective cloud systems,
whose differences are complex and subtle and whose classification is
complicated and sometimes inconsistent.

11. There is increasing evidence that attempts to modify clouds
in a prescribed target area have also induced changes outside the
target area, resulting in the so-called downwind or extended area
effect : reasons for this phenomenon and means for reducing negative
results need investigation.

1*2. There is the possibility that cloud seeding in a given area and
during a given time period has led to residual or extended time effects
on weather phenomena in the target area beyond those planned from
the initial seeding.

13. The conduct of independent cloud-seeding operations in adjacent
locations or in the neighborhood of weather modification experiments
may cause contamination of the atmosphere so that experimental
results or estimates of operational success are biased.

14. There have been and continue to be conflicting claims as to
the reliability with which one can conduct cloud-seeding operations
so that the seeding agent is transported properly from the dispensing
device to the clouds or portions of the clouds one seeks to modify.

27 Hosier. C. L.. "Overt Weather Modification.*' Reviews of Geophysics and Space Phys-
ics, vol. 12. Xo. 3, August 1974, p. 526.

28 Simpson. Joanne, "What Weather Modification Needs." In preprints of the Sixth
Conference on Planned and Inadvertent Weather Modification. Oct. 10-13, 1977. Cham-
paign. 111.. Boston. American Meteorological Society. 1977, p. 306.

29 Hosier, "Overt Weather Modification,' – 1974, p. 325.


15. There is need to develop, improve, and evaluate new and cur-
rently used cloud-seeding materials and to improve systems for deliv-
ery of these materials into the clouds.

16. There is need to improve the capability to measure concentra-
tions of background freezing nuclei and their increase through seed-
ing; there is poor agreement between measurements made with various
ice nucleus counters, and there is uncertainty that cloud chamber
measurements are applicable to real clouds. 30

IT. In order to estimate amounts of fallen precipitation in weather
modification events, a combination of weather radar and raingage
network are often used; results from such measurement systems have
often been unsatisfactory owing to the quality of the radar and its
calibration, and to uncertainties of the radar-raingage intercalibration.

18. There is continuing need for research in establishing seedability
criteria ; that is, definition of physical cloud conditions when seeding
will be effective in increasing precipitation or in bringing about some
other desired weather change.

10. Mathematical models used to describe cloud processes or account
for interaction of cloud systems and larger scale weather systems
greatly oversimplify the real atmosphere; therefore, model research
must be coupled with field research. 31


The basic problem which encompasses all governmental weather
modification issues revolves about the question of the respective roles,
if any, of the Federal, State, and local governments. Resolution of this
fundamental question puts into perspective the specific issues of where
m the several governmental levels, and to what extent, should goals be
set, policy established, research and/or operations supported, activities
regulated, and disputes settled. Part of this basic question includes
the role of the international community, considered in another section
on. international issues; 32 the transnational character of weather modi-
fication may one day dictate the principal role to international orga-

Role of the Federal Government

Because weather modification cannot be restricted by State bound-
aries and because the Federal Government has responsibilities for re-
source development and for reduction of losses from natural hazards,
few would argue that the Federal Government ought not to have some
interest and some purpose in development and possible use of weather
modification technolo public than the Federal Government — often as a result of more direct
contact and personal acquaintance with officials and through greater
actual or perceived control by the voters. Consequently, a number of
weather modification functions, for both reasons of practical effi-
ciency and social acceptance, may be better reserved for State and/or
local implementation. Since weather phenomena and weather modifica-
tion operations cannot be restricted by State boundaries or by bound-
aries within States, however, many functions cannot be carried out
in isolation. Moreover, because of the economy in conducting research
nnd development on a common basis — and perhaps performing other
functions as well — through a single governmental entity, such as an
agency or agencies of the Federal Government, it may be neither
feasible nor wise for State governments (even less for local jurisdic-
tions) to carry out all activities.

Thus, there are activities which might best be reserved for the States
(and possibly for local jurisdictions within States), and those which
more properly belong to the Federal Government. In the previous
l ist of issues on the role of the Federal Government, there was allusion
to a number of functions which might, wholly or in part, be the re-
sponsibility of either Federal or State governments; most of these
will not be repeated here. Issues and problems concerned primarily
with State and local government functions are listed below:

1. State weather modification laws. Where they exist, are nonuni-
form in their requirements and specifications for licensing, permitting,
inspection, reporting, liabilities, and penalties for violations. More-
over, some State laws and policies favor weather modification, while
ot hers oppose 1 he technology.

2. Authorities for funding operational and research projects with-
in States and local jurisdictions within States, through public funds


: " Changnon, "The Federal Role in Weather Modification." |p. 11.

37 ,f Local" bere refers broadly to any jurisdiction below the State level : it could laelucto
cities, townships, counties, groups of counties, water districts, or any other organized area
Operating under public authority.


or through special tax assessments, vary widely and, except in a few
States, do not exist.

3. Decisionmaking procedures for public officials appear to be often
lacking; these could be established and clarified, especially as the pos-
sibility of more widespread application of weather modification tech-
nology approaches.

4. Many public officials, usually not trained in scientific and en-
gineering skills, often do not understand weather modification tech-
nology, its benefits, and its potential negative consequences. Some
training of such officials could contribute to their making wise de-
cisions on the use of the technology, even without complete informa-
tion on which to base such decisions.

5. Many weather modification decisions have had strong political
overtones, with some legislators and other public officials expressing
their views or casting their votes allegedly on the basis of political
expediency rather than on the basis of present or potential societal

6. State and local authorities may need to provide for the education
of the general public on the rudiments of weather modification, on its
economic benefits and disbenefits. and on other societal aspects.

7. To keep communication channels open, mechanisms such as pub-
lic hearings could be established to receive comments, criticisms, and
general public sentiments on weather modification projects from in-
dividual citizens and from various interest groups.

8. Criteria and mechanisms have not been established for compen-
sating those individuals or groups within States who might be eco-
nomically injured from weather modification operations.

9. Questions of water rights within States, as well as between States,
have not been addressed and/or resolved in a uniform manner.


Legal issues in weather modification are complex and unsettled.
They can be discussed in at least four broad categories :

1. Private rights in the clouds ;

2. Liability for weather modification ;

3. Interstate legal issues ; and

4. International legal issues, 38

The body of law on weather modification is slight, and existing case
law offers few guidelines to determine these issues. It is often neces-
sary, therefore, to analogize weather modification issues to more set-
tled areas of law such as those pertaining to water distribution.

Private rights in the clouds

The following issues regarding private rights in the clouds may be
asked :

Are there any private rights in the clouds or in the water which
may be acquired from them ?

Does a landowner have any particular rights in atmospheric
water ?

Does a weather modifier have rights in atmospheric water \

^Questions on regulation or control of weather modification activities through licensing
and permitting, while of a basic legal nature, are related to important administrative func-
tions and are dealt with under issues concerned with Federal and State activities.


Some State statutes reserve the ownership or right to use atmospheric
water to the State. 39

There is no general statutory determination of ownership of atmos-
pheric water and there is no well-developed body of case law. Conse-
quently, analogies to the following general common law doctrines may
be helpful, but each has its own disadvantages when applied to weather
modification :

1. The doctrine of natural rights, basically a protection of the land-
owner's right to use his land in its natural condition (i.e., precipita-
tion is essential to use of the land as are air, sunlight, and the soil

2. The ad coelum doctrine which states that whoever owns the land
ought also to own all the space above it to an indefinite extent.

3. The doctrine of riparian rights, by which the one owning land
which abuts a watercourse may make reasonable use of the writer, sub-
ject to similar rights of others whose lands abut the watercourse.

4. The doctrine of appropriation, which gives priority of right based
on actual use of the water.

5. The two main doctrines of ownership in the case of oil and gas
(considered, like water, to be "fugitive and migratory" substances) ;
that is, (a) the non-ownership theory, by which no one owns the oil and
gas until it is produced and anyone may capture them if able to do so;
and (b) the ownership-in-place theory, by which the landowner has the
same interest in oil and gas as in solid minerals contained in his land.

6. The concept of "developed water," that is, water that would not
be available or would be lost were it not for man's improvements.

7. The concept of "imported water," that is, water brought from one
watershed to another.

Liability for weather modification

Issues of liability for damage may arise when drought, flooding, or
other severe weather phenomena occur following attempts to modify
the weather. Such issues include causation as well as nuisance, strict
liability, trespass, and negligence. Other issues which could arise relate
to pollution of the air or water through introduction of artificial nu-
cleants such as silver iodide, into the environment. While statutes of
10 States discuss weather modification liability, there is much varia-
tion among the specific provisions of the laws in those States. 40

Before any case can be made for weather modification liability
based upon causation it must be proven that the adverse weather con-
ditions were indeed brought about by the weather modifier, a very
heavy burden of proof for the plaintiff. In fact, the scientific uncer-
tainties of weather modi Heal ion are such that no one has ever been able
to establish causation of damage through these activities. As weal her
modification technology is improved, however, the specter of a host of
liability issues is expected to emerge as evidence for causation becomes
more plausible.

While the general defense of the weather modifier against liability
charges is that causation has not been established, he may also use as
further defense the arguments based upon immunity, privilege, con-
sent , and waste.

• Sec p. 4.">o, ch. 1 1. and app. n.

M Sec discussion p. 453 in ch. 11 and app. D.


Interstate legal issues

When weather modification activities conducted in one State affect
another State as well, significant issues may arise. The following-
problem categories are examples of some generally unresolved inter-
state issues in weather modification :

1. There may be the claim that cloud seeding in one State has removed
from the clouds water which should have fallen in a second State or
that excessive flooding in a neighboring State has resulted from seed-
ing in a State upwind.

2. Operation of cloud-seeding equipment near the border in one State
may violate local or State ordinances which restrict or prohibit weather
modification in an adjacent State, or such operations may conflict with
regulations for licensing or permitting of activities within the bor-
dering State.

Some States have attempted to resolve these issues through specific
legislation and through informal bilateral agreements. 41 Another ap-
proach would be through interstate compact, though such compacts re-
quire the consent of Congress. No compacts specifically concerned with
weather modification currently exist, though some existing compacts
allocating waters in interstate streams may be applicable to weather

International legal issues

Because atmospheric processes operate independent of national
borders, weather modification is inherently of international concern.
International legal issues have similarities to domestic interstate activi-
ties and dangers. The following serious international questions, which
have arisen in conjunction with a developing capability to modify the
weather, have been identified by Orfield : 42

Do countries have the right to take unilateral action in all
weather modification activities?

What liability might a country incur for its weather modifica-
tion operations which [might] destroy life and property in a
foreign State?

On what theory could and should that State base its claim ?
The primary international legal issue regarding weather modifica-
tion is that of liability for transnational injury or damage, which could
conceivably result from any of the following situations :

(1) injury or damage in another nation caused by weather
modification activities executed within the United States;

(2) injury or damage in another nation caused by weather
modification activities executed in that nation or a third nation by
the United States or a citizen of the United States ;

(3) injury or damage in another nation caused by weather
modification activities executed in an area not subject to the juris-
diction of any nation (e.g., over the high seas), by the United
States or a citizen thereof ; and

(4) injury or damage to an alien or an alien's property within
the United States caused by weather modification activities exe-
cuted within the United States.

41 See discussion p. 457 in ch. 11 and app. D.

42 Orfield, Michael B.. "Weather Genesis and Weather Neutralization: a New Approach
to Weather Modification," California Western International Law Journal, vol. 6, no. 2,
spring 1976, p. 414.

34-S57— 79 4


Whereas domestic weather modification law is confused and unset-
tled, international law in this area is barely in the formative stage. In
time, ramifications of weather modification may lead to major interna-
tionl controversy. 43


The potential for long-term economic gains through weather modi-
fication cannot be denied ; however, current, economic analyses are tenu-
ous in view of present uncertainty of the technology and the complex
nature of attendant legal and economic problems. Meaningful economic
evaluation of weather modification activities is thus limited to special,
localized cases, such as the dispersal of cold fog at airports, where bene-
fit-cost ratios greater than 5 to 1 have been realized through savings in
delayed or diverted traffic. Various estimated costs for increased pre-
cipitation through cloud seeding range from $1.50 to $2.50 per acre-
foot in the western United States.

fsy/es complicating economic analyses of weather modification

Costs of most weather modification operations are usually relatively
small and are normally believed to be only a fraction of the benefits
obtained through such operations. However, if all the benefits and all
the costs are considered, benefit-cost ratios may be diminished. While
direct costs and benefits from weather modification are reasonably
obvious, indirect costs and benefits are elusive and require further study
of sociological, legal, and ecological implications.

In analyzing benefit-cost ratios, some of the following considerations
need to be examined :

Weather modification benefits must be considered in terms of
the costs for achieving the same objectives as increased precipita-
tion, e.g., through importation of water, modified use of agricul-
tural chemicals, or introduction of improved plant strains.

Costs for weather modification operations are so low in compari-
son with other agricultural investments that farmers may gamble
in spending the 5 to 20 cents per acre for operations designed to
increase rainfall or suppress hail in order to increase yield per
acre, even though the results of the weather modification opera-
tions may be doubtful.

Atmospheric conditions associated with prolonged droughts are
not conducive to success in increasing precipitation; however,
under these conditions, it is likely that increased expenditures
may be made for operations which offer little hope of economic

Increased precipitation, obtained through a weather modifica-
tion program sponsored and funded by a group of farmers', can
also benefit other farmers who have not shared in the costs; thus,
the benefit-cost ratio to those participating in the program is
higher than it need be if all share in its costs.

As weather modification technology develops and programs be-
come more 1 sophisticated', increased costs for equipment and labor
will increase direct costs to clients: indirect costs resulting from
increased State license and permit fees and liability insurance for
operators will probably also be passed on to the customer.

I: s»'c ch. 10 on International aspects and i>. 4< ;s. ch. 11; on International legal aspects of wpa i her modification. 19 The sophistication of future programs will likely incur addi- tional costs for design, evaluation, and program information ac- tivities, along with supporting meteorological prediction services; these costs will be paid from public funds or by private clients, in either case reducing the overall benefit-cost ratios. Ultimate costs for compensation to those incurring disbenefits from weather modification operations will offset overall benefits and thus reduce bene fit -cost ratios. Weather modification and conflicting interests There are numerous cases of both real and perceived economic losses which one or more sectors of the public may suff er while another group is seeking economic advantage through some form of weather modi- fication. Overall benefits from weather modification are accordingly reduced when net gains are computed from such instances of mixed economic advantages and disadvantages. Benefits to the parties seek- ing economic gain through weather modification will be directly re- duced at such time when mechanisms are established for compensating those who have suffered losses. The following are some examples of such conflicting situations : Successful suppression of hail may be valuable in reducing crop damage for orchardists while other agricultural crops may suffer f rom decrease of rain concomitant with the hail decrease. Additional rainy days may be of considerable value to farmers during their growing season but may be detrimental to the finan- cial success of outdoor recreational enterprises. Increased snowpack from orographic cloud seeding may be beneficial to agricultural and hydroelectric power interests but increases the costs for maintaining free passage over highways and railroads in mountainous areas. Successful abatement of winds from severe storms, such as those of hurricanes, may result in decreased precipitation necessary for agriculture in nearby coastal regions or may redistribute the ad- verse storm effects, so that one coastal area is benefitted at the ex- pense of others. SOCIAL ISSUES It has been said that "weather modification is a means toward so- cially desired ends, not an end in itself. It is one potential tool in a set of possible societal adjustments to the vagaries of the weather. Iden- tifying when, where, and how to use this tool, once it is scientifically established, is the primary need in weather modification." 44 It is likely that, in the final analysis, the ultimate decisions on whether weather modification should and will be used in any given instance or will be adopted more generally as national or State programs depends on social acceptance of this tool, no matter how well the tool itself has been perfected. That this is increasingly the case has been Suggested by numerous examples in recent years. Recently Silverman said : Weather modification, whether it he research or operations, will not progress wisely, or perhaps at all, unless it is considered in a context that includes everyone M Fnrhar. Barbara C. "What Does Weather Modification Need ?" In preprints of the Sixth Conference on rianr.pd and Inadvertent Weather Modification. October 10-13, 1977. Cham- paign* 111. Boston. American Meteorological Society, 1977. p. 296. 20 that may be affected. We must develop and provide a new image of weather modification. 45 Regardless of net economic benefits, a program is hard to justify when it produces obvious social losses as well as gains. Research in the social science of weather modification has not kept pace with the development of the technology, slow as that has been. In time, this failure may be a serious constraint on further develop- ment and on its ultimate application. In the past, organized opposition has been very effective in retarding research experiments and in cur- tailing operational cloud-seeding programs. Thus, there is need for an expanded effort in understanding public behavior toward weather modification and for developing educational programs and effective decisionmaking processes to insure intelligent public involvement in eventual application of the technology. Social issues discussed in this section are those which relate to public behavior and public response to weather modification, while societal issues are generally considered to include economic, legal, and other nontechnical issues as Veil as the social ones. These other aspects of societal issues were discussed in preceding sections. In the subsections to follow there are summaries of social implications of weather modifi- cation, the need for public education, and the problem of decisionmaking. Social factors It has been said that social factors are perhaps the most elusive and difficult weather modification externalities to evaluate since such fac- tors impinge on the vast and complex area of human values and at- titudes. 46 Fleagle, et al., identified the following important social implications of weather modification, which would presumably be taken into account in formulation of policies : 47 1. The individuals and groups to be affected, positively or negatively, by tlie project must be defined. An operation beneficial to one party may actually barm another. Or an aggrieved party may hold the operation responsible * * ::: for damage * * * which might occur at the same time or following the modification. 2. The impact of a contemplated weather modification effort on the genera! well-being of society and the environment as a whole must be evaluated. Con- sideration should be given to conservationists, outdoor societies, and other citizens and groups representing various interests who presently tend to ques- tion any policies aimed at changes in the physical environment. It is reasonable and prudent to assume that, as weather modification operations expand, question- ing and opposition by the public will become more vocal. 3. Consideration must be given to the general mode of human behavior in response to innovation. There are cases where local residents, perceiving a cause and effect relationship between economic losses from severe weather and nearby weather modification operations, have continued to protest, and even to threaten violence, after all operations bave been suspended. 4. The uniqueness and complexity of certain weather modification operations must be acknowledged, and special attention should be given to their social and legal implications. The cases of hurricanes and tornadoes are especially perti- nent. Alteration of a few degrees in the path of a hurricane may result in its missing a certain area * * * and ravaging * * * instead, a different one. The decision on whether such an operation is justified can reasonably be made only at the highest level, and would need to be based on the substantial scientific finding thai the anticipated damages would be loss than those originally predicted h td the hurricane been allowed to follow its course. 1 b Silverman, Bernard A. "What Do We Need in Weather Modification?" In preprints of tli<' Sixth Conference on Planned and [nadvertenl Weather Modification, October 10—13, litTT. Champaign, ill.. Boston, American Meteorological Society. u»77. p. 310. ia Flengle, Crutchfleld, Johnson, and Abdo. "Weather Modification in the Public Interest." 1074. p. :',7-38. *• Ibid., p. 38-40. 21 5. Attention must be given to alternatives in considering a given weather modification proposal. The public may prefer some other solution to an attempt at weather tampering which may be regarded as predictable and risky. Further- more, alternative policies may tend to be comfortable extensions of existing policies, or improvements on them, thus avoiding the public suspicion of inno- vation. In an area such as weather modification, where so many uncertainties exist, and where the determination or assigning of liability and responsibility are far from having been perfected, public opposition will surely be aroused. Any alternative plan or combination of plans will have its own social effects, however, and it is the overall impact of an alternative plan and the adverse effects of not carrying out such a plan which, in the final analysis, should guide decisions on alternative action. 6. Finally, it is important to recognize that the benefits from a weather modi- fication program may depend upon the ability and readiness of individuals to change their modes of activity. The history of agricultural extension work in the United States suggests that this can be done successfully, but only with some time lag, and at a substantial cost. Social research studies suggest that public perception of flood, earthquake, and storm hazards is astonishingly casual. Need for public education on weather modification The previous listing of social implications of weather modification was significantly replete with issues derived from basic human atti- tudes. To a large extent these attitudes have their origin in lack of in- formation, misconceptions, and even concerted efforts to misinform by organized groups which are antagonistic to weather modification. As capabilities to modify weather expand and applications are more wide- spread, it would seem probable that this information gap would also widen if there are no explicit attempts to remedy the situation. "At the very least," according to Fleagle, et al., "a large-scale continuing pro- gram of education (and perhaps some compulsion) will be required if the potential social gains from weather modification are to be realized in fact," 48 Whether such educational programs are mounted by the States or by some agency of the Federal Government is an issue of jurisdiction and would likely depend on whether the Federal Govern- ment or the States has eventual responsibility for management of op- erational weather modification programs. Information might also be provided privately by consumer groups, professional organizations, the Aveather modification industry, or the media. It is likely that educational programs would be most effective if a variety of practical approaches are employed, including use of the news media, publication of pamphlets at a semitechnical level, semi- nars and hearings, and even formal classes. Probably the latter cate- gories would be most appropriate for civic groups, Government offi- cials, businessmen, or other interests who are likely to be directly affected by contemplated operations. The following list of situations are examples of public lack of under- standing which could, at least in part, be remedied through proper educational approaches : There is much apprehension over claims of potential d^rger of a long-lasting nature on climate, which could supposedly result from both inadvertent and planned modification of the weather, with little insight to distinguish between the causes and the scales of the effects. There have been extravagant claims, propagated through ig- norance or by deliberate distortion by antagonistic groups, about 48 Ibid., p. 40. 22 the damaging effects of cloud seeding on ecological systems, human lien 1th. and air and water quality. The controversies between opposing groups of scientists on the efficacy of weather modification technologies and between scien- tists and commercial operators on the readiness of these technolo- gies for application has engendered a mood of skepticism and even mistrust of weather modification on the part of a public which is largely uninformed on technical matters. The public has often been misinformed by popular news media, whose reporters seek to exploit the spectacular in popular weather modification "stories" and who, themselves usually uninformed in technical aspects of the subject, tend to oversimplify and distort the facts associated with a rather complex science and technology. There has been an organized effort on the part of groups opposed to weather modification to mount an educational program which runs counter to the objectives of informing the public about the potential benefits of a socially acceptable technology of weather modification. Portions of the public have acquired a negative impression that meteorologists and Government officials concerned with weather modification are irresponsible as a result of past use. or perceived present and future use. of the technology as a weapon of war. Lack of information to the public has sometimes resulted in citizen anger when it is discovered that a seeding project has been going on in their area for some time without their having been informed of it. Decisionmaking "The nature of wenther processes and the current knowledge about them require that most human decisions as to weather modification must be made in the face of uncertainty. This imposes special re- straints on public agencies and it increases the difficulty of predict- ing how individual farmers, manufacturers, and others who are directly affected by weather would respond to changes in leather Characteristics. 5 ' 49 The situation since 1965 when this statement was made has changed little with resrard to predictability of weather processes and their modification. There has also been little progress toward developing decisionmaking processes which can be applied, should the need arise, on whether or not weather modification should be emploved. A number of studies on social attitudes indicate that the preference of most cit izens is that decisionmaking in such areas as use or restraint from use of weather modification should be at the local level. owim>-
to the feeling that citizens' rights and property are best protected
when decisions are made bv officials over whom they have the most
direct; control. Farhar savs that evidence suggests that one important
condition for public acceptance of weather modification technology
is public involvement in the decision process, especially in civic
derisions.™ Procedures must then be developed for enabling {peal

49 Special Commission on Wcnther Modification. "Weather and Climate Modification."
NRF or, irto.~. p uc.

» F.-irlisir. Bar nun) P. "The Pnldie Derides Al

William Humphreys jDOsed a plausible explanation for the appar-
ently high correlation between such weather events and preceding
battles. He noted that plans were usually made and battles fought in
good weather, so that after the battle in the temperate regions of
Europe or North America, rain will often occur in accordance with
the natural 3- to 5-day periodicity for such events. 5 Even in modern
times there was the conviction that local and global weather had been
adversely affected after the explosion of the first nuclear weapons and
the various subsequent tests in the Pacific and elsewhere. Despite
statements of the U.S. Weather Bureau and others pointing out the
fallacious reasoning, such notions became widespread and persistent. 7

In addition to these somewhat rational though unscientific obser-
vations, many of which were accompanied by testimony of reliable
witnesses, there had been, and there still exist in some primitive cul-
tures, superstitions and magical practices that accompany weather
phenomena and attempts to induce changes to the weather. Daniel
Halacy relates a number of such superstitiouslike procedures which
have been invoked in attempts to bring rain to crops during a drought
or to change the 1 weather in some other way so as to be of particular
benefit to man : 8

Primitive rainmakers would often use various intuitive gestures, such as
sprinkling water on the soil that they wanted the heavens to douse, Mowing
mouthfuls of water into the air like rain or mist, hammering on drums to inu-
la re thunder, or throwing firebrands into the air to simulate lightning.

Women would carry water at night to the field and pour it out to coax the
skies to do likewise.

American Indians blew water from special pipes in imitation of the rainfall.

It was believed that frogs came down in the rain because many were seen
following rain : therefore, frogs were hung from trees so that the heavens would
pour down rain upon them.

Sometimes children were buried up to their necks in the parched ground and
then cried for rain, their tears providing the imitative magic.

Ward, R. !>«• < \. "Artificial Rain : a Review of the Subject to the Close of lSSft." Amor- lean Meteorological Journal; vol. s. May 1891-Aprtl *S92, p. 484. * Ibid., n. 408. s Humphreys. William -1 . "Rain Making and Other Weather Vagaries." Baltimore, The Williams and Wilkins Co.. 11*20. p. 31, "Byers, Horace i:.. 'History of Weather Modification." In Wilnot N. Hess (editor), "Weather and Climate Modification," New York. Wiley, 1!)74, p. 4. ~ T'.id « Halacy, Daniel S., Jr., "The Weather Changers," New York. Harper & Row. 1908. pp. 27 In China, huge paper dragons were part of religious festivals to bring rain; if- drought persisted, the dragon was angrily torn to bits. North American Indians roasted young women from enemy tribes over a slow fire, then killed them with arrows before eating their hearts and burying their remains in the fields they wanted irrigated with rainfall. Scottish witches conjured up the wind by beating a stone three times with a rag dipped in water, among intonations like those of characters in a Shake- spearean play. New Guinea natives used wind stones upon which they tapped with a stick, the force of the blow bringing anything from a zephyr to a hurricane. Pregnant women in Greenland were thought to be able to go outdoors, take a breath, and exhale it indoors to calm a storm. In Scandinavian countries witches sold knotted bits of string and cloth which, supposedly, contained the wind ; untying one knot at sea would produce a mod- erate wind, two a gale, and three a violent storm. Australian bushmen thought that they could delay the Sun by putting a clod of dirt in the fork of a tree at just the height of the Sun, or hasten its departure by blowing sand after it. Bells have been thought to prevent hail, lightning, and windstorms, and some- times they are still rung today for this purpose. EARLY SCIENTIFIC PERIOD James P. Espy was a 19th century American meteorologist known especially for his development of a theon^ of storms based on convec- tion. Recognizing that a necessary condition for rainfall is the formation of clouds by condensation of water vapor from rising air, Espy considered that rain could well be induced artificially when air is forced to rise as a result of great fires, reviving a belief of the pre- .scientific era but using scientific rationale. In the National Gazette in Philadelphia of April 5, 1839, he said : From principles here established by experiment, and afterward confirmed by observation, it follows, that if a large body of air is made to ascend in a column, a large cloud will be generated and that that cloud will contain in itself a self- sustaining power, which may move from the place over which it was formed, and cause the air over which it passes, to rise up into it, and thus form more cloud and rain, until the rain may become more general. 8 If these principles are just, when the air is in a favorable state, the bursting out of a volcano ought to produce rain ; and such is known to be the fact ; and I have abundant documents in my possession to prove it. So, under very favorable conditions, the bursting out of great fires ought to produce rain ; and I have many facts in my possession rendering it highly probable, if not certain, that great rains have sometimes been produced by great fires. 10 Later in the same article Espy stated that : From these remarkable facts above, I think it will be acknowledged that there is some connection between great fires and rains other than mere coincidence. But now. when it is demonstrated by the most decisive evidence, the evidence of experiment, that air, in ascending into the atmosphere in a column, as it must do over a great fire, will cool by diminished pressure, so much that it will begin to condense its vapor into cloud. 11 Espy postulated three mechanisms which could prevent great fires from providing rain at all times when they occur: (1) If there is a current of air at some height, it sweeps away the uprushing current of air; (2) the dew-point may be too low to produce rain at all: and (3) there may be an upper stratum of air so light that the rising 9 Espy. Tames P.. "Artificial Rains." National Gazette. Philadelphia. Apr. 5, lSf!9. Re- printed in James P. Espy, "Philosophy of Storms," Boston. Little & Brown. 1841. pd. 493-494. 10 Ibid., p. 494. 11 Ibid., p. 496. 28 column may not be able to rise far enough into it to cause rain. 12 He proposed an experiment in which he would set fire to a "large mass of combustibles," which would be ready for the right circumstances and at a time of drought. He added : "Soon after the fire commences, I will expect to see clouds begin to form * * *. I will expect to see this cloud rapidly increase in size, if its top is not swept off by a current of air at a considerable distance abov^e the Earth, until it becomes so lofty as to rain.'- 13 For over a decade Espy served as an adviser to the Congress on meteorological problems. He proposed in 1850 what is perhaps the first Fedora! project for large-scale weather modification. His plan included amassing large quantities of timber in the Western States along a 600- to 700-mile north-south line, to be set on fire simultaneously at regular T-day intervals. He believed that this fire could have started a "rain of great length" traveling toward the East, not breaking up until reaching "far over the Atlantic Ocean; that it will rain over the whole country east^of the place of beginning." The cost of this experiment would "not amount to half a cent a year to each individual in the United States." 14 Congress did not endorse the proposal for reasons which are unknown: however. Fleagle speculates that perhaps this failure was due to the fact that Congress had not yet accustomed itself to appropriating funds for scientific enterprises. 15 There was continuing controversy over whether or not fire could cause increased rainfall. In an article which appeared in Nature in 1871, J. K. Laughton stated that, "The idea that large fires do, in some way, bring on rain, is very old; but it was, I believe, for the first time stated as a fact and explained on scientific grounds by the late Pro- fessor Espy." 10 Laughton cited instances where burning brush in hot, dry weather did not result in any rainfall, and he concluded that : Large fires, explosions, battles, and earthquakes do tend to cause atmospheric disturbance, and especially to induce a fall of rain ; but that for the tendency to produce effect, it is necessary that other conditions should be suitable. With regard to storms said to have been caused by some of these agencies, the evidence is still more unsatisfactory ; and, in our present ignorance of the cause of storms generally, is quite insufficient to compel us to attribute any one particular gale, extending probably over a wide area, to some very limited and comparatively insignificant disturbance. 17 The 1871 Chicago fire also aroused interest, many believing that the fire was stopped by the rainfall which it had initiated. Ward cites a telegram of the time sent to London which read : This fire was chiefly checked on the third or fourth day by the heavy and con- tinuous downpour of rain, which it is conjectured is partly due to the great atmos- pheric disturbances which such an extensive lire would cause, especially wben we are told that the season just previous to the outbreak of the fire had been par- ticularly dry." u Ibid. 1 ■ I 'id., p. 400. « Espy, James P., "Second Reporl on Meteorology to the Secretary of the Navy." U.S. Senate. Executive Doctlmetats; No. 89, vol. 11, ."{1st Cong., 1st Bess. Washington, Wm. M Belt 1850. p. 20. us Fleagle. Robert O.. "Background and Present status of Weather Modification." In Robert (i. Flea pie (editor). "Weather Modification: Science and Public Policy." University of w ah inert on Press, Seattle 1968, p. 7. "' Lautrhton. J K., "Can Weather lie Influenced bv Artificial Means?" Nature, Feb. 10. 1871 i. :•(»(; 17 Ibid., p. 307. « Reported in Ward. "Artificial Rain : a Review of the Subject to the Close of 1889," 1*02. pp. 480-400. 29 On the other hand, Prof. I. A. Lapham, speaking of the Chicago fire, contradicted the previous account, saying : During all this time — 24 hours of conflagration — no rain was seen to fall, nor did any rain fall until 4 o'clock the next morning ; and this was not a very con- siderable downpour, but only a gentle rain, that extended over a large district of country, differing in no respect from the usual rains. It was not until 4 days afterward that anything like a heavy rain occurred. It is, therefore, quite certain that this case cannot be referred to as an example of the production of rain by a great fire. 19 Lapham goes on to say that, "The case neither confirms nor dis- proves the Espian theory, and we may still believe the well-authenti- cated cases where, under favorable circumstances of very moist air and absence of wind, rain has been produced by very large fires." 20 Prof. John Trowbridge of Harvard reported in 1872 on his experi- ments in which he investigated the influence of flares on atmospheric electricity. Noting that the normal atmospheric state is positive and that clearing weather is often preceded by a change from negative to positive charge, he suggested that perhaps large fires may influence the production of rain by changing the electrical state of the atmosphere, since, in his tests, his flame tended "to reduce the positive charge of electricity which generally characterizes the air of fine weather." 21 He concluded by saying: "The state of our knowledge, however, in regard to the part that electricity plays in atmospheric changes is very meager. The question of the truth of the popular belief that great fires are fol- lowed by rain still remains unanswered." 22 Meanwhile, H. C. Russel, president of the Royal Society of South Wales and government astronomer, attempted to dispel the ideas that both cannonading and great fires could be used to produce rain. He hypothesized that, if fire were to have such an effect, rain should arrive within 48 hours following the fire. Reviewing the records of 42 large fires (including two explosions) covering a 21-year period, Russel concluded that there was not one instance in which rain followed within 48 hours as an evident consequence of the fire. He further cal- culated that to get increased rainfall of 60 percent over a land surface of 52,000 square feet at Sidney would require 9 million tons of coal per day, in an effort to show what magnitude of energy expenditure was necessary and how futile such an attempt would be. 23 Toward the latter part of the 19th century there were a number of ideas and devices invented for producing rain artificially. In 1880 David Ruggles of Virginia patented what he said was "a new and use- ful mode of producing rain or precipitating rainfalls from rainclouds, for the purpose of sustaining vegetation and for sanitary purposes." His plan included a scheme by which balloons carrying explosives were sent up into the air, the explosives to be detonated in the upper air "by electric currents." 24 19 Lanham, I. A.. "The Great Fires of 1871 in the Northwest." The Journal of the Frank- lin Institute, vol. 64, No. 1. July 1872, pp. 46-47. 20 IMd., p. 47. 21 Trowlirirtge, John, "Great Fires and Rain-storms." The Popular Science Monthly, vol. 2, December 1872. p. 211. 22 Tbid. 23 Report of an address bv H. C. Russel was given in Science, vol. 3, No. 55, Feb. 22. 1884, pp. 229-230. 24 "New Method of Precipitating Rain Falls," Scientific American, vol. 43, Aug. 14. 1S80, p. 106. 30 G. H. Bell suggested a rainmaking device, consisting of a hollow tower 1.500 feet high, through which air was to be blown into the atmosphere, the volume of the up-rushing air to be increased through use of a s}^stem of tubes around the tower. The inventer consider that the same system could be used to prevent rain, by reversing the blower so that the descending air might "annihilate" the clouds. 25 Still other schemes and contrivances were proposed and patented. J. B. Atwater was granted a patent in 1887 for a scheme to dissipate tornadoes by detonating an explosive charge in their centers, and an- other was granted to Louis Gathman in 1891 for seeding clouds for rain by exploding a shell containing "liquid carbonic acid gas" at cloud height, 20 the latter concept antedating by over 50 years the more recent carbon dioxide seeding projects. There continued to be adherents to the idea that explosions could cause rainfall. This belief was reinforced by "evidence" of such a con- nection in a book by Edward Powers, called "War and the Weather," published in 1871 and 1890 editions, in which the author recounted the instances in which rain followed battles, mostly from North America and Europe during the 19th century. 27 Powers was convinced that : The idea that rain can be produced by human agency, though sufficiently startling, is not one which, in this age of progress, ought to be considered as impossible of practical realization. Aside from its connection with the supersti- tions of certain savage tribes, it is an opinion of comparatively recent origin, and is one which cannot be regarded as belonging, in any degree, to a certain class of notions which prevail among the unthinking; * * * on the contrary, it is one which is confined principally to those who are accustomed to draw conclusions only from adequate premises, and * * * founded on facts which have come under their own observation. 28 In tones somewhat reminding us of those urging a greater Federal research effort in recent years, Powers proposed that experiments be undertaken for economic benefit : Judging from the letters which I have received since commencing in 1870 an attempt to bring forward the subject of rains produced by cannon tiring. I believe that the country would regard with interest some experiments in the matter, and would not begrudge the expense, even if they should prove unsuccessful in leading to a practical use of the principle under discussion. In some matters connected w T ith science, the Government has justly considered that an expenditure of public funds was calculated to be of public benefit: but where, in anything of tiie kind it. has ever undertaken, has there been so promising a field for such actions as here? 20 Powers, upon examining the records of many battles, said : Let us proceed to facts — facts not one of which, perhaps, would be of a in- significance if it stood alone and unsupported by the others; but which, taken in the aggregate, furnish the strongest evidence that heavy artillery firing has an influence on the weather and tends to bring rain. 11 Perhaps influenced by the arguments of Powers and others, in 1890 the U.S. Congress had become so much interested in and gained Another Ka in Controller." Scientific American, vol. 4:{. Aug, 21. 1SSO. p 11M. 26 Harrington, Mark W.. "Weather-making, Ancient and Modern," Smithsonian Institu- tion Annual Report, to July 1894, pp. 249 1270. -'■ I'owers. IMward. "War and the Weather." Delavan. Wis.. 10. Powers. 1890, revised edition, 202 pp. (An earlier edition was published in Chicago in 1871. Incidentally, the plates for the first edition were deal roved in the Chicago lire, and I'owers did not have an opportunity to complete his revision until 1890. ) -* Ihid.. p. 5. ■ Ihid.. p. 143. * Ihid., p. 11. 31 such faith in the possibility of weather modification that funds we re appropriated to support experiments to be carried out under the auspices of the Forestry Division of the U.S. Department of Agriculture. The initial $2 ? 0p0 appropriated was increased first to $7,000, and finally to $10,000. in the first federally sponsored weather modification project. Of the total appropriated. $9,000 was to be spent on held experiments. Gen. Robert St. George Dyrenforth was selected by the Department of Agriculture to direct these tests, hav- ing earlier conducted tests near Utiea, X.Y., and Washington, D.C.. using balloons and rockets carrying explosives. The principal ex- periments were executed near Midland, Tex., using a variety of ex- plosive devices, detonated singly and in volleys, both on the ground and in the air. 31 According to an interesting account by Samuel Hopkins Adam-. Dyrenforth arrived in Texas on a hot day in August 1891 with a company of 80 workers, including "* * * chemists, weather observers, balloon operators, electricians, kitefiiers, gunners, minelayers, sap- pers, engineers, and laborers * * * together with some disinterested scientists, who were to serve as reporters." 32 Adams discusses the ap- paratus which Dyrenforth took with him : The expedition's equipment was impressive. There were 68 balloons of from 10 to 12 feet in diameter, and one of 20 feet — all to be hlled with an explosive mixture of hydrogen and oxygen. There were also sixty 6-inch mortars, made of pipe, and several tons of rackarock (a terrifying blend of potassium chlorate and nitro- benzol that, was the general's favorite "explodent" >, dynamite, and blasting
powder. Finally, there were the makings of a hundred kites, to be assembled on the
scene, and sent up with sticks of dynamite lashed to them. The congressional
$9,000 fell considerably short of sufficing for so elaborate an outfit, but expectant
Texans chipped in with liberal contributions and the railroads helped out by sup-
plying free transportation. 1 "

Dyrenforth carried out five series of trials during 1891 and 1892 :
one period of sustained cannonading coincided with a heavy down-
pour, and the apparent connection provided support to the credi-
bility of many people, who accepted the hypotheses as confirmed.
Dyrenforth gave optimistic and promising reports of his results:
however, meterologists and other scientists were critical of his work.
It does not appear that the Forestry Division was fervently ad-
vocating the research program for which it had responsibility. In
1891, Bernhard E. Fernow, Chief of the Division of Forestry, re-
ported to the Secretary of Agriculture his sentiments regarding the
experiments which were to be conducted in the coming summer, with
a caution reminiscent of the concerns of many meterologists of the
1970°s :

The theories in regard to the causes of storms, and especially their local and
temporal distribution, are still incomplete and unsatisfactory. It can by no means
be claimed that we know all the causes, much less their precise action in precipi-
tation. It would, therefore, be presumptuous to deny any possible effects of ex-
plosions ; but so far as we now understand the forces and methods in precipitating
rain, there seems to be no reasonable ground for the expectation that they will be
effective. We may say, then, that at this stage of meteorological knowledge we
are not justified in expecting any results from trials as proposed for the predtre-
tion of artificial rainfall, and that it were better to increase this knowledge first

31 Fleagle. "Background and Present Status of Weather Modification." 1968, pp. 7-8.

32 Adams. Samuel Hopkins. The New Yorker. Oct. 9, 1952, pp. 93-100.
*> Ibid., i«. !.'4.


by simple laboratory investigations and experiments preliminary to experiment
on a larger scale. 34

In 1893, the Secretary of Agriculture asked for no more public funds
for support of this project. 35

Fleagle tells about the use of 36 "hail cannons" by Albert Stiger, a
town burgomaster, on the hills surrounding his district in Austria in

Tbe hail cannon consisted of a vertically pointing three-centimeter mortar
above which was suspended the smokestack of a steam locomotive. This device
not only produced an appalling sound, but also created a smoke ring a meter or
more in diameter which ascended at about one hundred feet per second and
produced a singing note lasting about ten seconds. Initial successes were impres-
sive, and the hail cannon was widely and rapidly copied throughout central
Europe. Accidental injuries and deaths were numerous, and in 1902 an inter ua-
tional conference was called by the Austrian government to assess the effects of
the hail cannon. The conference proposed two tests, one in Austria and one in
Italy, the results of which thoroughly discredited the device. 36

Though unsuccessful, the work of Dyrenforth and others had in-
spired belief in the possibilities of drought alleviation such that a
number of unscrupulous "rainmakers" were able to capitalize on the
situation. Halacy gives an account of a famous rainmaker of the early
20th century, Charles Warren Hatfield, who operated for about 10
years in the western United States. With a 25-foot platform and a
secret device for dispensing chemicals, he claimed to create rain over
extensive areas. In 1916. Hatfield contracted with the city of San Diego
to alleviate drought conditions and was to be paid $1,000 for each inch
of rain produced. When 20 inches of rain coincidentally fell nearby,
the resulting floods destroyed a dam, killed 17 people, and produced
millions of dollars damage. Hatfield, faced with a choice of assuming
financial responsibility for the lawsuits or leaving the city without pay,
chose the latter. 37

One of Hatfield's accomplices was a colorful racetrack reporter from
Xew York, who met and joined Hatfield in California in 1912, named
James Stuart Aloysius MacDonald, alias Colonel Stingo, "the Honest
Rainmaker." Over his half -century career as a writer, mostly for var-
ious horseracing journals. MacDonald reportedly involved himself in
various schemes for quick profit, including weather changing projects
on both the west and east coasts. Contracts with clients were drawn up
with terms for remuneration that resembled very much the language
of success or failure at the racetrack. By his own admission, Mac-
Donald based his odds for success on past weather data for a given
area, which he obtained from records of the U.S. Weather Bureau or
the Xew York Public Library. 88 MacDonald, or Colonel Stingo, was
the inspiration for a Broadway play called "The Rainmaker" which
opened in 1954.


Espy's L839 proposal for an experiment on the production of con-
vection currents and water vapor condensation at high altitudes was

■ A Fernow, Rernhard E.. in report to Jeremiah McClain Rusk. Secretary of Agriculture,
1891, an reported in Ward, "Artificial Rain ; a Review of the Subject to the Close of 1889."
1882. p. 492.

• livers. "History of Weather .Modification." 1 1*74. p. 5.
38 Fleajcle. "Rackpronnd and Present Status of Weather Modification," 1968, p. 9.
:t7 Halacy, "The Weather Changers," 1968, pp. 68 69.
38 Liebling, A. J., "Profiles," The New Yorker, Sept. 20, 1902, pp. 43-71.


based on sound physical principles. Since knowledge of atmospheric
processes was expanding and unfolding rapidly at the time, Hartman
reminds us that the limited usefulness of Espy's weather modification
concepts should not be ascribed to faulty logic, but rather to the primi-
tive understanding at the time of the complex processes in precipita-
tion, many of which are still not understood satisfactorily. 39

The understanding which meteorologists have today about precipi-
tation has been learned slowly and sometimes painfull}^, and, while
many of the discoveries haA'e resulted from 20th century research,
some important findings of the latter part of the 19th century are
fundamental to these processes. Important results were discovered in
1875 by Coulier in France on foreign contaminant particles in the
normal atmosphere, and quantitative measurements of the concentra-
tions of these particles were achieved by Aitken in 1879. These events
established a basis for explaining the fundamental possibility for
occurrence of precipitation. Earlier, it had been learned that high
supersaturations were required for the formation of water droplets. 40
Aitken was the first to imply that there are two types of nuclei, those
with an affinity for water vapor (hygroscopic particles) and nuclei
that require some degree of supersaturation in order to serve as con-
densation centers. The Swedish chemist-meteorologists of the 1920's
developed a theory of condensation on hygroscopic nuclei and showed
the importance of sea-salt particles. In the 1930's in Germany and the
United Kingdom, a series of measurements were conducted on the
numbers and sizes of condensation nuclei by Landsberg, Judge, and
Wright. Data from measurements near Frankfurt, augmented sub-
sequently by results from other parts of the world, have been adopted
as the standard of reference for condensation nuclei worldwide. 41

At the beginning of the 1930's important aspects of cloud phys'
were not yet understood. In particular, the importance of thp ic ,ri phu
to precipitation was not yet clarified, though, ever since the turn of
the century meteorologists were aware that water droplets were abun-
dantly present in clouds whose temperatures were well below the freez-
ing point. Little was known about the microphysics of nucleation of ice
crystals in clouds ; however, it had been noted that rains fell only after
visible glaeiation of the upper parts of the clouds. Understanding
of these processes was essential before scientific seeding of clouds for
weather modification could be pursued rationally. In 1933 Tor Berg-er-
on presented and promulgated his now famous theory on the initiation
of precipitation in clouds containing a mixture of liquid and ice.
W. Findeisen expanded on Bergeron's ideas and published a clearer
statement of the theory in 1938 ; consequently, the concept is generally
known as the Bergeron-Findeisen theory. 42 in his investigation of the
formation of ice crystals, Findeisen was of the opinion that they crys-
talled directly from the vapor (that is, by sublimation) rather than
freezing from droplets. He also conjectured that quartz crystals might
be the nuclei responsible for this process and even foresaw that the
mechanism might be initiated artificially by introducing suitable
nuclei. 43

33 Hartman, "Weather Modification and Control," 1966, p. 13.

40 Ibid.

41 Bvers. "History of Weather Modification," 1974, p. 7.

42 Ibid., p. 8.

*» Ibid., pp. 8-9.

34-857—79 5


Findeisen stated emphatically that rain of any importance must
originate in the form of snow or hail, though Bergeron had admitted
the occurrence of warm rain in the tropics. Though many meteorolo-
gists doubted that the ice crystal process was an absolute requirement
for rain, they had been unable to collect evidence from aircraft obser-
vations. In Germany aerological evidence was obtained on the growth
of rain drops by the collision-coalescence process in "warm" clouds,
but the papers on this work were published in 1940, and World War
II restricted communication of the results to meteorologists world-
wide. Meanwhile in the United States, papers were published on the
theory of the warm rain process. In 1938, Houghton showed that pre-
cipitation could be started by either the Bergeron process or by the
collision-coalescence process. He noted that drops could be formed by
condensation on "giant" hygroscopic nuclei present in the air and that
growth of droplets to raindrop size was possible through collision.
G. C Simpson elucidated further on condensation and precipitation
processes in 1941, disagreeing with Findeiseivs rejection of "warm"
rain formation by the collision-coalescence process. 44


Starting about 1920 and continuing for about two decades until
the outbreak of World War II, there were a number of experiments
and operations intended to produce rain or modify the weather in
some other way. Although some of these activities were pusued in a
scientific manner, others were less so and were directed at producing
immediate results; all of these projects lacked the benefit of the funda-
mental knowledge of precipitation processes that was to be gained
later during this same period, the discoveries of which are discussed
in the preceding subsection. Various schemes during this period in-
cluded the dispensing of materials such as dust, electrified sand, dry
ice, liquid air, and various chemicals, and even the old idea that explo-
sions can bring rain. Field tests were conducted in the United States,
Germany, the Netherlands^ and the Soviet Union.

Byers tells .about the experimental work of Dr. E. Leon Chaffee,
professor of physics at Harvard, who became interested in the possi-
bility of making cloud particles coalesce by sprinkling electrically
charged sand over the clouds :

Dr. Chaffee became enthusiastic about the idea and developed in his laboratory
a nozzle tor charging sand and dispersing it from an airplane. The nozzle could
deliver sand grains having surface gradients of the order of 1.000 V/ein. Flight
experiments were carried out in August and Seprcmber of 1024 at Aberdeen,
Md.. with an airplane scattering the sand particles in the clear air above clouds
having tops at to 10,000 feet. Dr. Chaffee reported "success*' in the reverse
sense, in that several clouds were observed to dissipate after treatment. The tests
were well publicized in newspapers and scientific news journals, and this author,
then a freshman at the University of California, recalls that his physics pro-
fessors were enthusiastic about the idea. Chaffee's results probably would not
endure the type of statistical scrutiny to which experiments of this kind are
subject today. 43

Chaffee considered several trials successful, since clouds were dis-
sipated after being sprayed with the charged sand. It has been pointed

" Ibid . p. 9.
« Ibid., p. 5.


out, however, in view of the much greater experience in recent years,
that scientists must be extremely cautious in ascribing success in such
experiments, when the evidence is based largely on visual obser-
vations. 4 ' 1

In the Netherlands, August Veraart successfully produced rain by
seeding clouds with dry ice from a small aircraft in 1930. This was
16 years before the work at General Electric in the United States, when
clouds were also seeded with dry ice, initiating the modern period in
the history of weather modification. Since Veraart probably did not
understand the mechanism involved in the precipitation process which
he triggered, ho did not realize that the dry ice was effective in develop-
ment of ice crystals by cooling supercooled clouds, and his success was
likely only a coincidence. Byers observes that Veraart's vague con-
cepts on changing the thermal structure of clouds, modifying tem-
perature inversions, and creating electrical effects were not accepted,
however, by the scientific community. 47 He claimed to be a true rain-
maker and made wide, sweeping claims of his successes. He died in
19o*2, a year before Bergeron's theory appeared, not aware of the theo-
retical basis for his work. 48

Partly successful experiments on the dissipation of fog were con-
ducted by the Massachusetts Institute of Technology in the 1930s,
under the direction of Henry G. Houghton. At an airfield near Round
Hill, Mass., fog was cleared using sprays of water-absorbing solutions,
particularly calcium chloride, as well as fine particles of dry hygro-
scopic material. Results of these experiments, which predated some of
the present-day foo- dispersal attempts bv some 30 vears, were reported
in 1938. 19

Weather Modification Sixce 1946


The following chronology of "critical events" relating to weather
modification policy, compiled by Fleagle. unfolds only some of the
major events and activity periods which have occurred since the his-
toric discoveries of 1946 : 50

1946 : Schaefer demonstrated seeding: with dry ice.

1947 : Vonnegut demonstrated seeding with silver iodide.

1947-55 : Irving Langmuir advertised weather modifieaton widely and aggres-

1947- 53: General Electric field experiments ("Cirrus") extended evidence
that clouds can he deliherately modified, but failed to demonstrate large effects.

1948- 50: Weather Bureau Cloud Physics Project on cumulus and stratiform
clouds resulted in conservative estimate of effects.

1948-52 : Commercial operations grew to cover 10 percent of United States.

1950: Report of Panel on Meteorology of Defense Department's Research and
Development Board (Haurwitz, Chairman) was adverse to Langmuir's claims.

1953: Public Law 83-256 established President's Advisory Committee on
Weather Control.

45 McDonald. James E.. "An Historical Note on an Early Cloud-Modification Experiment.
Bulletin of the American Meteorological Society, vol. 42. No. 3, March 1961, p. 19o.

47 Byers. "History of Weather Modification." 1947. p. 6.

48 Hartman. "Weather Modification and Control." 1966. p. 15. , , „

» Houghton. Henrr G.. and W. H. Radford. "On the Local Dissipation of Natural bog.
Papers in Physical Oceanography and Meteorology. Massachusetts Institute of Technology
and Woods Hole Oceanographic Institution, vol. 6, No. 3. Cambridge and Woods Hole, Mass.,
October 1938, 63 pp. , „ – .. „ „ .

50 Fleagle. Robert G . "An Analysis of Federal Policies in \\ eather Modification. Back-
ground paper prepared for use by the U.S. Department of Commerce Weather Modification
Advisory Board. Seattle. Wash., March 1977. pp. 3-5.


1953-54: "Petterssen" Advisory Committee organized field tests on storm sys-
tems, convective clouds, and cold and warm fog (supported by the Office of
Naval Research, the Air Force, the Army Signal Corps, and the Weather
Bureau). These statistically controlled experiments yielded results which have
been substantially unchanged in subsequent tests.

1957: Report of Advisory Committee (Orville, Chairman) concluded that tests
showed 15 percent increase in orographic winter precipitation.

1957 : Major cut in research support across the board by Defense Department
sends major perturbation through research structure.

195S: Public Law 85-510 assigned lead agency responsibility to the National
Science Foundation (NSF).

1959: Commercial operations had diminished to cover about one percent of
the United States.

1961 : First hurricane seeding under Project Stormfury.

1961 : Bureau of Reclamation authorized by Congress to conduct research in
weather modification.

1961 : RAND report on weather modification emphasized complexity of atmos-
pheric processes and interrelation of modification and prediction.

1962-70: Randomized field experiments established magnitude of orographic

1964: Preliminary report of National Academy of Sciences/Committee on
Atmospheric Sciences (NAS/CAS) roused anger of private operators and stimu-
lated the evaluation of operational data.

1964-present : Department of the Interior pushed the case for operational seed-
ing to augment water supplies.

1966: NAS/CAS report 1S50 laid the basis for expanded Federal programs.

1966 : Report of NSF Special Commission on Weather Modification and an NSF
symposium called attention to social, economic, and legal aspects.

1966: Interdepartmental Committee for Atmospheric Sciences (ICAS) report
f Newell, Chairman) proposed expanded Federal support to $90 million by 1970.

1966- 68 : Efforts of the Departments of Commerce and Interior to gain lead
agency status were unsuccessful.

1967: ICAS recommended that Commerce be designated as lead agency.
1967: S. 2916, assigning lead agency responsibility to the Department of Com-
merce : passed the Senate but did not become law.

1967- 72 : Military operational programs conducted in Vietnam.
1968: Public Law 90-407 removed the NSF mandate as lead agency.
1968 : Detrimental effects of acid rain reported from Sweden.

1969: Public Law 91-190 (National Environmental Policy Act) required im-
pact statements.

1970; Massachusetts Institute of Technology Study of Critical Environmental
Problems called attention to inadvertent effects on climate.

1970 : Stratospheric contamination by SST's suggested.

1971 : Departments of Commerce and Interior carried out operational programs
in Oklahoma and Florida.

1971 : Public Law 92-205 required filing of reports of non-Federal weather
modification activities with the Department of Commerce.

1971 : International Study of Man's Impact on Climate raised this issue to inter-
national level.

1971 : NAS/CAS report on priorities for the 1970's emphasized need for atten-
tion to management and policy problems of weather modification.

1971: Federal Council for Science and Technology approved seven national
projects under various lead agencies.

1971-72: First technological assessments of weather modification projects are
favorable to operational programs.

1971-74 : Climate impact assessment program ( CTAP) of Department of Trans-
portation indicates potentially serious consequences of large SST fleet but sug-
gests ways to ameliorate the problem.

1972: Failure of Soviet wheat crop and drought in Sahel emphasized critical
need for understanding climate and the value of effective weather modification.

1973: Weather modification budget reduced by impoundment from $25.4 million
to $20.2 million.

1973 : Five national projects deferred or terminated.

1973: NAS/CAS report on weather and climate modification confirmed earlier
conclusions and recommended lead agency status for NOAA.


1974 : Stratospheric contamination by freon reported.

1974 : Domestic Council organized panels in climate change and weather

1974 : General Accounting Office report on weather modification criticized
weather modification program and pointed to need for lead agency.

1974 : Defense Department released information on operations in Vietnam.

1974 : The United States and the U.S.S.R. agreed to a joint statement intended
"to overcome the dangers of the use of environmental modification techniques for
military purposes."

1975 : World Meteorological Organization Executive Committee proposed cumu-
lus experiment perhaps in Africa or Iran.

1975 : Department of Transportation CIAP report indicated that a fleet of 500
SST's would deplete ozone significantly, but suggested that cleaner engines could
be developed.

1976: Chinese disapproval resulted in abandoning plans for Stormfury in the
western Pacific.

1976 : Hearings held on three weather modification bills by Senate Commerce

1976: The National Weather Modification Policy Act of 1976 (Public Law 94-
859) enacted requiring study of weather modification.

1977 : Exceptionally dry winter in the west stimulates State operational pro-
grams intended to increase mountain snowpack.

Since the completion of Fleagle's list above in March 1977, at least
three other activities of equivalent significance ought to be noted :

1977 : The U.S. Department of Commerce Weather Modification Advisory Board
established in April 1977 and initiated a major study on a recommended national
policy and Federal program of research in weather modification, in accordance
with requirements to be fulfilled by the Secretary of Commerce under Public Law
94-490, the National Weather Modification Policy Act of 1976.

1977 : The United Nations General Assembly approved a treaty banning environ-
mental modification activities for hostile purposes on May 18, 1977 ; and the treaty
opened for signature by the member nations.

1978 : The Report of the Commerce Department's Weather Modification Advi-
sory Board transmitted through the Secretary of Commerce to the Congress.

The history of the modern period of weather modification which
follows is essentially that of the two decades following the monumental
discoveries of 1946. An excellent account of the history of weather
modification, which emphasizes this period, has been prepared by
Byers. 51 This work has been very helpful in some of the material to
follow and is referenced frequently. The late 1960's and the 1970's are
so recent that events during this period are discussed in various sections
of the report as ongoing activities or events leading to current activities
in weather modification research programs, operations, and policy
decisions rather than in this chapter as an integral part of an updated
history of the subject.


The modern era of scientific weather modification begaai in 1946,
when a group of scientists at the General Electric Co. demonstrated
that, through "seeding," a cloud of supercooled water droplets could
be transformed into ice crystals and precipitation could be induced.
These were not traditional meteorologists, though their leader. Dr.
Irving Langmuir, was a famous physicist and Nobel laureate. He and
his assistant, Vincent J. Schaefer, had been working for 3 years on
cloud physics research, however, in which they were studying particle
sizes, precipitation static, and icing. Their field research was carried on

Byers, "History of Weather Modification," 1974, pp. 3-44.


at the summit of Mt. Washington., X.H.. where they observed super-
cooled clouds which often turned into snowstorms. 52

In an attempt to simulate field conditions. Schaefer contrived a
laboratory setup using a home freezer lined with black velvet, with a
light mounted so as to illuminate ice crystals that might happen to
form in the box. Breathing into the box, whose temperature was about
— 23° C, produced fog but no ice crystals, even when various sub-
stances — including sand, volcanic dust, sulfur, graphite, talc, and
salt — were dropped in as possible sublimation nuclei. 53 On July 12.
19-16, Schaefer wanted to lower the freezer temperature somewhat, so
he inserted a large piece of dry ice. and. in an instant, the air was
full of millions of ice crystals. He discovered that even the tiniest
piece of dry ice produced the same etfect. In fact, dry ice had no
direct effect on the supercooled cloud; producing an air temperature
below – 39° C was critical. 54

In his paper on the laboratory experiments, published in the No-
vember 15, 1946. issues of^Science v Schaefer stated :

It is planned to attempt in the near future a large-scale conversion of super-
cooled clouds in the atmosphere to ice crystal clouds, by scattering small frag-
ments of dry ice into the cloud from a plane. It is believed that such an opera-
tion is practical and economically feasible and that extensive cloud systems can
be modified in this way. 53

Two days before the paper appeared, on Xovember 13, 1946,
Schaefer made his historic flight, accomplishing man's first scientific
seeding of a supercooled cloud, as he scattered three pounds of dry ice
along a 3-mile line over a cloud to the east of Schenectady, X.Y. At
14.000 feet the cloud temperature was —20° C. and in about § minutes
after seeding the entire cloud turned into snow, which fell 2,000 feet
before evaporating. 56

Dr. Bernard Vonnegut had also worked on aircraft icing research
and in 1946 at General Electric was pursuing a variety of nueleation
problems ; but. after Schaefer's laboratory experiments, he again
turned his attention to ice nueleation research. He discovered that
silver iodide and lead iodide had crystal structures close to that of ice
and were also insoluble in water, and after repeated initial failures,
owing to impurities in the material, Vonnegut was able to produce ice
crystals, using very pure silver iodide powder, at temperatures only a
few degrees below freezing. Soon means were developed for generating
silver iodide smokes, and man's first successful attempt at artificial
nueleation of supercooled clouds was accomplished. 57

Langmuir explained that dry ice could make ice crystals form by
lowering the temperature to that required for natural nueleation on
whatever might be present as nuclei, or even in the absence of all
nuclei; however, the silver iodide provided a nucleus that was much
more efficient than those occurring naturally. 58

" Ibid., pp. 9-10.

" Halacy, "The Weather Changers/' ions. pp. S2-S3.

« langmuir. Irvinp. "The Growth of Particles in Smoke, and Clouds and the Production
of Snow from Supercooled Clouds. Proceedings of the American Philosophical Society, vol.
92, no. 3, July 1048, p. 182. ' , , _ ,

Schaefer, Vincent J.. "The Production of Ice Crystals in a Cloud of Supercooled Water
Droplets.' – Science, vol. U>4. No. 2707. Nov. 15. 1946, p. 459.

" Byers, "History of Weather Modification," 1074. p. 12.

57 H>id . p. 13.

M Langmuir, Irvine. "Cloud Seeding by Menus of Dry Ice. Silver Iodide, and Sodium
Chloride." Transactions of the New York Academy of Sciences, ser. II, vol. 14. November
1951, p. 40.


Following Schaefer's successful flight of November 13, 1946, and in
the months and immediate years thereafter, Langmuir was quoted in
the popular press as being very optimistic in his predicted benefits
from weather modification. In a 1948 paper he said that k> * * * it
becomes apparent that important changes in the whole weather map
can be brought about by events which are not at present being con-
sidered by meteorologists." 59 His publications and informal statements
of this character touched off years of arguments with professional
meteorologists, by whom refutation was difficult in view of Langmuir s
standing in the scientific community. His enthusiasm for discussing
the potential extreme effects from weather control was unrestrained
until his death in 1957. 60


Project Cirrus

Although the business of the General Electric Co. had not been in
meteorology, it supported the early research of Langmuir and his
associates because of the obvious importance of their discoveries.
Realizing that weather modification research was more properly a con-
cern of the Federal Government, the company welcomed the interest
of, and contract support from, the U.S. Army Signal Corps in
February 1947. Subsequently, contract support was augmented by the
Office of Naval Research, the U.S. Air Force provided flight support,
and the U.S. Weather Bureau participated in a consultative role. The
entire program which followed, through 1951, under this arrangement,
including the field activities by Government agencies and the labora-
tory work and general guidance by General Electric, was designated
''Project Cirrus." 61 According to Byers :

The most pronounced effect produced by Project Cirrus and subsequently sub-
stantiated by a number of tests by others, was the clearing of paths through
supercooled stratus cloud layers by means of seeding from an airplane with dry
ice or with silver iodide. When such clouds were not too thick, the snow that was
artificially nucleated swept all the visible particles out of the cloud. * * * In one
of the first flights, * * * the supercooled particles in stratus clouds were removed
using only 12 pounds of dry ice distributed along a 14-mile line. In later flights
even more spectacular results were achieved, documented by good photography. BL '

Initial Project Cirrus studies were made during the summer of
1947 on cumulus clouds near Schenectady, but the important seeding
experiments were conducted the following year in New Mexico. Also
during 1947, there was an attempt on October 13 to modify a hurricane
east of Jacksonville, Fla., through seeding with dry ice. 63 Visual ob-
servations, reported by flight personnel, seemed to indicate a pro-
nounced change in the cloud deck after seeding, and, shortly there-
after, the hurricane changed its course and headed directly westward,
striking the coasts of Georgia and South Carolina. Even though there
was precedent for such erratic behavior of hurricanes, there was
speculation about the effect of seeding on the storm path, and the pos-
sibility of legal responsibility for damages which might be caused by

59 Lanfrmuir. Irvinp. "The Production of Rain by a Chain Reaction in Cumulus Clouds at
Temperatures Above Freezing." Journal of Meteorology, vol. 5. No. 5. October 1948. p. 192.
6°T?vprs. "Historv of Weather Modification." 1974. pp. 13-14.

61 ThH.. p. 14.

62 Thirl.

M See discussion of Project Stormfury in ch. 5. p. 290 ff.


such experiments in the future provided reason to avoid seeding
thereafter any storms with the potential of reaching land. The legal
counsel of the General Electric Co. admonished Langmuir not to
relate the course of the hurricane to the seeding; however, throughout
the remainder of his career he spoke of the great benefit to mankind of
weather control and of the potential ability to abolish evil effects of
hurricanes. As a result, it was expected that the U.S. Weather Bu-
reau would undertake massive efforts in weather control. Meteorolo-
gists within and without of the Bureau were in a defensive position,
with many other scientists, impressed by Langmuirs arguments, op-
posing their position. Thus great controversies which developed
between Langmuir and the Weather Bureau and much of the meteoro-
logical community followed these and other claims, and often
resulted from the fact that Langmuir did not seem to fully comprehend
the magnitude and the mechanisms of atmospheric phenomena. 04

Langmuir wanted to ^work where he thought storms originated
rather than in upstate New York. He chose Xew Mexico as operations
area for Project Cirrus, also taking advantage of the opportunity to
collaborate there with Dr. E. J. Workman at the New Mexico Institute
of Mining and Technology, whose thunderstorm research included
radar observations and laboratory experiments on the effects of ire
on storm electrification. After cloud-seeding flights there in October
1948, Langmuir reported that, as a result of the seeding, rainfall had
been produced over an area greater than 40,000 square miles (about
one-fourth the area of the State of New Mexico) . 63

The Project Cirrus group returned to Xew Mexico in July 1040,
and 10 additional seeding nights were conducted. When Langmuir
learned that Vonnegut was dispensing silver iodide from a ground
generator in the same area and had, in fact, also been doing so during
the flights of the previous October, he concluded that both the July
1919 results and the widespread effects of October 1948 were caused
by the silver iodide rather than the dry ice seeding as he had theorized
previously. Spectacular results continued to be reported by him.
spurred on by meteorologists' challenges to his statistical methods
and conclusions. Noting that Vonnegut had operated the ground
generator only on certain days, Langmuir observed that rainfall
responses corresponded to generator "on" times, leading him to his
controversial "periodic seeding experiment.'' to which the remainder
of his life was devoted. 66

In the periodic seeding experiment, the silver iodide generators were
operated in an attempt to effect a 7-day periodicity in the behavior of
various weather properties. Langmuir was convinced that unusual
weekly weather periodicities in early 1950 resulted from periodic seed-
ings begun in Xew Mexico in December 1949. concluding that the effects
were more widespread than he felt earlier and that temperatures and
pressures thousands of miles away were also affected. Meteorologists
observed that, while these correlations were the most striking seen, yet
such periodicities were not uncommon. 67 The Weather Bureau under-
took a study of records from 1919 to 1951 to see if such weather perio-

" Ibid., pp. 14-16.
■ Ibid., p. 1«.
w Ibid., p in.
r ~ Ibid., pp. in 20.


dickies had occurred in the past. Glenn W. Brier, author of the report
on this study, indicated that a T-day component in the harmonic anal-
ysis of the data appeared frequently, though seldom as marked as dur-
ing the periodic seeding experiment. 68 Byers' opinion is that the evi-
dence appeared just as reliable for occurrence of a natural periodicity
as for one controlled artificially. He contends that the most important
discoveries in cloud physics and weather modification were made in the
General Electric Research Laboratory before Project Cirrus was orga-
nized, that the effect of clearing stratus decks was shown soon after the
project was underway, and that the seeding experiments thereafter
became more of a "program of advocacy than of objective proof." The
project * * failed to demonstrate that seeding of cumulus clouds
increased rainfall, that seeding initiates self -propagating storms, that
the atmosphere responds periodically to periodic seeding, or that a
hurricane could be deflected in its path by seeding." 69

Seeding under Project Cirrus ended in 1951 and the final report
appeared in 1953. After the close of the project, Langmuir continued
his analyses and wrote two more papers before his death in 1957. The
final paper was titled "Freedom — the Opportunity To Profit From the
Unexpected." a report that Byers feels provided a fitting philosophical
close to his career. 70 The Defense Department sponsored another series
of experiments, called the Artificial Cloud Xucleation Project, from
1051 to 1953.

Tlie Weather Bureau Cloud Physics project

Amid increasing publicity and spectacular claims of results from
cloud seeding in Project Cirrus, the U.S. Weather Bureau initiated in
1048 a project to test cloud seeding, with the cooperation of the Na-
tional Advisory Committee for Aeronautics, the Navy, and the Air
Force. The Cloud Phvsics Project, the first systematic series of seeding
experiments in stratiform and cumuliform clouds, continued for 2
years, with flight operations in Ohio, California, and the Gulf States.
Findings of Project Cirrus were substantiated in that striking visual
cloud modifications occurred: however, there was no evidence to show
spectacular precipitation effects, and the experiments led to a conserva-
tive assessment of the economic importance of seeding. 71 Cloud dissi-
pation rather than new cloud development seemed to be the general
result from seeding, the only precipitation extractable from clouds was
that contained in the clouds themselves, and cloud seeding methods did
not seem to be promising for the relief of drought. 72

Bosults of the cloud physics experiment had almost no effect on
the prevalent enthusiasm at the time for rainmaking through cloud
soedino-, oxcent in the "hard core" of the meteorology community. 73
As r result of thes< * experiments and the interpretation of the results, the TToather Bureau and its successor organizations in the Commerce Department, the Environmental Science Services Administration and the "National Oceanic and Atmospheric Administration, have been os Brier. Glenn W.. "Seven-Dar Periodicities in May 19.~2." Bulletin of the American Me^eorolosricPl Societr. vol. 35. No. 3. March 1954. pp. 118-121. p? B^ers. "History of Weather Modification." 1974. pp. 20-21. 70 Ibid., p. 20.. " Flpfisrle. Robert G.. "Background and Present Status of Weather Modification." 196S. pp 0-10. ■ 2 B-ers. "^'storv of Weather Modification." 1074. pp. 10-17. »» Ibid,, p. 17. 42 regarded by some critics as unimaginative and overconservative on weather modification. 74 The U.S. experiments of 1953-54 In 1951 the Weather Bureau, the Army, the Navy, and the Air Force appointed an advisory group, chaired by Dr. Sverre Petterssen of the University of Chicago, under whose advice and guidance the following six weather modification projects were initiated : 75 1. Seeding of extratropical cyclones, sponsored by the Office of Naval Research and conducted by Xew York University. 2. Seeding of migratory cloud systems associated with fronts and cyclones, conducted by the Weather Bureau. 3. Treatment of connective clouds, supported by the Air Force and conducted by the University of Chicago. 4. Research on the~dissipation of cold stratus and fog, conducted by the Army Signal Corps. 5. Studies of the physics of ice fogs, sponsored by the Air Force and conducted by the Stanford Research Institute. 6. Investigation of a special warm stratus and fog treatment svs- tem, sponsored by the Army and conducted by Arthur D. Little, Inc. Field experiments on these projects were carried out in 1953 and 1954, and reports were published under the auspices of the American Meteorological Society in 195T. 76 The purpose of the extratropical cyclone seeding project, called Project Scud, was to "* * * ascertain whether or not it would be possible to modify the development and behavior of extratropical cyclones by artificial nucleation. * * *" 77 Analysis obtained in Scud from Florida to Long Island showed that "* * * the seeding in this experiment failed to produce any effects which were large enough to be detected against the background of natural meteorological variance." 7S The Weather Bureau project on migratory cloud systems was con- ducted in western Washington on cloud systems that enter the area from the Pacific during the rainy winter months. This project was criticized by commercial seeders since it was conducted in the West, which was considered "their territory," and by those who accused the Weather Bureau of seeking a negative result to support their conserva- tive view toward weather modification. Byers feels that there was an attempt to avoid this negative impression by giving a more positive interpretation to the results than the data possibly justified. 79 In sum- marizing results. Hall stated: Considering the results as a whole there is no strong evidence to support a con- clusion that the seeding produced measurable changes in rainfall. * * * the eval- uations do not necessarily furnish information on what the effect might have been with more or less intense seeding activity, rate of release of dry ice, etc. Also it 71 Pleagle. "Background and Present Status of Weather Modification.'' 1998, p 10» « Byers, "History of Weather Modification," 1074. p. 25. 7.) Prtterssen, Sverre. Jerome Sp;ir. Ferguson Hall. Roscoe R. Braham. Jr., Louis J. Rat- tan. Horace R. Byers, H. J. aufm Kamoe. J. J. Kelly, and H. K. Welcfcraann. "Cloud and Weather Modification; a Croup of Field Experiments." Meteorological Monographs, vol. 2. No 11 American Meteorological Society, Boston. 10."»7. Ill pp. "Petterssen, Sverre. "Reports on Experiments with Artificial Cloud Nucleation: Intro- ductory Note." In Petterssen et al . "Cloud and Weather Modification : ii Croup of Field Experiments," Meteorological Monographs, vol. 2. No. n. American Meteoroio.^icnl Society. Boston. 1957, p, S. T" Spar. Jerome "Prolecl Send." in Petterssen et al.. "Cloud mid Weather Modification ; :i Group of Field Experiments." Meteorological Monojrra plis. vol. 2. No. 11. American Mete- orological Society, P.oston. ior>7, n 22.

"Byers. "History of Weather Modification," 1074. p. 26.


might be speculated that the seeding increased rainfall on some occasions and
decreased it on others. 80

The aim of the University of Chicago Cloud Physics project was as
follows : 81

The formulation of a consistent and immediately applicable picture of the
processes of formation of cumulus clouds, charged centers, and precipitation with
a view toward testing the possibility that one can modify these processes and
influence the natural behavior of clouds.

So that as many cumulus clouds as possible could be tested, work was
conducted in the Middle West in the summer and in the Caribbean in
the winter, realizing that the warm trade-wind cumulus clouds in the
latter region might be amenable to seeding with large hygroscopic
nuclei or water spray, and that the ice-crystal process would operate to
initiate precipitation in the colder clouds of the Middle West. 82, Of the
numerous conclusions from this project 83 a few will serve to indicate
the value of the project to the understanding of cloud phenomena and
weather modification. In the Caribbean tests, water spray from an air-
craft was seen to increase rainfall as determined by radar echoes ; anal-
ysis showed that the treatment doubled the probability of occurrence of
a radar echo in a cloud. From tests on dry ice seeding in the Middle
West it was found that in the majority of cases treated clouds showed
an echo, while untreated ones did not, although the sample was consid-
ered too small to be significant. In all cases clouds were considered in
pairs, one treated by seeding and the other untreated, and only those
clouds showing no echo initially were chosen for study. 84

The seeding experiments with supercooled stratus clouds by the
Army Signal Corps essentially substantiated the results of Project
Cirrus; however, from these carefully conducted tests a number of
new relationships w^ere observed with regard to seeding rates, spread
of glaciating effect, cloud thickness, overseeding, and cloud formation
after seeding. S5 The report on this project carefully summarized these
relationships and conclusions for both dry ice and silver iodide
seeding. 86

The Air Force project on the physics of ice fogs, conducted by
Stanford Research Institute, was intended to learn the relationship
to such fogs of synoptic situations, local sources of water, and pollu-
tion. Investigations in Alaska at air bases showed that most fogs
developed from local sources of water and pollution. In the Arthur L).
Little investigation for the Army attempts were made to construct
generators which were capable of producing space charges, associated
with aerosols, that could bring about precipitation of the water drop-
lets in warm fogs and stratus. 87

» Hail, Ferguson. "The Weather Bureau ACN Project." In Petterssen et al., "Cloud and
Weather Modification ; a Group of Field Experiments," Meteorological Monographs, vol. 2.
No. 11. American Meteorological Society. Boston. 1957. pp. 45-46.

sl Braham. Roscoe R., Jr.. Louis J. Battan. and Horace R. Byers. "Artificial Nucleation
of Cumulus Clouds." In Petterssen et al.. "Cloud and Weather Modification : a Group of
Field Experiments," 1957, p. 47.

& Byers, "History of Weather Modification," 1974, pp. 26-27.

83 Conclusions are precisely spelled out in somewhat technical terms in : Braham, Battan.
and Byers. "Artificial Nucleation of Cumulus Clouds," 1957, pp. S2-S3.
fi Byers, "History of Weather Modification," 1974, p. 27.

86 IMd. . » ,

86 aufm Kampe, H. J., J. J. Kelly, and H. K. Weickmann, "Seeding Experiments m Sub-
cooled Stratus Clouds." In Petterssen et al.. "Cloud and Weather Modification : a Group of
Field Experiments." Meteorological Monographs, vol. 2, No. 11. American Meteorological
Society. Boston, 1957, p. 93. , T . , .

57 Petterssen, "Reports on Experiments With Artificial Cloud Nucleation: Introductory
Note," 1957, p. 4.


Brers, in retrospect, wonders why the results of this series of six
experiments, which were carefully controlled statistically, did not
receive more attention than was accorded them. He attributes some
of this lack of visibility to the publication in the somewhat obscure
monograph of the American Meteorological Society 88 and to the delay
in publishing the results, since the Petterssen committee held the manu-
scripts until all were completed, so that they could be submitted for
publication together. 89

Arizona mountain cumulus experiments

After 1954, the University of Chicago group joined with the Insti-
tute of Atmospheric Physics at the University of Arizona in seeding
tests in the Santa Catalina Mountains in southern Arizona. These
experiments were conducted in two phases, from 1957 through 1960
and from 1901 through 1964, seeding mostly summer cumulus clouds,
but some winter storms, with silver iodide from aircraft. In the first
phase, analysis of precipitation data from the first 2 years revealed
more rainfall during seeded than on nonseeded days ; however, during
the latter 2 years, considerably more rainfall was achieved on non-
seeded days. Combining all data for the 4 years of the first phase
yielded overall results with more rain on unseeded days than on seeded
days; hence, the experiments were modified and the second phase
undertaken. Of the 3 years in the second phase, only one showed more
rain on seeded days than on nonseeded ones. None of the analyses
attempted could support the hypothesis that airborne silver iodide
seeding increased precipitation or influenced its area! extent. Byers
suggests that the failure to increase rainfall may have been due to the
fact that precipitation initiation resulted from the coalescence process
rather than the ice-crystal process. 90

Project Whitetop

According to Byers, perhaps the most extensive and most sophisti-
cated weather modification experiment (at least up to the time of
Byers' historical review in 1973) was a 5-year program of summer
convective cloud seeding in south-central Missouri, called Project
Whitetop. Conducted from 19G0 through 1964 by a group from the
University of Chicago, led by Dr. Roscoe 11. Braham, the purpose of
Whitetop was to settle with finality the question of whether or not
summer convective clouds of the Midwest could be seeded with silver
iodide to enhance or initiate precipitation. Experimental days were
divided into seeding and no seeding days, chosen randomly from
operational days suitable for seeding, based on certain moisture cri-
teria. Another feature of the project was the attempt to determine the
extent of spreading of silver iodide smoke plumes from the seeding
line. Precipitation effects were evaluated by radar and by a rain-gage
network. 01

Final analysis of all of the Project Whitetop data showed that the
overall effect was that, in the presence of silver iodide nuclei, the rain-
fall was less than in the unseeded areas. Byers attributes these negative

88 Petterssen et al.. "Cloud and Weather Modification; a Group of Field Experiments,"

*> livers. "History of Weather Modification," 11)74, p. 2S.

»° Il)ld., p. 29.

« Ibid., pp. 20-30.


results to the physical data obtained from cloud-physics aircraft. "Most
of the Missouri clouds produced raindrops by the coalescence process
below the freezing line, and these drops were carried in the updrafts
and frozen as ice pellets at surprisingly high subf reezing temperatures
( — 5° C to —10° C)." He further points out that the measured con-
centrations of ice particles, for the range of sizes present, were already
in the natural unseeded conditions equivalent to those hoped for with
seeding; consequently, the silver iodide only had the effect of over-
seeding. 92

Climax experiments

Following the initial General Electric experiments, it was concluded
by Bergeron 93 that the best possibility for causing considerable rain-
fall increase by artifical means might be found in seeding orographic 94
cloud systems. Consequently, there were almost immediate efforts to
increase orographic precipitation, the greatest concentration of such
work being in the Western United States. Commercial groups such
as power companies and irrigation concerns took the early initiative in
attempts to augment snowfall from orographic cloud systems in order
to increase streamflow from the subsequent snowmelt.

Colorado State University (CSU) began a randomized seeding
experiment in the high Rocky Mountains of Colorado in 1960, under
the direction of Lewis O. Grant, to investigate snow augmentation
from orographic clouds. The project was designed specifically to
(1) evaluate the potential, (2) define seedability criteria, and (3) de-
velop a technology for seeding orographic clouds in central Colorado. 95
It followed the 1957 report of the President's Advisory Committee for
Weather Control, in which it had been concluded that seeding of oro-
graphic clouds could increase precipitation by 10 to 15 percent, basing
this judgment, however, on data from a large number of seeding pro-
grams that had not been conducted on a random basis. 96

The first group of the CSU seeding experiments took place from
1960 to 1965 in the vicinity of Climax, Colo., and has been designated
Climax I. A second set of tests in the same area from 1965 to 1970
has been referred to as Climax II. The Climax experiments are impor-
tant in the history of weather modification because they were the first
intensive projects of their kind and also because positive results
were reported. 97 The precipitation for all seeded cases was greater than
for all of the unseeded cases by 9, 13, and 39 percent, respectively, for
Climax I, Climax II, and Climax IIB. The latter set of data are a
subsample of those from Climax II, from which possibly contaminated
cases due to upwind seeding by other groups were eliminated. 98

Ibid., p. 30.

93 Bergeron, Tor, "The Problem of an Artificial Control of Rainfall on the Globe ; General
Effects of Ice Nuclei in Clouds." Tellus, vol. 1, No. 1, February 1949, p. 42.

94 A definition of orographic clouds, a discussion of their formation, and a summary of
attempts to modify them are found in ch. 3, p. 71 ff.

95 Grant, Lewis O., and Archie M. Kahan, "Weather Modification for Augmenting Oro-
graphic Precipitation." In Wilmot N. Hess (editor), "Weather and Climate Modification,"
New York, Wiley, 1974, p. 295.

98 Advisory Committee on Weather Control. Final Report of the Advisory Committee on
Weather Control, Washington, D.C., U.S. Government Printing Office, Dec. 31, 1957, vol. I,
p. vi. (The establishment of the Advisory Committee and its activities leading to publica-
tion of its final report are discussed in ch. 5, under activities of the Congress and of the
executive branch of the Federal Government, see pp. 195. 214, and 236.)

97 Byers, "History of Weather Modification," 1974, pp. 30-31. „

98 Grant and Kahan, "Weather Modification for Augmenting Orographic Precipitation,
1974, p. 298.


Lightning suppression experiments

From 1947 until the close of Project Cirrus, interspersed with his
other activities, Vincent Schaefer visited U.S. Forest Service instal-
lations in the northern Rockies in order to assist in attempts to sup-
press lightning by cloud seeding. As early as 1949 an attempt was
made to seed thunderstorm clouds with dry ice, dumping it from the
open door of a twin-engine aircraft flying at 25,000 feet." This
stimulated curiosity among those involved, but also showed that light-
ning-prevention research w T ould require a long and carefully planned
effort. These early activities led to the formal establishment of Proj-
ect Skyfire in 1953, aimed at lightning suppression, as part of the
overall research program of the Forest Service. Throughout the his-
tory of the project, research benefited from the cooperation and sup-
port of many agencies "and scientific groups, including the National
Science Foundation, the Weather Bureau, Munitalp Foundation, the
Advisory Committee on Weather Control, the National Park Service,
General Electric Research Laboratories, Meteorology, Inc., and sev-
eral universities. The project was phased out by the Forest Service
in the 1970's, since results of years of tests were inconclusive, although
there had been some reports of success. Skyfire was the longest con-
tinuing Federal weather modification research project, enduring for
about 20 years. 1

Fog dispersal research

Experiments were conducted on clearing supercooled fog from run-
ways at Orly Airport in Paris since 1962, using sprays of liquid pro-
pane. Soon after these successful tests, the method became operational
and has already succeeded in various U.S. Air Force installations. The
dissipation of cold fog is now operational also at many locations,
including some in North America and in the Soviet Union. Warm fogs,
however, are more common over the inhabited globe, and efforts to
dissipate them had not advanced very far, even by 1970. 2

Hurricane modification

In an earlier discussion of the work of Langmuir and his associates
under Project Cirrus, an attempt at hurricane modification was men-
tioned. 3 The historical unfolding of hurricane research in the United
States thereafter will not be reported here since it is discussed in detail
in chapter 5, under Project Stormfury, now a major weather modifica-
tion research program of the National Oceanic and Atmospheric Ad-
ministration of the U.S. Department of Commerce. 4

Hail suppression

The principal lead in research to suppress hail during the 1950's and
1960's was not in the United States, but mainly elsewhere, particularly
in Switzerland, France, Italy, tho U.S.S.R., Argentina, Bulgaria,
Yugoslavia, Kenya, and Canada. Hail suppression is based on the

86 Barrows J S. "Preventing Fire from the Sky." In U.S. Department of Agriculture,
"The Yearbook of Agriculture, 1968: Science for Better Living." Washington. D.C., U.S.
Government Printing Office, 1968, p. 219.

1 For a more detailed discussion of Project Skyfire, see p. 309, under the weather modi-
fication program of the Department of Agriculture in ch. r>.

2 Byers, "History of Weather Modification," 1974, p. 33.

3 See p. 39.

* See p. 296.


hypothesis that, if a cloud is supplied with a superabundance of ice
nuclei, the available water will be used to form a great number of snow
crystals, thus depriving the hailstones of sufficient water to grow
to damaging size. Most of the early foreign attempts to suppress hail
using explosive rockets or ground-based silver iodide generators
proved disappointing. 5

In the Soviet Union, the Caucasus hail suppression experiments of
the mid-1960's were of great interest to cloud physicists. Using radar
to locate the zone of greatest water content in convective clouds and
rockets with explosive warheads to deliver lead iodide with precision
into this zone, the Russians claimed success in suppressing hailstorms,
based on statistical reduction in crop damages. Operational hail sup-
pression activity is now conducted on a large scale in the Soviet
Union. 6 – 7 Most hail suppression efforts in the United States in the
1960's were commercial operations which did not produce data of any
significant value for further analysis.

Foreign weather modification research

While the Russians and some other countries have concentrated on
hail suppression research, Australia, like the United States, has been
principally concerned with augmenting precipitation. Very shortly
after Schaefer first seeded a natural cloud with dry ice, Krauss and
Squires of the Australian Weather Bureau seeded stratonimbus clouds
in February 1947 near Sidney. The Commonwealth Scientific and
Industrial Research Organization (CSIRO) subsequently organized,
under Dr. E. G. Bowen, what might then have been the world's out-
standing group of cloud physics and weather modification scientists.
Byers feels that probably "* * * no other group contributed more to
practical cloud physics during the period approximately from 1950 to
1965." 8

The Snowy Mountain project in Australia, whose object was to pro-
duce a significant precipitation increase over the mountains by silver
iodide seeding, has attracted most attention. For a 5-year period from
1955 through 1959, this experiment was conducted during the colder
part of the Southern Hemisphere year, using silver iodide dispensed
from aircraft. Although initial experimental reports indicated suc-
cessful increases in precipitation over the target, the final 1963 re-
port after complete analysis stated that results were encouraging but
inconclusive. 9

Interesting experiments were carried out in Israel during the 1960's,
using airborne silver iodide seeding of mostly cumulus clouds. Statis-
tical analysis of data from the first 5% years of tests revealed an in-
crease of 18 percent in rainfall. 10

A project called Gross versuch III was conducted on the southern
slopes of the Alps in Switzerland. Although initiated as a randomized
hail suppression experiment, using ground-based silver iodide gen-
erators, the analysis indicated that hail frequency was greater on

5 Byers, "Histry of Weather Modification," pp. 31-32.

6 Ibid., p. 32.

7 The hail suppression efforts of the U.S.S.R. are discussed in more detail under the status
of hail suppression technology in ch. 3, p. 88, and under foreign programs in ch. 9, 412.

8 Byers, "History of Weather Modification," 1974, p. 23.

9 Ibid., pp. 23-24.
" Ibid., p. 31.


seeded than on nonseeded days, but that the average rainfall on seeded
days was 21 percent greater than on nonseeded days. 11


In the weeks and months following Schaefer's first cloud seeding
experiment public interest grew, and Langmuir and Schaefer spoke
before and consulted with groups of water users, farmers and ranchers,
city officials, Federal program directors, and scientific societies. As a
result there was a burgeoning of new cloud-seeding efforts initiated by
commercial operators, industrial organizations, water districts, and
groups of farmers. Some used ground generators for dispensing silver
iodide obviating the need for airplanes and their attendant high costs,
so that many such opepations became quite profitable. Many rain-
makers were incompetent and some were unscrupulous, but their activi-
ties flourished for a while, as the experiments of Shaefer and Lang-
muir were poorly imitated. Some of the more reliable companies are
still in business today, and their operations have provided data valu-
able to the development of weather modification technology. 12

Byers relates a few instances of early commercial operations of
particular interest. 13 In 1949-50 the city of New York hired Dr. Wal-
lace E. Howell, a former associate of Langmuir, to augment its water
supply by cloud seeding. New York's citizenry became interested and
involved in discussions over Howell's activities as the news media made
them known. This project was also the first case where legal action was
taken against cloud seeding by persons whose businesses could be
adversely affected by the increased rain. Although rains did come and
the city reservoirs were filled, Howell could not prove that he was re-
sponsible for ending the drought. 14 Howell subsequently seeded in
Quebec in August 1953 in an attempt to put out a forest fire and in
Cuba to increase rainfall for a sugar plantation owner. 15

The Santa Barbara project in California, also a commercial opera-
tion designed to increase water supply, received a great deal of atten-
tion. In this period water was increased through augmenting rain and
snow in the mountains north and northeast of the city. The project
was evaluated by the California State Water Resources Board and
was unique among commercial contract operations, inasmuch as the
clients permitted randomization (that is, random selection of only
some storms for seeding) in order to allow adequate evaluation. 16

In the West the earliest commercial operations were developed
under Dr. Irving P. Krick, formerly head of the Department of Mete-
orology at the California Institute of Technology. Asked to monitor
aerial dry ice seeding over Mt. San Jacinto in 1947, Krick became
interested in weather modification, left Caltech, and formed his own
company. Seeding projects were carried out during 1948 and 1949 for
ranchers in San Diego County, Calif., in Mexico, and in Arizona. In
1050 lie moved to Denver and formed a new company, which began
seeding activity over the Great Plains, elsewhere in the West, and in

" Ibid.

12 Ibid., pp. 17, 21. 22.
" Ibid., pp. 22-23.
w Ibid., p. 22.

15 Hnlacv. "The Weather Chancers, " 1968, pp. 96-97.
"Ibid., pp. 22-23.


other countries. A number of former students of Krick joined him or
formed other cloud seeding companies, mostly in the West during the
1950's. 17 By 1953 Krick had operated 150 projects in 18 States and 6
foreign countries and amassed over 200,000 hours of seeding time. For
three winters — 1949, 1950, and 1951 — his company claimed that they
had increased the snowpack in the Rockies around Denver from 175 to
288 percent over the average of the previous 10 years. After 6 months
of seeding in Texas in 1953, the water in a drainage basin near Dallas
had increased to 363 percent of the January 1 level, while in nearby
nonseeded basins water ranged from a 22-percent deficit to an increase
of 19 percent. 18

At the start of extensive seeding in the early 1950's there was a sharp
increase in commercial operations, accompanied by great publicity as
drought began in the Great Plains. During the middle and latter 1950's,
however, seeding diminished as did the drought. The some 30 annual
seeding projects in the United States during the mid and latter 1950's
and the 1960's (excluding fog clearing projects) were conducted for
the most part by about five firms, on whose staffs there were skilled
meteorologists, cloud physicists, and engineers for installing and main-
taining ground and air systems. Most of these projects were in the
categories of enhancing rain or snowfall, with a distribution in a
typical year as follows : About a dozen in the west coast States, half
a dozen in the Rocky Mountains-Great Basin area, half a dozen in
the Great Plains, and the remainder in the rest of the United States.
Of the projects in the West, six to nine have been watershed projects
sponsored by utility companies. Most of these projects endured for
long periods of years and many are still underway. 19

Fleagle notes that by the early 1950's, 10 percent of the land area
of the United States was under commercial seeding operations and
$3 million to $5 million was being expended annually by ranchers,
towns, orchardists, public utilities, and resort operators. The extent
of such commercial operations receded sharply, and by the late 1950's
business was only about one-tenth or less than it had been a decade
earlier. As noted above, public utilities were among those who con-
tinued to sponsor projects throughout this period. 20

Figure 1 shows the purposes of weather modification operations for
various sections of the United States for the period July 1950 through
June 1956. For each geographical section the column graphs represent
the percentage of the total U.S. seeding for each of five purposes that
was performed in that section. The bar graph in the inset shows the
percentage of total U.S. cloud-seeding effort that is undertaken for
each of these five purposes. Figure 2 shows the total area coverage
and the percent of U.S. territory covered by cloud seeding for each
year from July 1950 through June 1956. Both figures are from the
final report of the President's Advisory Committee on Weather
Control. 21

17 Elliott, Robert D., "Experience of the Private Sector," 1974, p. 47.

18 Halacy, "The Weather Changers," 1968, p. 96.

19 Elliott, "Experience of the Private Sector," 1974, p. 46-48.

20 Fleagle, "Background and Present Status of Weather Modification." 1968, p. 11.

21 Advisory Committee on Weather Control, Final Report, 1958, vol. II. Figures lacing
p. 242 and 243.

Figure 1 — Purposes of weather modification operations conducted in various
geographical sections of the United States, July 1950 through June 1956. (From
Final Report of the Advisory Committee on Weather Control, 1958.)




1950- 1951- 1952- 1953- (954- 1935-

1951 1952 1953 1954 1955 1936

Figure 2. — Total area coverage and percent of area coverage for the 48 cotermi-
' nous States of the United States by weather modification operations for each

year, July 1950 through June 1956. (From Final Report of the Advisory

Committee on Weather Control, 1958.)

Table 1 is a summary of weather modification operations for fiscal
years 1966, 1967, and 1968, compiled by the National Science Founda-
tion from field operators' reports which the Foundation required to be
filed. Figure 3 shows the locations in the continental United States for
both operational and research weather modification projects during
fiscal year 1968. In September 1968, as provided by Public Law 90-407,
the National Science Foundation was no longer authorized to require
the submission of reports on operational weather modification proj-
ects. 22 Weather modification activities are now reported to the Depart-
ment of Commerce, under provisions of Public Law 92-205, and sum-
mary reports of these activities are published from time to time. 23

22 See discussions of this law and of the activities of the National Science Foundation as
lead weather modification acency through September 1968. pp 196 and 215 in ch. 5.

23 See discussions of Public Law 92-205 and of the weather modification activities report-
ing program in ch. 5, 197 and 232. The activities summarized in the latest available
Department of Commerce report are discussed in ch. 7 and listed in app. G.



Area treated Number of Number of Number of

(square miles) projects States 2 operators 2

Purpose 1966 1967 1968 1966 1967 1968 1966 1967 1968 1966 1967 1968

Rain augmentation and snow-
pack increase 61,429 62,021 53,369 35 41 37 21 20 21 22 25 23

Hail suppression 20,566 20,556 13,510 3 4 4 3 3 5 3 4 4

Fog dissipation 100 118 145 22 15 15 15 13 9 17 15 10

Cloud modification 19,345 28,300 18,600 9 18 8 8 12 7 8 14 6

Lightning suppression 314 314 314 1 1 1 1 1 1 1 1 1

Totals… 101,744 111,383 85,938 70 79 65 30 23 25 46 44 37

1 Data for fiscal year 1968 include reports received to Sept. 1, 1968.

2 Totals are not the sum of the items since many States and operators are involved in more than one type of activity.

An early commercial hail suppression project was begun in Colorado
in 1958. Eventually it involved 5 seeding aircraft and about 125
ground-based generators "making it the largest single cloud-seeding
project up to that time. Results of the project were examined at Colo-
rado State University and presented at the International Hail Con-
ference in Verona, Italy, in 1960. This project stimulated the interest
of scientists and provided historical roots for what later was estab-
lished as the National Hail Research Experiment in the same area over
a decade later by the National Science Foundation. 2 ' 4 ' 25

During the 1960's, clearing of cold airport fog through cloud seed-
ing became an operational procedure. Since the techniques used can
only be applied to cold fog, they were used at the more northerly
or high-altitude airports of the United States, where about 15 such
projects were conducted, and are still underway, each winter. 2,6

2 * Elliott, "Experience of the Private Sector," 1974, p. 48.

23 The National Hail Research Experiment is discussed in detail under the weather modi-
fier lion program ol" the Xationa' Science Foundation in ch. 5 ; se p. 274ff.
28 Elliott, "Experience of the Private Sector," 1974. pp. 48-49.


Figure 3. — Weather modification projects in the United States during fiscal year
1968. (From NSF Tenth Annual Report on weather modification, 1968.)



In the various discussions under activities of the Congress and the
executive branch of the Federal Government in chapter 5, there are
historical accounts of legislative actions pertinent to weather modifica-
tion, of the establishment and functioning of special committees in
accordance with public laws or as directed by the executive agencies,
and of the policy and planning studies and reports produced by the
special committees or by the agencies. Inclusion of a separate historical
account of these Federal activities at this point would be largely repeti-
tive, and the reader is referred to the various sections of chapter 5, in
which historical developments of various Federal activities are un-
folded as part of the discussions of those activities.




(By Robert E. Morrison, Specialist in Earth Sciences, Science Policy Research
Division, Congressional Research Service)


Although the theoretical basis for weather modification was laid to
a large extent during the 1930's, the laboratory and field experiments
which ushered in the "modern era" occurred in 1946 and in the years
immediately thereafter. By 1950, commercial cloud seeding had become
widespread, covering an estimated total U.S. land area of about 10 per-
cent. 1 By the mid-1950's, however, it was apparent that the funda-
mental atmospheric processes which come into play in weather
modification are very complex and were far from being understood. A
period of retrenchment and reevaluation began, the number of com-
mercial operators had decreased dramatically, and weather modifica-
tion had fallen into some disrepute among many meteorologists and
much of the public. A period of carefully designed experiments was
initiated about two decades ago, supported by increased cloud physics
research and increasingly more sophisticated mathematical models and
statistical evaluation schemes.

Meanwhile, a small group of commercial operators, generally more
reliable and more responsible than the typical cloud seeder of the 1950
era, has continued to provide operational weather modification services
to both public and private sponsors. These operators have attempted to
integrate useful research results into their techniques and have pro-
vided a bank of operational data useful to the research community.
The operational and research projects have continued over the past two
decades, often in a spirit of cooperation, not always characteristic of
the attitudes of scientists and private operators in earlier years. Often
the commercial cloud seeders have contracted for important roles in
major field experiments, where their unique experiences have been
valuable assets.

Through the operational experiences and research activities of the
past 30 years, a kind of weather modification technology has been
emerging. Actually, though some practices are based on common theory
and constitute the basic techniques for meeting a number of seeding
objectives, there are really a series of weather modification technol-
ogies, each tailored to altering a particular atmospheric phenomenon
and each having reached a different state of development and opera-
tional usefulness. At one end of this spectrum is cold fog clearing, con-
sidered to be operational now, while the abatement of severe storms, at

1 Fleagle. Robert G., "Background and Present Status of Weather Modification." In
"Weather Modification : Science and Public Policy," Seattle, University of Washington
Press, 1968, p. 11.



the other extreme, remains in the initial research phase. Progress to
date in development of these technologies has not been nearly so much
a function of research effort expended as it has depended on the funda-
mental atmospheric processes and the ease by which they can be altered.
There is obvious need for further research and development to refine
techniques in those areas where there has been some success and to
advance technology were progress has been slow or at a virtual


Recently, the following summary of the current status of weather
modification technology was prepared by the Weather Modification
Advisory Board :

1. The only routine operational projects are for clearing cold fog.
Research on warm fog has yielded some useful knowledge and good
models, but the resulting technologies are so costly that they are usable
mainly for military purposes and very busy airports.

2. Several long-running efforts to increase winter snowpack by
seeding clouds in the mountains suggest that precipitation can be
increased by some 15 percent over what would have happened

3. A decade and a half of experience with seeding winter clouds on
the U.S. west coast and in Israel, and summer clouds in Florida, also
suggest a 10- to 15-percent increase over "natural'' rainfall. Hypotheses
and techniques from the work in one area are not directly transferable
to other areas, but will be helpful in designing comparable experiments
with broadly similar cloud systems.

4. Xumerous efforts to increase rain by seeding summer clouds in the
central and western parts of the United States have left many ques-
tions unanswered. A major experiment to try to answer them — for the
High Plains area — is now in its early stages.

5. It is scientifically possible to open holes in wintertime cloud layers
by seeding them. Increasing sunshine and decreasing energy con-
sumption may be especially relevant to the northeastern quadrant of
the United States.

6. Some $10 million is spent by private and local public sponsors for
cloud-seeding efforts, but these projects are not designed as scientific
experiments and it is difficult to say for sure that operational cloud
seeding causes the claimed results.

7. Knowledge about hurricanes is improving with good models of
their behavior. But the experience in modifying that behavior is primi-
tive so far. It is inherently difficult to find enough test cases, especially
since experimentation on tvphoons in the "Western Pacific has been
blocked for the time being by international political objections.

8. Although the Soviets and some U.S. private oi>erators claim some
success in suppressing hail by seeding clouds, our understanding of the
physical processes that create hail is still weak. The one major U.S.
field experiment increased our understanding of severe storms, but
otherwise proved mostlv the dimensions of what we do not vet know.

0. There have been many efforts to suppress lightning by seeding
thunderstorms. Our knowledge of the processes involved is fair, but


the technology is still far from demonstrated, and the U.S. Forest
Service has recently abandoned further lightning experiments. 2

Lewis O. Grant recently summarized the state of general disagree-
ment on the status of weather modification technology and its readiness
for application.

There is a wide diversity of opinion on weather modification. Some believe
that weather modification is now ready for widespread application. In strong
contrast, others hold that application of the technology may never be possible
or practical on any substantial scale. 3

He concludes that —

Important and steady advances have been made in developing technology for
applied weather modification, but complexity of the problems and lack of ade-
quate research resources and commitment retard progress. 4

In 1975, David Atlas, then president of the American Meteorologi-
cal Society, expressed the following pessimistic opinion on the status
of weather modification technology :

Almost no one doubts the economic and social importance of rainfall augmenta-
tion, hail suppression, fog dissipation, and severe storm abatement. But great
controversy continues about just what beneficial modification effects have been
demonstrated or are possible. Claims and counterclaims abound. After three
decades of intense research and operational weather modification activities, only
a handful of experiments have demonstrated beneficial effects to the general
satisfaction of the scientific community.

To describe weather modification as a "technology" is to encourage misunder-
standing of the state of the weather modification art. The word "technology"
implies that the major substantive scientific foundations of the field have been
established and. therefore, that all that is required is to develop and apply tech-
niques. But one of the conclusions of the special AMS study on cloud physics was
that "the major bottleneck impeding developments of useful deliberate weather
modification techniques is the lack of an adequate scientific base." 5

At a 1975 workshop on the present and future role of weather modi-
fication in agriculture, a panel of 10 meteorologists assessed the ca-
pabilities for modifying various weather and weather-related phenom-
ena, both for the present and for the period 10 to 20 years in the fu-
ture. Conclusions from this assessment are summarized in table 1. The
table shows estimated capabilities for both enhancement and dissipa-
tion, and includes percentages of change and areas affected, where
appropriate. 6

A recent study by Barbara Farhar and Jack Clark surveyed the
opinions of 551 scientists, all involved in some aspect of weather modi-
fication, on the current status of various weather modification technol-

2 Weather Modification Advisory Board. "A U.S. Policy to Enhance the Atmospheric
Environment." Oct. 21, 1977. In testimony by Harlan Cleveland "Weather Modification."
he-ring before the Subcommittee on the Environment arid the Atmosphere. Comnrtee on
Science and Technology. U.S. House of Representatives. 95th Cong.. 1st sess.. Oct. 26, 1977.
Washington. DC U.S. Government Prfnt'nsr Office. 1077. pp. 28-30.

3 Grant. Lewis 0., "Scientific and Other Uncertainties of Weather Modification." In Wil-
liam A. Thomas (editor). "Legal and Scientific Uncertainties of Weather Modification.'
Proceedings of a symposium convened at Duke University, Mar. 11-12. 1976, by the
National Conference of Lawyers and Scientists. Durham. N.C., Duke University Press.
1977. p. 7. .

4 Ibid., p. 17.

5 Atlas. David. "Selling Atmospheric Science. The President's Page." Bulletin of the
American Meteorological Societv. vol. 56. No. 7. July 1975. p. 6SS.

6 Grant. Lewis O. and John D. Reid (compilers). "Workshop for an Assessment of the
Present and Potential Role of Weather Modification in Agricultural Production." Colorado
State Universitv. Fort Collins. Colo., July 15-1S. 1975. August 1975. PB-245-633. pp.


ogies. 7 Table 2 is a summary of the assessments of the level of develop-
ment for each of 12 such technologies included in the questionaire to
which the scientists responded, and table 3 shows the estimates of ef-
fectiveness for 7 technologies where such estimates are pertinent. Re-
sults of this study were stratified in accordance with respondents' af-
filiation, specific education, level of education, age, and responsibility
or interest in weather modification, and tabulated summaries of
opinions on weather modification in accordance with these variables ap-
pear in the report by Farhar and Clark. 8


[From Grant and Reid, 1975)

Enhancement Dissipation

Amount Amount

change Area change Area

(per- (square (per- (square

Modified variable Now 10 to 20 yr cent) miles) Now 10 to 20 yr cent) miles)

I. Clouds:

1. Cold stratus No (8)

2. Warm stratus No (10)

3. Fog, cold Yes (10)

4. Fog, warm Yes (10)

5. Fog, artifical (for

temperature con-
trol) Yes (10)

6. Contrails Yes (10)

7. Cirrus… Yes (5)

8. Carbon black No (10)

9. Aerosol Yes (7)

II. Convective precipitation:

1. Isolated small Yes (7)

2. Isolated large No (6)

3. Squall lines Yes (5)

4. Nocturnal Yes (5)

5. Imbedded cyclonic. . Yes (9)

6. Imbedded Oro-

graphic Yes (9)

III. Stratoform precip-

1. Orographic Yes (10)

2. Cyclonic No (10)

3. Cloud water collec-

tion Yes (10)

IV. Hazards:

1. Hail Yes (5)

2. Lightning Yes (7)

3. Erosion— wind

gradient No (10)

4. Erosion— water

drop size Yes (5)

5. Wind— hurricane No (5)

6. Tornado. No (10)

7. Blowdown No (5)

8. Floods— symoptic … No (10)

9. Floods— mesoscale… No (9)

10. Drought No (10)

V. Other:

1. Albedo Yes (5)

2. Surface roughness… No (6)

3. Topography changes. No (6)

Yes (7) 1-1000

No (5)

Yes (10) 1-10

Yes (10) 1-100

Yes (10) 1-10

Yes (10) 100-1000

Yes (10) 100-1000

No (6)

Yes (10)

Yes (10) 100 10-100
Yes (7) 15 100-1000
Yes(S) 20 100-10,000
Yes (6) 100 100-1000
Yes (10) 30 300-6000

Yes (10) 20 300-6000

Yes (10) Yes (10) 1-1000

No (8) Yes (9)

Yes (10) Yes (10) 1-1000

Yes (10) Yes (10) 1-1


No (10) No (10)

No (10) No (8)



Yes (5) Yes (8) 100 10-100

Yes (5) Yes (8) 15 10-1000

No (8) Yes (5) 20 100-10,000

No (8) Yes (5) 100 100-1000

Yes (8) Yes (10) <5 300-6000

Yes (8) Yes (10) 20 300-6000

Yes (10) 10 100-3000 Yes (10) Yes (10) 10 100-3000
No (6) No (10) No (6)

Yes (10) ….

Yes (7) (i)

Yes (9) (■)

No(10) ….


100-60,000 Yes
40,000 Yes (7)


Yes (9)


No (10) No (10)

Yes (7) 0) 10,000 Yes (5)

Yes (6) No (6)

Yes (5) No (10)

Yes (5) No (9)

No (10) No (10)

Yes (6) No (9)

No (10) Yes (5)

Yes (7) 10,000

Yes (6)

Yes (5)

Yes (5)

No (3)

Yes (6)

Yes (6)

Yes (10)
Yes (6)
Yes (5)

Yes (5)
No (6)
No (6)

Yes (10)

Yes (6)

Yes (5) 10-100

1 Uncertain.

7 Farhar. Barbnra C. and Jack A. Clark. "Can Wp Modify the Weather? a Survey of
Scientists " Final report, vol. 3 (draft), Institute of Behavioral Science. University of Colo-
rado. Boulder, Colo.. January 1078. (Based on research supported by the National Science
Foundation under grants No*. ENV74-1R013 AOS. 01-35452, GI-44087. and BRT74-18613,
as part of "A Comparative Analysis of Public Support of and Resistance to Weather Modi-
fication Projects.") 89 pp.

* Ibid.



[From Farhar and Clark, 1978]

Operations 1 Research 2 Neither Don't know Other







Weather modification technology












Cold fog dispersal









Precipitation enhancement, winter oro-



1 1
1 1




Precipitation enhancement, winter oro-

graphic, maritime










Hail suppression










Precipitation enhancement, summer convec-

tive, continental .










Precipitation enhancement, summer convec-

tive, maritime










Warm fog dispersal…









Precipitation enhancement with hail sup-












Precipitation enhancement, general storms..











Lightning suppression










Hurricane suppression











Severe storm mitigation











1 This category is a combination of two responses: "The technology is ready for operational application" and "The
technology can be effectively applied; research should continue."

2 This category is a combination of two responses: "The technology is ready for field research only" and "The technology
should remain at the level of laboratory research."




In a previous review of weather modification for the Congress, three
possible classifications of activities were identified — these classifica-
tions were in accordance with (1) the nature of the atmospheric proc-
esses to be modified, (2) the agent or mechanism used to trigger or
bring about the modification, or (3) the scale or dimensions of the
region in which the modification is attempted. 9 The third classifica-
tion was chosen in that study, where the three scales considered were
the microscale (horizontal distances, generally less than 15 kilometers) ,
the mesoscale (horizontal distances generally between 15 and 200
kilometers), and the macroscale (horizontal distances generally
greater than 200 kilometers). 10 Examples of modification of processes
on each of these three scales are listed in table 4, data in which are
from Hartman. 11 Activities listed in the table are illustrative only,
and there is no intent to indicate that these technologies have been
developed, or even attempted in the case of the listed macroscale


[Information from Hartman, 19661
Scale Horizontal dimensions Examples of modification processes

Microscale Less than 15 km

Mesoscale 15 to 200 km.

Macroscale Greater than 200 km.

Modification of human microclimates.
Modification of plant microclimates.
Evaporation suppression.
Fog dissipation.
Cloud dissipation.
Hail prevention.

Precipitation through individual cloud modification.

Precipitation from cloud systems.

Hurricane modification.

Modification of tornado systems.

Changes to global atmospheric circulation patterns.

Melting the Arctic icecap.

Diverting ocean currents.

In this chapter the characteristics and status of weather modifica-
tion activities will be classified and discussed according to the nature
of the processes to be modified. This seems appropriate since such a
breakdown is more consonant with the manner the subject has been
popularly discussed and debated, and it is consistent with the direc-
tions in which various operational and research activities have moved.
Classification by the second criterion above, that is, by triggering
agent or mechanism, focuses on technical details of weather modi-
fication, not of chief interest to the public or the policymaker, although
these details will be noted from time to time in connection with dis-
cussion of the various weather modification activities.

In the following major section, then, discussion of the principles
and the status of planned weather modification will be divided accord-

9 Hartman. Lawton M.. "Characteristics and Scope of Weather Modification. In U.S.
Congress, Senate Committee on Commerce. "Weather Modification and Control," TV ashing-
ton. D.C., U.S. Government Printing Office. 1966. (89th Cone:.. 2d sess., Senate Kept. JSo.
1139. prepared by the Legislative Reference Service, Library of Congress), p. 20.

10 Ibid.

" Ibid., pp. 21-31.

34-857 O – 79 – 7


ing to the major broad categories of phenomena to be modified; these
will include :

Precipitation augmentation.

Hail suppression.

Fog dissipation.

Lightning suppression.

Severe storm mitigation.
In subsequent major sections of this chapter there are reviews of
some of the specific technical problem areas common to most weather
modification activities and a summary of recommenced research

In addition to the intentional changes to atmospheric phenomena
discussed in this chapter, it is clear that weather and climate have also
been modified inadvertently as the result of man's activities and that
modification can also be brought about through a number of natur-
ally occurring processes. These unintentional aspects of weather and
climate modification will be addressed in the following chapter of
this report. 12

Principles and Status of Weather Modification Technologies

Before discussing the status and technologies for modification of
precipitation, hail, fog, lightning, and hurricanes, it may be useful to
consider briefly the basic concepts of cloud modification. The two major
principles involved are (1) colloidal instability and (2) dynamic ef-
fects. Stanley Changnon describes how each of these principles can
be effective in bringing about desired changes to the atmosphere : 13

Altering colloidal stability. — The physical basis for most weather modification
operations has been the belief that seeding with certain elements would produce
colloidal instability in clouds, either prematurely, to a greater degree, or with
greater efficiency than in nature. Most cloud seeding presumes that at least a por-
tion of the treated cloud is supercooled, that nature is not producing any or
enough ice at that temperature of the cloud, and that treatment with chemical
agents of refrigerants will change a proportion of the cloud to ice. The resultant
mixture of water and ice is unstable and there is a rapid deposition of water
vapor upon the ice and a simultaneous evaporation of water from the super-
cooled droplets in the cold part of the cloud. The ice crystals so formed become
sufficiently large to fall relative to remaining droplets, and growth by collection
enhances the probability that particles of ice or water will grow to be large
enough to fall from the cloud and become precipitation.

This process of precipitation enhancement using ice nucleants has been dem-
onstrated for the stratiform type cloud, and generally for those which are oro-
graphically-produced and supercooled. Cumulus clouds in a few regions of the
United States have also been examined for the potential of colloidal instability in
their supercooled portions. This has been founded on beliefs that precipitation
(1) can be initiated earlier than by natural causes, or (2) can be produced from
a cloud which was too small to produce precipitation naturally.

Seeding in the warm portion of the cloud, or in "warm clouds" (below the
freezing level), has also been attempted so as to alter their colloidal instability.
Warm-cloud seeding has primarily attempted to provide the large droplets neces-
sary to initiate the coalescence mechanism, and is of value in clouds where insuffi-
cient large drops exist. In general alteration of the coalescence process primarily
precipitates out the liquid water naturally present in a cloud, whereas the ice-
crystal seeding process also causes a release of latent energy that conceivably
results in an intensification of the storm, greater cloud growth, and additional

Alirrhifj cloud dynamics. — The effects to alter the colloidal instability of
clouds, or their microphysical processes, have been based on the concept of rain

1L ' Sof p. 145.

13 Chnncrnon. Stanley A.. Jr. "Prosont and Future of Woathor Modification ; Peprtonal
Issues." The Journal of Woathor Mortification, vol. 7. No. 1, April 1075, pp. 154-156.


increase through increasing the precipitation efficiency of the cloud. Simpson
and Dennis (1972) showed that alterations of cloud size and duration by "dynam-
ic modification" could produce much more total rainfall than just altering the
precipitation efficiency of the single cloud. In relation to cumulus clouds,
"dynamic seeding" simply represents alteration one step beyond that sought
in the principle of changing the colloidal stability. In most dynamic seeding
efforts, the same agents are introduced into the storm but often with a greater
concentration, and in the conversion of w r ater to ice, enormous amounts of
latent heat are hopefully released producing a more vigorous cloud which will
attain a greater height with accompanying stronger updrafts, a longer life, and
more precipitation. Seeding to produce dynamic effects in cloud growth, whether
stratiform or cumuliform types, is relatively recent at least in its serious in-
vestigation, but it may become the most important technique. If through con-
trolled cloud seeding additional uplift can be produced, the productivity in terms
of rainfall will be higher whether the actual precipitation mechanism involved
is natural or artificial.

It has been proposed that the selective seeding of cumulus clouds also can
either (a) bring upon a merger of tw T o or more adjacent clouds and a much
greater rainfall production through a longer-lived, larger cloud * * * or (b) pro-
duce eventually an organized line of clouds (through selective seeding of ran-
domized cumulus). The latter could allegedly be accomplished by minimizing and
organizing the energy into a few vigorous systems rather than a larger number of
isolated clouds.

Essentially, then, dynamic seeding is a label addressed to processes involved
in altering cloud microphysics in a selective and preferential way to bring
upon more rainfall through an alteration of the dynamical properties of the
cloud system leading to the development of stronger clouds and mesoscale
systems. Actually, dynamic effects might be produced in other ways such as
alterations of the surface characteristics to release heat, by the insertion of
chemical materials into dry layers of the atmosphere to form clouds, or by re-
distribution of precipitation through microphysical interactions in cloud processes.

The various seeding materials that have been used for cloud modi-
fication are intended, at least initially, to change the microphysical
cloud structure. Minute amounts of these materials are used with the
hope that selected concentrations delivered to specific portions of the
cloud will trigger the desired modifications, through a series of rapid
multiplicative reactions. Seeding materials most often used are classi-
fied as (1) ice nuclei, intended to enhance nucleation in the super-
cooled part of the cloud, or (2) hygroscopic materials, designed to
alter the coalescence process. 14

Glaciation of the supercooled portions of clouds has been induced
by seeding with various materials. Dry ice injected into the subfreezing
part of a cloud or of a supercooled fog produces enormous numbers of
ice crystals. Artificial ice nuclei, with a crystal structure closely re-
sembling that of ice, usually silver iodide smoke particles, can also
produce glaciation in clouds and supercooled fogs. The organic fer-
tilizer, urea, can also induce artificial glaciation, even at temperatures
slightly warmer than freezing. Urea might also enhance coalescence in
warm clouds and warm fogs. Water spray and fine particles of sodium
chloride have also been used in hygroscopic seeding, intended to alter
the coalescence process. There have been attempts to produce co-
alescence in clouds or fog using artificial electrification, either with
chemicals that increase droplet combination by electrical forces, or
with surface arrays of charged wires whose discharges produce ions
which, attached to dust particles, may be transported to the clouds. 15

Problems of cloud seeding technology and details of seeding deliv-
ery methods are discussed in a later section of this chapter, as are

14 Ibid., p. 156.

15 Ibid., pp. 156-157.


some proposed techniques for atmospheric modification that go beyond
cloud seeding. 16


The seeding of clouds to increase precipitation, either rainfall or
snowfall, is the best known and the most actively pursued weather
modification activity. Changes in clouds and precipitation in the
vicinity of cloud seeding operations have shown unquestionaBly that
it is possible to modify precipitation. There is evidence, however,
that such modification attempts do not always increase precipitation,
but that under some conditions precipitation may actually be de-
creased, or at best no net change may be effected over an area. Never-
theless, continued observations of clouds and precipitation, from both
seeded and nonseeded regions and from both experiments and com-
mercial operations, are beginning to provide valuable information
which will be useful for distinguishing those conditions for which
seeding increases, decreases, or has no apparent effect on precipita-
tion. These uncertainties were summarized in one of the conclusions
in a recent study on weather modification by the National Academy
of Sciences : 17

The Panel now concludes on the basis of statistical analysis of well-designed
field experiments that ice-nuclei seeding can sometimes lead to more precipita-
tion, can sometimes lead to less precipitation, and at other times the nuclei
have no effect, depending on the meteorological conditions. Recent evidence has
suggested that it is possible to specify those microphysical and mesophysical
properties of some cloud systems that determine their behavior following
artificial nucleation.

Precipitation enhancement has been attempted mostly for two gen-
eral types of cloud forms, both of which naturally provide precipita-
tion under somewhat different conditions. Convective or cumulus
clouds are those which are formed by rising, unstable air, brought
about by heating from below or cooling in the upper layers. Under
natural conditions cumulus clouds may develop into cumulo-nimbus
or "thunderheads," capable of producing heavy precipitation. Cu-
mulus clouds and convective systems produce a significant portion
of the rain in the United States, especially during critical growing
seasons. Attempts to augment this rainfall from cumulus clouds
under a variety of conditions have been underway for some years
with generally uncertain success. The other type of precipitation-
producing clouds of interest to weather modifiers are the orographic
clouds, those which are formed when horizontally moving moisture-
laden air is forced to rise over a mountain. As a result of the cooling
as the air rises, clouds form and precipitation often falls on the
windward side of the mountain. Through seeding operations, there
have been attempts to augment precipitation through acceleration
of this process, particularly in winter, in order to increase mountain

Figures 1 and 2 show regions of the coterminous United States
which are conducive to precipitation management through seeding
of spring and summer convective clouds and through seeding oro-
graphic cloud systems, respectively. The principles of precipitation

16 See pp. 115 and 129.

17 National Academy of Sciences, National Research Council, Committee on Atmospheric
Sciences, "Weather and Climate Modification : Problems and Progress," Washington, D.C.,
1973, p. 4.


enhancement for both cumulus and orographic clouds, and the present
state of knowledge and technology for such modification, are dis-
cussed in the following sections.

Figure 1. — Regions where preciptation management may be applied to enhance
rainfall from spring and summer showers.

Figure 2.— Regions where precipitation management may be applied to enhance
snowfall from winter orographic weather systems, thus augmenting spring and
summer runoff from mountain snowpacks.


Currmlus clouds

If air containing moisture is cooled sufficiently and if condensation
nuclei such as dust particles are present, precipitation may be pro-
duced. This process occurs when air is forced to rise by convection,
so that the water vapor condenses into clouds. Cumulus clouds are the
woolly vertical clouds with a flat base and somewhat rounded fop,
whose origin can always be traced to the convection process. They can
most often be observed during the summer and in latitudes of high
temperature. When updrafts become strong under the proper con-
ditions, cumulus clouds often develop into cumulonimbus clouds, the
principal producer of precipitation. About three-fourths of the rain
in the tropics and subtropics and a significant portion of that falling
on the United States is provided from cumulus clouds and convective

The science of cloud study, begun in the 1930's and greatly expanded
following World War II, includes two principal aspects — cloud micro-
physics and cloud dynamics. Though once approached separately by
different groups of scientists, these studies are now merging into a
single discipline. In cloud physics or microphysics the cloud parti-
cles — such as condensation and freezing nuclei, water droplets, and ice
crystals — are studied along with their origin, growth, and behavior.
Cloud dynamics is concerned with forces and motions in clouds, the
prediction of cloud structure, and the life cycle of updrafts and down-
drafts. 18

For cloud modification purposes, present theories of microphysical
processes provide an ample basis for field seeding experiments ; how-
ever, further work is still needed on laboratory experiments, improved
instrumentation, and research on assumptions. On the other hand,
the processes in cloud dynamics are not completely understood and
require continued research. 19

Most cumulus clouds evaporate before they have had opportunity
to produce precipitation at the Earth's surface. In fact many clouds
begin to dissipate at about the same time that rain emerges from their
bases, leading to the impression that they are destroyed by the forma-
tion of precipitation within them. This phenomenon is not yet fully
understood. Cumulus clouds have a life cycle; they are born, mature,
and eventually age and die. Small cumuli of the trade regions live only
about 5 to 10 minutes, while medium-sized ones exist for about 30 min-
utes. On the other hand, a giant cumulonimbus cloud in a hurricane
or squall line may be active for one to several hours. In its lifetime it
may exchange over 50 million tons of water, producing heavy rain,
lightning, and possibly hail. At all times, however, a cumulus cloud
struggles to exist; there is a precarious balance between the forces
aiding its growth and its destruction. 20

The increasing capability to simulate cloud processes on the com-
puter has been a major advance toward understanding cloud modifi-
cation. The ways in which cloud microphysics influences convective

18 Simpson Joanne and Arnett S. Dennis, "Cumulus Clouds and Their Modification. In
Wilmot N. Hess (ed.), "Weather and Climate Modification." New York, John Wiley & Sons,

^'^Mo'schandreas, Demetrios J . and Irving Leichter. "Present Capabilities to Modify
Cumulus Clouds." Geomet. Inc. report No. EF-46.H. Final report for U.S. Navy Environ-
mental Prediction Research Facility, Mar. :U), 1976. p. 209. .

20 Simpson and Dennis, "Cumulus Clouds and Their Modification, 1947, pp. 234-23o.


dynamics are not well documented or modeled, however. Feedback
mechanisms are dynamic and thermodynamic. Dynamically, the buoy-
ancy is reduced by the weight of the particles formed within the
cloud, sometimes called "water loading/' Modeling suggests that
thermodynamic feedback from the microphysics can be even more
important, as evaporation at the edges of the cloud produces cooling
and thus induces downdrafts. Observations confirm this important
influence of evaporation, particularly where the cloud environment is
relatively dry, but the effect is minimized in humid tropical regions. 21

Cumulus modification experiments

An enormous amount of energy is expended in natural atmospheric
processes. As much energy as the fusion energy of a hydrogen super-
bomb is released in a large thunderstorm, and in a moderate -strength
hurricane the equivalent of the energy of 400 bombs is converted each
clay. In his attempt to modify precipitation from clouds, man must
therefore look for some kind of a trigger mechanism by which such
energetically charged activities can be controlled, since he cannot hope
to provide even a fraction of the energy involved in the natural proc-
ess. A major problem in evaluating modification efforts is the large
natural variability in atmospheric phenomena. A cumulus cloud can,
in fact, do almost anything all by itself, without any attempt to mod-
ify its activity by man. This high variability has led the layman to
overestimate grossly what has been and can be done in weather modifi-
cation. In designing an experiment, this variability requires that there
be sound statistical controls. 22

Precipitation is formed by somewhat different processes in warm
clouds and in subfreezing clouds. In the former, droplets are formed
from condensation of water vapor on condensation nuclei and grow
through collision and coalescence into raindrops. In subfreezing
clouds, such as the cumuli under discussion, supercooled water drop-
lets are attached to ice nuclei which grow into larger ice particles.
When large enough, these particles fall from the cloud as snow or sleet
or may be converted to rain if the temperature between the cloud and
the Earth's surface is sufficiently warm. Increasing precipitation
through artificial means is more readily accomplished in the case of
the subfreezing clouds. In addition, attempts have been made to pro-
mote the merging of cumulus clouds in order to develop larger cloud
systems which are capable of producing significantly more precipita-
tion than would be yielded by the individual small clouds.

Nearly all cumulus experiments have involved "seeding" the clouds
with some kind of small particles. Sometimes the particles are dis-
persed from the ground, using air currents to move them into the
clouds. Most often the materials are dispensed from aircraft, by releas-
ing them upwind of the target clouds, by dropping them into the cloud
top, by using the updraft from beneath the cloud, or by flying through
the cloud. Although more expensive, aircraft seeding permits more
accurate targeting and opportunity for measurements and observa-
tions. In the Soviet Union, cumulus clouds have been seeded success-

21 Simpson. Joanne, "Precipitation Augmentation from Cumulus Clouds and Systems :
Scientific and Technical Foundations." 1975. Advances in Geophysics, vol. 19. Xew York.
Academic Press, 1976. pp. 10-11.

122 Simpson and Dennis, "Cumulus Clouds and Their Modification," 1974, pp. 240-241.


fully with artillery shells and rockets, using radar to locate parts of
the clouds to be seeded. 23

Augmentation of precipitation in cumulus clouds has been attempted
both by accelerating the coalescence process and by initiating ice parti-
cle growth in the presence of supercooled water. In fact, these processes
are essentially identical in cumuli where the tops extend above the
freezing level.

Prior to the 1960's nearly all supercooled seeding experiments and
operations were concerned with attempting to increase precipitation
efficiency, based on consideration of cloud microstructure. 24 This is
essentially a static approach, intended to produce precipitation by in-
creasing the total number of condensation nuclei, through the intro-
duction of artificial nuclei injected by seeding into or under the clouds.
This approach has been moderately successful in convective storms
with conducive cloud microstructure in a number of locations — Cali-
fornia, Israel, Switzerland, and Australia — where clouds are often
composed of small supercooled droplets, typical of winter convection
and of continental air masses. 25 On the other hand, the large cumulus
clouds originating in tropical and subtropical ocean regions, which are
evident over much of the eastern United States during the summer, are
much less influenced by this static approach. A technique known as
dynamic seeding has shown promise in enhancing precipitation from
clouds of this type.

According to dynamic seeding philosophy, the strength, size, and
duration of vertical currents within the cloud have stronger control on
cumulus precipitation than does the microstructure. In this technique,
first demonstrated in the 1960 ? s, the seeding provides artificial nuclei
around which supercooled water freezes, liberating large quantities of
latent heat of fusion, within the clouds, causing them to become more
buoyant and thus to grow to greater heights. This growth invigorates
circulation within the cloud, causes increased convergence at its base,
fosters more efficient processing of available moisture, and enhances
rainfall through processes by which cumuli ordinarily produce such
precipitation. Results of the Florida Area Cumulus Experiment
(FACE) , conducted by the U.S. Department of Commerce, seem to in-
dicate that dynamic seeding has been effective in increasing the sizes
and lifetimes of individual cumuli and the localized rainfall resulting
from them. 20

Success thus far in rain enhancement from dynamic seeding of
cumulus has been demonstrated through seeding techniques applied
to single, isolated clouds. In addition to the experiments in Florida,
dynamic seeding of single clouds has been attempted in South Dakota,
Pennsylvania, Arizona, Australia, and Africa, with results similar to
those obtained in Florida. 27 It appears, however, that a natural process
necessary for heavy and extensive convective rainfall is the merger
of cloud groups. Thus, this process of cloud merger must be promoted
in order for cloud seeding to be effective in augmenting rainfall from

23 Ibid., p. 242.

24 Ibid., 1974, pp. 246-247.

25 Ibid., p. 247. , – „

26 William L. Woodley. Joanne Simpson. Ronald Biondini, and Joyce Berkeley. "Rainfall
Results. 1970-I97. r > ; Florida Area Cumulus Experiment," Science, vol. ID'S. No. 4280. Feb. 2f>.
1077. p. 735.

-~ Simpson and Dennis, "Cumulus Clouds and Their Modification." 1974, p. 261.


cumulus clouds. The FACE experiment has been designed to investi-
gate whether dynamic seeding can induce such cloud merger and in-
creased rainfall. 28 Area wide cumulus cloud seeding experiments are
also planned for the U.S. Department of the Interior's High Plains
Cooperative program (HIPLEX), being conducted in the Great
Plains region of the United States. 29 30 There has been some indication
that desired merging has been accomplished in the Florida experi-
ment. 31 Though this merging and other desirable effects may be
achieved for Florida cumulus, it must be established that such mergers
can also be induced for other connective systems which are found over
most of the United States east of the Great Plains. Changnon notes
that, "The techniques having the most promise for rain enhancement
from convective clouds have been developed for single, isolated types
of convective clouds. The techniques have been explored largely
through experimentation with isolated mountain-type storms or with
isolated semitropical storms. * * * Weather modification techniques
do not exist for enhancing precipitation from the multicellular con-
vective storms that produce 60 to 90 percent of the warm season
rainfall in the eastern two-thirds of the United States." 32

Effectiveness of precipitation enhancement research and operations

A major problem in any precipitation enhancement project is the
assessment of whether observed increases following seeding result from
such seeding or occur as part of the fluctuations in natural precipita-
tion not related to the seeding. This evaluation can be attempted
through observations of physical changes in the cloud system which
has been seeded and through statistical studies.

Physical evaluation requires theoretical and experimental investi-
gations of the dispersal of the seeding agent, the manner that seeding
has produced changes in cloud microstructure, and changes in gross
characteristics of a cloud or cloud system. Our understanding of the
precipitation process is not sufficient to allow us to predict the magni-
tude, location, and time of the start of precipitation. Hence, because
of this lack of detailed understanding and the high natural variability
of precipitation, it is necessary to use statistical methods as well. There
is a closer physical link between seeding and observable changes in
cloud microstructure ; however, even the latter can vary widely with
time and position in natural, unseeded clouds, so that statistical evalua-
tion is also required with regard to the measurement of these
quantities. 33

It should first be determined whether the seeding agent reached
the intended region in the cloud with the desired concentration rather

^Woodley, et al.. "Rainfall Results, 1970-1975; Florida Area Cumulus Experiment,
1977. p. 735.

29 Bureau of Reclamation. U.S. Department of the Interior. "High Plains Cooperative
Program : Progress and Planning Report No. 2," Denver. March 1976. p. 5.

30 The history, purposes, organization, and participants in the FACE and HIPLEX pro-
grams are discussed along with other programs of Federal agencies in chapter o or tms
report. _ . L „

31 William L. Woodley and Robert I. Sax. "The Florida Area Cumulus Experiment : Ka-
tionale. Design. Procedures. Results, and Future Course." U.S. Department of Commerce.
National Oceanic and Atmospheric Administration, Environmental Research Laboratories.
NOAA technical report ERL 354-WMPO 6. Boulder, Colo., January 19 , 6 pp. 41-4o.

32 Changnon, Stanley A.. Jr., "Present and Future of Weather Modification : Regional

ISS33J Warn 9 e 7 r°'j PP '"Th 9 e ~Deteetabilitv of the Effects of Seeding." In World Meteorological Or-
ganization. Weather Modification Programme, position papers used in the Preparation of
the plan for the Precipitation Enhancement Experiment (PEP), Precipitation Enhancement
Project Report No. 2. Geneva, November 1976, annex I, p. 43.


than spreading into other areas selected as controls. When the agent
has been delivered by aircraft, this problem is usually minimized,
though even in this case, it is desirable to learn how the material has
diffused through the cloud. When ground-based seeding generators
are used, the diffusion of the material should be studied both by
theoretical studies and by field measurements. Such measurements
may be made on the seeding agent itself or on some trace material
released either with the seeding agent or separately ; this latter might
be either a fluorescent material such as zinc sulphide or any of various
radioactive materials. Sometimes the tracer might be tracked in the
cloud itself, while in other experiments it may be sufficient to track
it in the precipitation at the surface. 34

In looking for cloud changes resulting from seeding, the natural
cloud behavior is needed as a reference; however, since the character-
istics of natural clouds vary so widely, it is necessary to observe a
number of different aspects of the properties and behavior of seeded
clouds against similar studies of unseeded clouds in order to be able
^o differentiate between the two. It is further desirable to relate such
behavior being studied to predictions from conceptual and numerical
models, if possible. Direct observations should be augmented by radar
studies, but such studies should substitute for the direct measurements
only when the latter are not possible. 35

A statistical evaluation is usually a study of the magnitude of the
precipitation in the seeded target area in terms of its departure from
the expected value. The expected quantity can either be determined
from past precipitation records or through experimental controls. Such
controls are established by dividing the experimental time available
roughly in half into periods of seeding and nonseeding, on a random
basis. The periods may be as short as a day or be 1 or 2 weeks in dura-
tion. The precipitation measured during the unseeded period is used as
a measure of what might be expected in the seeded periods if seeding
hadn't occurred. In another technique, control areas are selected where
precipitation is highly correlated with that in the target area but
which are never seeded. The target area is seeded on a random basis
and its rainfall is compared with that of the control area for both
seeded and unseeded periods. Another possibility includes the use of
two areas, either of which may be chosen for seeding on a random basis.
Comparisons are then made of the ratio of precipitation in the lirst
area to that in the second with the first area seeded to the same ratio
when the second is also seeded. There are many variations of these
basic statistical designs, the particular one being used in a given experi-
ment depending on the nature of the site and the measuring facilities
available. As with the seeding techniques employed and the physical
measurements which are made, experimental design can only be final-
ized after a site has been selected and its characteristics studied. 36

Results achieved through cumulus modification

Cumulus modification is one of the most challenging and controver-
sial areas in weather modification. In some cases randomized seeding
efforts in southern California and in Israel have produced significant

Ibid., p. 44.
33 Ibid.

M Ibid., p. 47).


precipitation from bands of winter cyclonic storms. However, attempts
have been less promising in attributing increased rain during summer
conditions to definitive experiments. There has been some success in
isolated tropical cumuli, where seeding has produced an increase in
cloud height and as much as a twofold to threefold increase in rain-
fall. 37

In the Florida area cumulus experiment (FACE), the effects on
precipitation over a target area in southern Florida as a result of
seeding cumuli moving over the area is being studied under the spon-
sorship of the National Oceanic and Atmospheric Administration
(NOAA). Analysis of the data from 48 days of experimentation
through 1975 provided no evidence that rainfall over the fixed target
area of 13,000 square kilometers had been altered appreciably from
dynamic seeding. On the other hand, there is positive evidence for
increased precipitation from seeding for clouds moving through the
area. 38

When FACE data from the 1976 season are combined with previous
data, however, increasing the total number of experimental days to 75,
analysis shows that dynamic seeding under appropriate atmospheric
conditions was effective in increasing the growth and rain production
of individual cumulus clouds, in inducing cloud merger, and in pro-
ducing rainfall increases from groups of convective clouds as they
pass through the target area. A net increase seemed to result from the
•seeding when rainfall on the total target area is averaged. 39

Further discussion of FACE purposes and results is found under
the summary of weather modification programs of the Department of
Commerce in chapter 5. 40

Recent advances in cumulus cloud modification

In the past few years some major advances have been achieved in
cumulus experimentation and in improvement of scientific under-
standing. There has been progress in (1) numerical simulation of
cumulus processes and patterning; (2) measurement techniques; (3)
testing, tracing, delivery, and targeting of seeding materials; and (4)
application of statistical tools. Recognition of the extreme difficulty of
cumulus modification and the increased concept of an overall systems
approach to cumulus experimentation have also been major advances. 41

Orographic clouds and precipitation

In addition to the convection clouds, formed from surface heating,
clouds can also be formed when moist air is lifted above mountains
as it is forced to move horizontally. As a result, rain or snow may fall,
and such precipitation is said to be orographic, or mountain induced.
The precipitation results from the cooling within the cloud and charac-

37 Sax. R. I.. S. A. Changnon. L. O. Grant. W. F. Hitschfeld. P. V. Hobbs. A. M. Kanan.
and J. Simnson, "Weather Modification: Where Are We Now and Where Should \\ e Be
Going? An Editorial Overview." Journal of Applied Meteorology, vol. 14. No. o, August 1975,
P- 662.

38 Woodlev, et al., "Rainfall Results, 1970-1975 ; Florida Area Cumulus Experiment.
1977. p. 742. , „ . .

^Woodley. William L.. Joanne Simpson. Ronald Biondini. and Jill Jordan. NOAA s
Florida Area Cumulus Experiment; Rainfall Results. 1970-1976 " In preprints from the
Sixth Conference on Planned and Inadvertent Weather Modification, Champaign, 111..
Oct. 10-13. 1977. Boston, American Meteorological Society, 1977, p. 209.

40 gee p 292

41 Sax. et.' ai. "Weather Modification : Where Are We Now and Where Should We Be
Going? An Editorial Overview," 1975, p. 663.


teristically falls on the windward side of the mountain. As the air
descends on the leeward side of the mountain, there is warming and
dissipation of the clouds, so that the effect of the mountains is to pro-
duce a "rain shadow" or desert area. The Sierra Nevada in western
North America provide such conditions for orographic rain and snow
along the Pacific coast and a rain shadow east of the mountains when
moisture laden air generally flows from the Pacific eastward across
this range.

The western United States is a primary area with potential for
precipitation augmentation from orographic clouds. This region re-
ceives much of its annual precipitation from orographic clouds during
winter, and nearly all of the rivers start in the mountains, deriving
their water from melting snowpacks. The major limitation on agricul-
ture here is the water supply, so that additional water from increased
precipitation is extremely valuable. Streamflow from melting snow
is also important for the production of hydroelectric power, so that
augmentation of precipitation during years of abnormally low natural
snowfall could be valuable in maintaining required water levels neces-
sary for operation of this power resource. Orographic clouds provide
more than 90 percent of the annual runoff in many sections of the
western United States. 42

Figure 3 (a) and (b) are satellite pictures showing the contrast
between the snow cover over the Sierra Nevada on April 28, 1975, and
on April 19, 1977. This is a graphical illustration of why much of Cali-
fornia was drought stricken during 1977. The snowpack which custo-
marily persists in the highest elevations of the Sierras until July had
disappeared by mid-May in 1977. 43

The greatest potential for modification exists in the winter in this
region, while requirements for water reach their peak in the summer ;
hence, water storage is critical. Fortunately, the snowpack provides a
most effective storage, and in some places the snowmelt lasts until early
July. Water from the snowmelt can be used directly for hydroelectric
power generation or for irrigation in the more arid regions, while
some can be stored in reservoirs for use during later months or in sub-
sequent dry years. In some regions where the snowpack storage is not
optimum, offseason orographic precipitation is still of great value,
since the water holding capacity of the soil is never reached and addi-
tional moisture can be held in the soil for the following groAving season.

Orographic clouds are formed as moist air is forced upward hy
underlying terrain. The air thus lifted, containing water vapor, cools
and expands. If this lifting and cooling continue, the air parcels will
frequently reach sal mat ion. If the air becomes slightly supersaturated,
small droplets begin to form by condensation, and a cloud develops,
which seems to hang over the mountain peak. The location where this
condensation occurs can be observed visually by the edge of the cloud
on the windward side of the mountain. Upon descent in the lee of the
mountain the temperature and vapor capacity of the air parcel again

"Grant, Lewis O. and Archie M. Kahan, "Weather Modification for Augmenting Oro-
graphic Precipitation." In Wilmot N. Hess (editor), "Weather and Climate Modification,"
New York. Wiley. 1974. p. 2S5.

4:1 U.S. Department of Commerce, news release, NOAA 77-234. NO A A Public Affairs Office,
Rockville, Md., Aug. 17, 1077.


increase, so that any remaining liquid droplets or ice crystals
evaporate. 44

(a) April 28, 1975

Figure 3. — NOAA-3 satellite pictures of the snowcover on the Sierra Nevada
Mountains in (a) April 1975 and (b) April 1977. (Courtesy of the National
Oceanic and Atmospheric Administration.)

44 Sax. et al.. "Weather Mortification : Where Are We Now and Where Should We Be
Going?" an editorial overview, 1975, pp. 657-658.



(b) April 19, 1977

The supercooled cloud droplets exist as liquid at temperatures down
to about -20° C ; but at temperatures colder than -20° C, small ice
crystals begin to form around nuclei that are naturally present in the
atmosphere. Once formed, the ice crystals grow rapidly because the
saturation vapor pressure over ice is less than that over water. As the
crystals increase they may fall and eventually may reach the ground
as snow. The temperature at the top of the cloud is an important
factor in winter storms over mountains, since natural ice crystals will
not form in large quantities if the cloud top is warmer than —20° C.
If the temperature is below —20° C, however, a large fraction of the
cloud particles will fall as snow from natural processes. 45

45 Weisbecker, Leo W. (compiler), "The Impacts of Snow Enhancement; Technology
Assessment of Winter Orographic Snowpack Augmentation in the Upper Colorado River
Basin," Norman, Okla., University of Oklahoma Press, 1974, pp. 64-66.


Orographic precipitation modification

According to Grant and Kalian, " * * * research has shown that
orographic clouds * * * provide one of the most productive and
manageable sources for beneficial weather modification." 46 In a re-
cent study by the National Academy of Sciences, it was concluded
broadly that orographic clouds provide one of the "main possibilities
of precipitation augmentation,*' based on the considerations below : 47
A supply of cloud water that is not naturally converted into
precipitation sometimes exists for extended periods of time ;

Efficient seeding agents and devices are available for treating
these clouds;

Seeding agents can sometimes (not always) be delivered to
the proper cloud location in proper concentrations and at the
proper time;

Microphysical cloud changes of the type expected and neces-
sary for seeding have been demonstrated;

Substantial increases in precipitation with high statistical sig-
nificance have been achieved in some well-designed randomized
experiments for clouds that, based on physical concepts, should
have seeding potential; and

Augmentation of orographic precipitation can have great eco-
nomic potential.

Although natural ice crystals will not form in sufficient numbers if
the cloud top is warmer than —20° C, it has been shown that particles
of silver iodide smoke will behave as ice nuclei at temperatures some-
what warmer than — 20° C, so that ice crystals can be produced by such
artificial nuclei in clouds with temperatures in the range of —10° to
— 20° C. Whereas in the natural state, with few active nuclei at these
temperatures, the cloud particles tend to remain as water droplets,
introduction of the silver iodide can quickly convert the supercooled
cloud into ice crystals. Then, the natural growth processes allow the
crystals to grow to sufficient size for precipitation as snow. 48

Meteorological factors which favor increased snowfall from oro-
graphic clouds through cloud seeding are summarized by
Weisbecker : 49

The component of the airflow perpendicular to the mountain
ridge must be relatively strong.

The air must have a high moisture content. Generally, high
moisture is associated with above-normal temperatures.

The cloud, including its upper boundary, should be at a temp-
erature warmer than — 20° C. Since temperature decreases with
increasing altitude, this temperature criterion limits the altitude
of the cloud top. However, it is advantageous for the cloud base
to be low, since the water droplet content of the cloud will then
be relatively large.

46 Grant and Kahan, "Weather Modification for Augmenting Orographic Precipitation,"
1974. p. 282.

* 7 Committee on Climate and Weather Fluctuations and Agricultural Production, National
Research Council, "Climate and Food ; Climatic Fluctuation and U.S. Agricultural Produc-
tion." National Academy of Sciences. Washington, D.C., 1976, p. 136.

48 Weisbecker, "The Impacts of Snow Enhancement ; Technology Assessment of Winter
Orographic Snowpack Augmentation in the Upper Colorado Basin," 1974, p. 66.

» Ibid. pp. 66-67.


It must be possible to disperse silver iodide particles within the
cloud in appropriate numbers to serve as ice crystal nuclei. If
ground generators are used, the silver iodide smoke must be dif-
fused by turbulence and lifted by the airflow into cloud regions
where temperatures are colder than — 10° C.

The ice crystals must have time to grow to a precipitable size
and to fall to Earth before reaching the downdrafts that exist on
the far side of the mountain ridge.
The meteorological conditions which are ideally suited for augment-
ing artificially the snowfall from a layer of orographic clouds are
depicted in figure 4. The figure also shows the optimum location of
ground-based silver iodide smoke generators upwind of the target area
as well as the spreading of the silver iodide plume throughout the cloud
by turbulent mixing. Although there are several seeding agents with
suitable properties for artificial ice nuclei, silver iodide and lead iodide
appear to be most effective. Owing to the poisonous effects of lead com-
pounds, lead iodide has not had wide use. The optimum silver iodide
particle concentration is a function of the temperature, moisture, and
vertical currents in the atmosphere ; it appears to be in the range from
5 to 100 nuclei per liter of cloud. 50 While the most common means of
dispersing silver iodide in mountainous areas is by ground-based gen-
erators, other methods of cloud seeding make use of aircraft, rockets,
and balloons.

In contrast to convective clouds, ice crystal formation in orographic
clouds is thought to be static, depending primarily on cloud micro-
physics, and that orographic cloud seeding has little effect on the
general patterns of wind, pressure, and temperature. On the other
hand, clouds formed primarily by convection, such as summer cumulus
or hurricane clouds, are believed to be affected dynamically by seeding
as noted above in the discussion of modification of convective clouds. 51
Since the lifting of the air in winter mountain storms is mainly caused
by its passage over the mountain barrier, the release of latent energy
accompanying this lifting has little effect upon the updraft itself. In
convective cases, however, heat released through seeding increases
buoyancy and lifting, with attendant effects on the wind and pressure
fields. The static nature of the processes involved in orographic cloud
modification therefore suggests that there is less chance that the storm
dynamics downwind of the target area will be altered appreciably as a
result of the modification activities. 52

60 Ibid., p. 68.

si See p. 68.

52 Ibid., pp. 70-71.


Figure 4. — Idealized model showing meteorological conditions that should lead
to increased snowfall if clouds are seeded with silver iodide particles. (From
Weisbecker, 1974.)

Orographic seeding experiments and seeddbility criteria

A randomized research weather modification program with winter
orographic storms in central Colorado was initiated by Colorado State
University in 1959. Data on precipitation and cloud physics were col-
lected for 16 years under this Climax program, named for the location
of its target area near Climax, Colo. Analysis of data has shown pre-
cipitation increases between 100 and 200 percent when the average
temperatures of seeded clouds at the 500 millibar level were — 20°C or
warmer. When corresponding temperatures were — 26°C to — 21°C,
precipitation changes ranged between —5 and +6 percent. For tem-
peratures colder than — 26°C, seeded cloud systems produced decreases
in precipitation ranging from 22 to 46 percent. 53

While the results of Climax have provided some useful guidelines in
establishing seedability criteria of certain cloud systems, it has been
learned from other experimental programs that direct transfer of the
Climax criteria to other areas is not warranted. 54 In particular, this
nontransferability has been evident in connection with analysis of re-
sults from the Colorado River Basin Pilot Project, conducted from
1970 through 1975 in the San Juan Mountains of southwest Colorado,
sponsored by the Bureau of .Reclamation of the U.S. Department of
the Interior. 55

Difficulties are frequently encountered in attempting to evaluate ex-
perimental cloud-seeding programs. A major problem in assessing
results of all cold orographic cloud-seeding projects stems from the
high natural variability of cloud properties. Frequent measurements
are therefore required in order to monitor these properties carefully
and consistently throughout the experiment. Another set of problems
which have troubled investigators in a number of experimental pro-
grams follow from improper design. Such a deficiency can easily re-

53 Hjermstad. Lawrence M.. "San Juan and Climax." In proceedings of Special Weather
Modification Conference; Augmentation of Winter Orographic Precipitation in the West-
ern United States, San Francisco, Nov. 11-13, 1975, Boston, American Meteorological
Society. 1975, p. 1 (abstract).

~ 4 Ibid., pp. 7-S. . …

53 This nroiect. part of Project Skywater of the Bureau of Reclamation, is discussed along
with other programs of Federal agencies in chapter 5 of this report, see p. 2o4.

34-857 O – 79 – 8


suit, for example, if insufficient physical measurements have been taken
prior to establishment of the design of the experiment. 56

Under Project Sky water the Bureau of Reclamation has carried out
an analysis of data from seven past weather modification projects in
order to identify criteria which define conditions when cloud seeding
will increase winter snowfall in mountainous terrain and when such
seeding would have no effect or decrease precipitation. The seven
projects examined in the study were conducted in the Rocky Moun-
tains, in the Sierra Nevada, and in the southern coast range in Cali-
fornia during the 1960's and 1970 ? s, in areas which represent a wide
range of meteorological and topographical conditions. 57

Figure 5 shows the locations of the seven projects whose results were
analyzed in the Skywater study, and table 5 includes more detailed
information on the locations and dates of seeding operations for these
projects. General seedability criteria derived from this study were
common to all seven projects, with the expectation that the criteria
will also be applicable to all winter orographic cloud-seeding projects.
While there have been other efforts to integrate results from several
projects into generalized criteria, based only on a few meteorological
variables, Vardiman and Moore considered 11 variables which depend
on mountain barrier shapes and sizes and on characteristics of the
clouds. Some of these variables are physically measurable while others
are derived from simple computations. 58

Figure 5. — Locations of winter orographic weather modification projects whose
results were used to determine generalized cloud seeding criteria. (From Vardi-
man and Moore, 1977.

M Hobbs. Peter V, "Evaluation of Cloud Seeding Experiments; Some Lessons To Be
i.earned From the Cascade and San Juan Projects." In proceedings of Special Weather
Modification Conference ; Augmentation of Winter Orographic Precipitation in the West-
Society 1976 . af Francisco, Nov. 11-13, 1975. Boston, American Meteorological

"Vardiman. Tarry and James A. Moore. "Generalized Criteria for Seeiing Winter Oro-
graphic Cloudy' Skywater monograph No. 1, U.S. Department of the Interior, Bureau of
133 -Division of Atmospheric Water Resources Management, Denver, July 1977.

■ Ibid., p. 15.



[From Vardiman and Moore, 1977]

Project Site Seeding operations

Bridger Range Project (BGR) Rocky Mountains, Montana 1969-70 to 1971-72 (3 seasons).

Climax Project (CMX) Rocky Mountains, Colorado 1960-61 to 1969-70 (10 seasons).

Colorado River Basin Pilot Project Rocky Mountains, Colorado 1970-71 to 1974-75 (5 seasons).


Central Sierra Research Experiment Sierra Nevada, California 1968-69 to 1972-73 (5 seasons).


Jemez Mountains Project (JMZ) Rocky Mountains, New Mexico 1968-69 to 1971-72 (4 seasons).

Pyramid Lake Pilot Project (PYR) Sierra Nevada, California/Nevada 1972-73 to 1974-75 (3 seasons).

Santa Barbara Project (SBA) Southern Coast Range, California 1967-68 to 1973-74(7 seasons).

Detailed analyses were conducted on four variables calculated from
topography and vertical distributions of temperature, moisture, and
winds. These are (1) the stability of the cloud, which is a measure of
the likelihood that seeding material will reach a level in the cloud
where it can effect the precipitation process; (2) the saturation mixing
ratio a£ cloudbase, a measure of the amount of water available for
conversion to precipitation; (3) the calculated cloud top temperature,
a measure of the number of natural ice nuclei available to start the
precipitation process; and (4) the calculated trajectory index, a meas-
ure of the time available for precipitation particles to form, grow, and
fall to the ground. 59

Results of the study thus far are summarized below :

Seeding can increase precipitation at and near the mountain crest under the
following conditions:

Stable clouds with moderate water content, cloud top temperatures between
—10 and —30° C, and winds such that the precipitation particles would be
expected to fall at or near the crest of the mountain barrier.

Moderately unstable clouds with moderate-to-high water content, cloud
top temperatures between —10 and —30° C, and a crest trajectory for the pre-

Seeding appears to decrease precipitation across the entire mountain barrier
under the following condition:

Unstable clouds with low water content, cloud top temperatures less
than —30° C, and winds such that the precipitation particles would
be carried beyond the mountain crest and evaporate before reaching the

59 Bureau of Reclamation. Division of Atmospheric Water Resources Management, "Sum-
mary Report ; Generalized Criteria for Seeding Winter Orographic Clouds.'" Denver. March
1977, p. 1. (This is a summary of the report by Vardiman and Moore which is referenced
above. )

80 Ibid., pp. 1-2.

Rime ice conditions at sensing device which measures intensity of snowfall.
(Courtesy of the Bureau of Reclamation.)


Results quoted above represent only a portion of the analyses which
are to be carried out. Seeding "window" bounds must be refined, and
the expected effect must be converted into estimates of additional pre-
cipitation a target area might experience during a winter season. It is
very unlikely that observed effects could have occurred by chance in
view of the statistical tests which were applied to the data. 61

Operational orographic seeding projects

For several decades commercial seeding of orographic clouds for
precipitation augmentation has been underway in the western United
States, sponsored by specific users which include utility companies,
agricultural groups, and State and local governments. Much of the
technology was developed in the late forties and early fifties by com-
mercial operators, with some improvements since. The basic technique
most often used involves release of silver iodide smoke, usually from
ground-based generators, along the upwind slopes of the mountain
where clouds are seeded, as shown schematically in figure 6. It is the
opinion of Grant and Kahan that this basic approach still appears
sound for seeding orographic clouds over many mountain barriers, but
that in all aspects of these operating programs, there have been "sub-
stantial improvements" as a result of research and development pro-
grams. 62 They summarized the following major deficiencies of past
operational orographic seeding programs :

1. The lack of criteria for recognizing the seedability of specific

2. The lack of specific information as to where the seeding
materials would go once they are released.

3. The lack of specific information as to downwind or broader
social and economic effects from the operations.

4. The lack of detailed information on the efficiency of seeding
generators and material being used for seeding clouds with differ-
ing temperatures. 63

Figure 6. — Schematic view of silver iodide generators placed upwind from a tar-
get area in the mountains, where orographic clouds are to be seeded for pre-
cipitation enhancement (From Weisbecker, 1974.)

61 Ibid., p. 2.

63 Grant and Kalian, "Weather Modification for Augmenting Orographic Precipitation,"
1974, p. 307.

« Ibid., pp. 307-308.


Results achieved through orographic precipitation modification

Results from several projects in the western United States have
shown that winter precipitation increases of 10 to 15 percent are pos-
sible if all suitable storms are seeded. 64 From randomized experiments
at Climax, Colo., precipitation increases of 70 to 80 percent have been
reported. These results, based on physical considerations, are repre-
sentative of cases which have a high potential for artificial
stimulation. 65

64 U.S. Department of the Interior, Bureau of Reclamation, "Reclamation Research in the
Seventies," Second progress report. A water resources technical publication research report
No. 28, Washington, U.S. Government Printing Office, 1977, p. 2.

65 National Academy of Sciences, "Climate and Food ; Climatic Fluctuation and U.S. Agri-
cultural Production," 1976, p. 136.




The hail problem

Along with floods, drought, and high winds, hail is one of the major
hazards to agriculture. Table 6 shows the estimated average annual
hail loss for various crops in the United States, for each of the 18
States whose total annual crop losses exceed $10 million. Also included
in the table are total losses for each crop and for each of the 18 States
and the aggregate of the remaining States.

The following vivid description of a hailstorm conveys both a sense
of its destructiveness and some notion of its capricious nature :

At the moment of its happening, a hailstorm can seem a most disastrous event.
Crashing stones, often deluged in rain and hurled to the surface by wind, can
create instant destruction. Picture windows may he broken, cars dented, or a
whole field of corn shredded before our eyes.

Then quite quickly, the storm is over. Xow the damage is before us. we per-
ceive it to be great, and we vow to do something to prevent its happening again.

But what we have experienced is "our" storm. Hail did not happen perhaps a
mile away. We may see another the same day. or never again. Thus, the concept
of hail suppression is founded in a real or perceived need, but the assessment of
this solution must be considered in terms of the nature of hail. 06


[In millions of dollars] 1



and veg-







grains 2






































North Dakota.







North Carolina
















South Dakota










































South Carolina











. 1


















Other States


















1 1973 production and price levels.

2 Coarse grains: Barley, rye, oats, sorghum.

Source: "National Hail Research Experiment" from Boone (1974).

A major characteristic of hail is its enormous variability in time,
space, and size. Some measure of this great variability is seen in figure
7, which shows the average annual number of days with hail at points
within the continental United States. The contours enclose points with
equal frequency of hail days. 67

00 Chanson, Stanley A.. Jr.. Ray Jay Davis, Barbara C. Farhar. J. Eupene Haas, J.
Lorena Ivens. Marvin V. Jones, Donald A. Klein, Dean Mann. Griffith M. Morgan. Jr.. Steven
T. Sonka. Earl R. Swanson. C. Robert Taylor, and Jon Van Blokland. "Hail Suppression :
Impacts and Issues." Final report — "-Technology Assessment of the Suppression of Hail
fTASH ) ." Urbana, 111.. Illinois State Water Survey. April lt>77 (sponsored by the National
Science Foundation, Research Applied to National Needs Program), p. 9.

« Ibid.


Hail forms in the more active convective clouds, with large vertical
motions, where large quantities of water vapor condense under condi-
tions in which large ice particles can grow quickly. The kinds of con-
vective clouds from which hail can be formed include (1) supercells
(large, quasi-steady-state, convective storms, (2) multicell storms
(active convective storms with multiple cells), (3) organized convec-
tive storms of squall lines or fronts, and (4) unstable, highly convective
small cumuli (primarily occurring in spring). 68 While hail generally
occurs only in thunderstorms, yet only a small proportion of the world's
thunderstorms produce an appreciable amount of hail. Based upon sev-
eral related theories, the following desciption of the formation of hail
is typical :

Ice crystals or snowflakes, or clumps of snowflakes, which form above the
zone of freezing during a thunderstorm, fall through a stratum of supercooled
water droplets (that is, water droplets well below 0° O). The contact of the ice
or snow particles with the supercooled water droplets causes a film of ice to form
on the snow or ice pellet. The pellet may continue to fall a considerable distance
before it is carried up again by a strong vertical current into the stratum of
supercooled water droplets where another film of water covers it. This process
may be repeated many times until the pellet can no longer be supported by the
convective updraft and falls to the ground as hail. 69

( Note: The lines enclose points (stations) that have equal frequency of hail days )

Figure 7. — Average annual number of days with hail at a point, for the contiguous
United States. (From Changnon, et al., TASH, 1977.)

68 National Academy of Sciences, "Climate and Food ; Climatic Fluctuation and U.S.
Agricultural Production." 1976. p. 141.

89 Koeppe. Clarence E. and George C. de Long, "Weather and Climate," New York, Mc-
Graw-Hill, 1958, pp. 79-80.


Modification of hail

According to D. Ray Booker, "Hail modification seeding has been
done operationally for decades in the high plains of the United States
and in other hail prone areas of the world. Thus, there appears to be a
significant market for a hail-reduction technology." 70 In the United
States most attempts at hail suppression are conducted by commercial
seeders who are under contract to State and county governments and to
community associations. There are also extensive hail suppression op-
erations underway in foreign countries. Although some successes are
reported, many important questions are still unanswered with regard
to mitigation of hail effects, owing largely to lack of a satisfactory
scheme for evaluation of results from these projects.

In theory, it should be possible to inhibit the formation of large
ice particles which constitute hailstones by seeding in order to increase
the number of freezing nuclei so that only smaller ice particles will
develop. This would then leave the cloud with insufficient precipita-
tion water to allow the accretion of supercooled droplets and the
formation of hail of damaging size. This simplistic rationale, how-
ever, does not provide insight into the many complications with
which artificial nail suppression is fraught ; nor does it explain the
seemingly capricious responses of hailstorms to seeding and the incon-
sistent results which characterize such modification attempts. As with
all convective systems, the processes involved are very complex. They
are controlled by the speed of movement of the air parcels and precipi-
tation particles, leading to complicated particle growth, evaporation,
and settling processes. 71 As a result, according to Changnon, the con-
clusions from various hail suppression programs are less certain than
from those for attempts to enhance rain from convective clouds, and
they are best labeled "contradictory." 72

Changnon identifies two basic approaches that have been taken
toward hail modification :

»Most common has been the intensive, high rates of seeding of the potential
storm with silver iodide in an attempt to transform nearly all of the super-
cooled water into ice crystals, or to "glaciate" the upper portion of the clouds.
However, if only part of the supercooled water is transformed into ice, the
storm could actually be worsened since growth by accretion is especially rapid
in an environment composed of a mixture of supercooled drops and ice crystals.
Importantly, to be successful, this frequently used approach requires massive
seeding well in advance of the first hailstone formation.

The second major approach has been used in the Soviet Union and * * * in the
National Hail Research Experiment in Colorado. It involves massive seeding
with silver iodide, but only in the zone of maximum liquid water content of the
cloud. The hope is to create many hailstone embryos so that there will be in-
sufficient supercooled water available to enable growth to damaging stone sizes."

70 Booker, D. Ray, "A Marketing Approach to Weather Modification," background paper
prepared for the U.S. Department of Commerce Weather Modification Advisory Board.
Feb. 20, 1977. p. 4.

i National Academy of Sciences, "Climate and Food; Climatic Fluctuation and U.S.
Agricultural Production." 1070. p. 143.

72 Changnon, "Present and Future of Weather Modification ; Regional Issues," 1975,
p. 102.

™ Ibid.


Precipitation instrument site, including, from left to right, hailcube, anemom-
eter, rain/hail separator, and Belfort weighing precipitation gage. (Courtesy of
the National Science Foundation. )

Hail seeding technologies

The most significant field programs in hail suppression during recent
years have included those conducted in the Soviet Union, in Alberta,
in South Africa, and in northeastern Colorado (the National Hail
Research Experiment). In the course of each of these projects, some
of which are still underway, various procedural changes have been
initiated. In all of them, except that in South Africa, the suppression
techniques are based on increasing the number of hail embryos by


seeding the cloud with ice nuclei. Usually, the seeding material is
silver iodide, but the Russians also use lead iodide, and on occasion
other agents such as sodium chloride and copper sulfate have been
used. The essential problems in seeding for hail suppression are re-
lated to how, when, and where to get the seeding agent into potential
hail clouds and how to identify such clouds. 74

Soviet suppression techniques are based on their hypothesis that
rapid hail growth occurs in the "accumulation zone," just above the
level of maximum updraft, where liquid water content can be as
great as 40 grams per cubic meter. To get significant hail, the maximum
updraft should exceed 10 to 15 meters per second, and the temperature
in this zone must be between and —25° C. Upper large droplets
freeze and grow, combining with lower large droplets, and an increase
in particle size from 0.1 cm to 2 or 3 cm can occur in only 4 to 5 minutes.
In the several Russian projects, the seeding agent is introduced at
selected cloud heights from rockets or antiaircraft shells ; the number
of volleys required and the position of injection being determined by
radar echo characteristics and past experience in a given operational
region. 75

In other hail suppression projects, seeding is most frequently carried
out with aircraft, from which flares containing the seeding agent are
released by ejection or dropping. Each flare may contain up to 100
grams of silver iodide ; and the number used as well as the spacing and
height of ignition are determined from cloud characteristics as well as
past experience in a given experiment or operation. In each case it
is intended to inject the seeding material into the supercooled portion
of the cloud.

Evaluation of hail suppression technology

It appears that mitigation of the effects of hail has some promise,
based on the collection of total evidence from experiments and opera-
tions around the world. In the Soviet Union, scientists have been
reporting spectacular success (claims of 60 to 80 percent reduction) 76
in hail suppression for nearly 15 years; however, their claims are not
universally accepted, since there has not been careful evaluation under
controlled conditions. Hail-seeding experiments have had mixed results
in other parts of the world, although a number of commercial seeders
have claimed success in hail damage reduction, but not with convincing
evidence. 77

Successful hail suppression reports have come from a number of
operational programs in the United States as well as from weather
modification activities in the Soviet Union and in South Africa. Often
the validity of these results is questionable in view of deficiencies in
project design and data analysis; nevertheless, the cumulative evidence
suggests that hail suppression is feasible under certain conditions.
There are also reports of negative results, for example, in foreign pro-
grams and in the National Hail Research Experiment in the United

7 *Chan*rnon. Stanlev A.. Jr.. and Griffith M. Moroni. Jr.. "Desipn of an Experiment To
Suppress Hail In Illinois." Illinois State Water Survey. TSWS/R 01 /7fi. RnHetln 01. State ot
Illinois. Department of Registration and Education, Urbana, 1970. pp. 82-S3.

75 Ibid., p. S3.

70 Chancrnon. "Present and Future of Weather Modification," 107". p 102.

77 Rattan. Louis J. statement submitted to Subcommittee on Environment and Atmos-
phere Committee on Science and Technology, U.S. House of Representatives, at hearings.
June 18, 1970, pp. 7-8.


States, which indicate that under some conditions seeding induces
increased hail. 78

Atlas notes that this apparent dichotomy has until recently been
attributed to different approaches to the techniques and rates of seed-
ing. However, lie observes that both positive and negative results
have been obtained using a variety of seeding methods, including
ground- and cloud-based generators, flares dropped from above the
cloud top, and injection by rockets and artillery. 79 In discussing the
reasons for increased hail upon seeding, Atlas states :

There are at least four physical mechanisms by which seeding may produce
increased hail. Two of these occur in situations in which the rate of supply of
supercooled water exceeds that which can be effectively depleted by the com-
bination of natural and artificially produced hail embryos. This may occur in
supercell storms and in any cold-base storm in which the embryos are graupel
rather than frozen raindrops. Moreover, present seeding methods are much more
effective in warm-base situations in which the hail embryos are frozen raindrops.
Increased hail is also probable when partial glaciation of a cloud is produced
and the hail can grow more effectively upon the ice-water mixture than upon
the supercooled water alone. Similarly, increases in the amount of hail may
occur whenever the additional latent heat resulting from nucleation alters the
undraft profile in such a manner as to increase its maximum velocity or to
shift the peak velocity into the temperature range from —20° to —30° C, where
the accreted water can be more readily frozen. A probable associated effect is
the redistribution of precipitation loading by the combination of an alternation
in the updraft velocity and the particle sizes such that the hail embroyos may
grow for longer durations in a more favorable growth environment. 80

Surreys of hail suppression effectiveness

Recently, Changnon collected information on the effectiveness of
hail suppression technology from three different kinds of sources. One
set of data was based on the results of the evaluations of six hail sup-
pression projects; another was the collection of the findings of three
published assessments of hail modification ; and the third was obtained
from two opinion surveys conducted among weather modification
scientists. 81 The principal statistics on the estimated capabilities for
hail suppression from each of these groups of sources are summarized
in table 7. Where available, the estimated change in rainfall accom-
panying the hail modification estimates are also included. Such rain-
fall changes might have been sought intentionally as part of a hail sup-
pression activity or might result simply as a byproduct of the major
thrust in reducing hail. In the table, a plus sign* indicates an estimated
percentage increase in hail and/or rainfall while a minus sign signifies
a percentage decrease.

The six evaluations in part A of table 7 are from both experimental
and operational projects, each of which was conducted for at least 3
years in a single locale and in each of which aircraft seeding tech-
niques were used. Thus, the results of a number of earlier experiments,
using ground-based seeding generators, were not considered in the
estimations. Furthermore, change in hail due to suppression activities
was defined on the basis of crop-loss statistics rather than on the basis
of frequency of hail days, since Changnon does not consider the latter,

7S Atlas. David, "The Paradox of Hail Suppression," Science, vol. 195, No. 4274, Jan. 14.
1977. p. 195.
79 Ibid.

60 Ibid., pp 195-196.

81 Chanjrnon. Stanlev A.. Jr.. "On tbe Status of Hail Suppression." Bulletin of the Amer-
ican Meteorological Society, vol. 58, No. 1, Jan. 1977, pp. 20-28.


along with other criteria such as number and size of hailstones, hail
mass, and radar echo characteristics, to be a reliable indicator. 82 Note
that five of the six projects listed indicate a hail suppression capability
ranging from 20 percent to 48 percent. Changnon notes, however, that
most of these results are not statistically significant at the 5 percent
level, but that most scientists would classify the results as "opti-
mistic." 83

Table 7— Status of Hail Suppression and Related Rainfall Modification
(Based on information from Changnon. On the Status of Hail Suppression.


1. Texas: Hail modification was —48 percent (crop-loss cost value) ; no change
in rainfall.

2. Southwestern North Dakota : Hail modification was —32 percent (crop-hail
insurance rates) ; no rain change information available.

3. North Dakota pilot project : Hail modification was —30 percent (a composite
of hail characteristics, radar, and crop-loss data) ; change in rainfall was +23

4. South Africa : Hail modification was —40 percent (crop-loss severity ;
change in rainfall was —4 percent.

5. South Dakota "Statewide" project : Hail modification was —20 percent
(crop loss) ; increase in rainfall was +? percent.

6. National hail research experiment in Colorado :

Increase in hail mass was +4 percent to +23 percent, with median of
+23 percent :
Increase in rainfall was +25 percent.


1. American Meteorological Society : Positive but unsubstantiated claims and
growing optimism.

2. National Academy of Sciences: 30 to 50 percent reductions in U.S.S.R. and
15 percent decreases in France — neither result proven by experimentation.

3. Colorado State University Workshop :

—30 percent modification nationwide ;

—30 percent modification in the High Plains, with ± 10-percent change in
rain ; unknown results in the Midwest ; also unknown rainfall effects.


1. Farhar-Grant questionnaire (214 answers) : —25 percent crop-hail damage
nationwide, although majority — 59 percent — admit they do not know.

2. Illinois State Water Survey questionnaire (63 answers) :

—30 percent hail loss, with +15 percent rain increasein the Great Plains:
—20 percent hail loss, with +10 percent rain increase in the Midwest.

The results, shown in part B of table 7, from the recent published
assessments of capability in hail suppression reveal a position of
"guarded optimism;" however, there is no indication of definitive
proof of hail suppression contained in those assessments. 84 These pub-
lished assessments are comprised of a statement, on the status of
weather modification by the American Meteorological Society, 85 the
conclusions of a study on the progress of weather modification by the

82 Ibid., p. 22.
*»Th1rt.. p. 26.
"* Ibid.

" American Meteorological Society. "Policy Statement of tbo American Meteorological
Rocietv on Purposeful and Inadvertent Modifier Hon of Woatbcr nnd Climate," Bulletin of
tbo American Meteorological Society, vol. , r )4. No. 7, July 1073. pp. 694-695.


National Academy of Sciences, 86 and a report on a workshop at Colo-
rado State University on weather modification and 'agriculture. 87

The third view (part C, table 7) resulting from two opinion surveys,
indicates wide-ranging but basically "bipolar" attitudes among the
scientists surveyed. The majority of the experts queried felt that a hail
suppression capability could not be identified; however, a sizable
minority were of the opinion that a moderate capability for modifying
hail (greater than 20-percent decrease) does now exist. Changnon says
that the results of these opinion surveys show at best that the con-
sensus must be considered to be a pessimistic view of a hail suppres-
sion capability. 88

In his conclusions on the status of hail suppression technology,
Changnon states :

These three views of the current status of hail suppression, labeled as (1) opti-
mistic, (2) slightly optimistic, and (3) pessimistic, reflect a wide range of opin-
ion and results. Clearly, the present status of hail suppression is in a state of
uncertainty. Reviews of the existing results from 6 recent operational and ex-
perimental hail suppression projects are sufficiently suggestive of a hail sup-
pression capability in the range of 20 to 50 percent to suggest the need for an
extensive investigation by an august body of the hail suppression capability
exhibited in these and other programs.

One of the necessary steps in the wise experimentation and future use of hail
suppression in the United States is to cast the current status in a proper light.
This can only be accomplished by a vigorous in-depth study and evaluation of
the results of the recent projects. 88

Conclusions from the TASH study

Sponsored by the Eesearch Applied to National Needs program of
the National Science Foundation, a major technology assessment of
hail suppression in the United States was conducted from 1975 through
1977, by an interdisciplinary research team. 90 This Technology Assess-
ment of the Suppression of Hail (TASH) study was intended to bring
together all of the considerations involved in the application of hail
suppression, in the present and in the future, to ascertain the net value
of such technology to society. The goals of the study were :

To describe the current knowledge of hail suppression.
To identify long-range expectations for such a technology.
To estimate the societal impacts that might be generated by its wide use.
To examine public policy actions that would most equitably direct its beneficial

From its interdisciplinary study of hail suppression and its impacts
the TASH team reached the following broad conclusions on the effects
of hail and on the potential technology for suppression of hail :

The United States experiences about $850 million in direct crop and property
hail losses each year, not including secondary losses from hail. The key character-
istic of hail is its enormous variability in size, time, and space.

Among the alternative ways of dealing with the hail problem, including crop
insurance, hail suppression, given a high level of development, appears to be the
most promising future approach in high hail loss areas. Economic benefits from
effective hail suppression vary by region of the country, with the most benefit to

66 National Academy of Sciences. National Research Council. Committee on Atmospheric
Sciences. "Weather and Climate Modification : Problems and Progress," Washington, D.C.,
1973. pp. 100-106.

87 Grant and Reid, "Workshop for an Assessment of the Present and Potential Role of
Weather Modification in Agriculture Production." 1975. pp. 33-45.

88 Changnon. "On the Status of Hail Suppression," 1977, p. 26.
68 Ibid., pp. 26-27.

90 Changnon. et al.. "Hail Suppression ; Impacts and Issues." Technology Assessment of
the Suppression of Hail (TASH) , 1977, 432 pp.


be derived in the Great Plains area. Any alterations in rainfall resulting from
hail suppression would importantly affect its economic consequences.

The effects of cloud seeding on rainfall are more significant than its effects on
hail from economic and societal standpoints.

At the present time there is no established hail suppression technology. It may
be possible to reduce damaging hail about 25 percent over the growing season in a
properly conducted project.

Reducing the scientific uncertainties about hail suppression will require a sub-
stantial commitment by the Federal Government for long-term funding of a sys-
tematic, well-designed program of research. For the next decade or so, monitoring
and evaluation of operational programs will be important.

Benefit-cost analysis revealed that investment in development of the high-level
technology would result in a ratio of 14 :1, with the present value of benefits esti-
mated to total $2.8 billion for 20 years. The low-level technology showed a nega-
tive benefit-cost ratio. Research and development to provide the high-level
technology is the best choice from an economic standpoint; a minimal level of
support would be nonbeneficial. In a word, if we are going to develop hail suppres-
sion technology, we would need to do it right.

Effective hail suppression will, because of the hail hazard, technological
approach, patterns of adoption, and institutional arrangements, lead to regionally
coherent programs that embrace groups of States, largely in the Great Plains.

Some would gain and others would lose from widespread application of an
effective hail suppression technology. Farmers within adopting regions would
receive immediate benefits from increased production. After several years this
economic advantage would be diminished somewhat, but increased stability of
income would remain. Farmers growing the same crops outside the adopting areas
would have no advantages and would be economically disadvantaged by commod-
ity prices lower than they would have been with no hail suppression. The price
depressing effects result from increased production in adopting areas. Consumers
would benefit from slightly decreased food prices. The impacts generated by a
highly effective technology include both positive and negative outcomes for vari-
ous other stake-holder groups in the Nation. For the Nation as a whole, the
impacts would be minor and beneficial. On balance, the positive impacts outweigh
the negative impacts if a high-level technology can be developed.

An adequate means of providing equitable compensation on an economically
sound basis for persons suffering from losses due to cloud seeding has not been
developed. Some better procedure for compensating losers will be necessary. In
addition, present decision mechanisms and institutional arrangements are inade-
quate to implement the technology in a socially acceptable manner. Some mecha-
nism for including potential opponents in the decisionmaking process will be

It is unlikely that widespread operational hail suppression programs would
have serious adverse environmental impacts, although lack of sufficient knowledge
indicates that adverse impacts should not be ruled out. Long-term environmental
effects are not known at the present time. 91


Fog poses a hazard to man's transportation activities, particularly
to aviation, where as a result of delays air carriers lose over $80 million
annually. Highway accidents attributed to fog are estimated to cost
over $300 million per year. 92 Most often the impetus to develop effec-
tive fog and stratus cloud dispersal capabilities has come from the
needs of commercial and military aircraft operations.

There are two basic kinds of fog, and the suppression of each re-
quires a different approach. Supercooled fog and stratus clouds are
comprised of liquid water droplets whose temperature is below f reez-

81 Farhar. Barbara C, Stanley A. Changnon, Jr., Earl R. Swanson, Ray J. Davis, and
J Eugene Haas. "Hail Suppression and Societv. Summary of Technology Assessment of Hail
Suppression," Urbana. 111.. "Illinois State Water Survey, June 1977." pp. 21-22. (This
document is an executive summary of the technology assessment by Changnon, et al., "Hail
Suppression ; Impacts and Issues.")

92 National Oceanic and Atmospheric Administration, "Summary Report : Weather Modi-
fication ; Fiscal Years 1969, 1970, 1971," Rockville, Md., May 1973, p. 72.


ing (i.e., 0° C or below). Supercooled fogs account for only about 5
percent of all fog occurrences in the United States, although they are
prevalent in certain parts of northeastern and northwestern North
America. The remainder of North American fogs are warm fogs (water
droplets warmer than 0° C). 93 Although cold fog has been amenable
to modification, so that there essentially exists an operational tech-
nology for its dissipation, practical modification of warm fogs, on an
economical basis, has not yet been achieved.

Cold fog modification

Dispersal of cold fog by airborne or ground-based techniques has
been generally successful and has become an operational weather modi-
fication technology. In the United States cold fog dispersal operations
have been conducted, for example, by commercial airlines, usually with
dry ice as the seeding agent. The U.S. Air Force has also operated
ground-based liquid propane systems, at domestic and foreign bases,
which have been effective in dissipating cold fog over runways, thus
reducing flight delays and diversions. 94 Conducted largely at airports,
cold fog suppression is usually accomplished using aircraft, which drop
various freezing agents, such as dry ice or silver iodide as they fly over
the fog-covered runways. The agents initiate ice crystal formation and
lead to precipitation of the growing crystals. 95 Ground-based systems
for cold fog dispersal have also been used and have some advantages
over airborne systems. Such a system can operate continuously for ex-
tended time periods more economically and more reliably.

Warm fog modification

The remainder of North American fogs are "warm fogs" for which
a suitable dispersal capability remains to be developed. Crutchfield
summarizes the status of warm fog dispersal technology and its eco-
nomic potential :

The much more extensive warm fogs which cause delays, accidents, and costly
interruptions to every type of transportation have proved intractable to weather
modification thus far. Some success has been achieved on occasion by heavy
seeding with salt and other materials, but results have not been uniformly good,
and the materials used have presented environmental problems in the areas
treated. Heating airport runways has been of some benefit in dealing with warm
fog, but at present is not generally effective in cost-benefit terms and can inter-
rupt air traffic.

Nevertheless, the research and technology problems involved in the dispersal
of warm fog appear to be of manageable proportions, and the benefits from an
environmentally acceptable and predictable technique for dealing with warm
fog would be of very real interest in terms of economic gain. 96

A number of field techniques have been attempted, with some meas-
ure of success, for artificial modification of warm fogs. Seeding is
one technique, where the seeding agents are usually hygroscopic parti-
cles, solution drops, or both. There are two possible desired effects of
seeding warm fogs, one being the evaporation of fog droplets, resulting
in visibility improvement. A second desired effect of seeding, results
from the "coalescence" process, in which the solution droplets, falling

93 Changnon, "Present and Future of Weather Modification," 1975, p. 165.

94 National Oceanic and Atmospheric Administration "Summary Report : Weather Modi-
fication ; Fiscal Year 1973." Rockville, Md., December 1974, pp. 39-40.

9a Changnon. "Present and Future of Weather Modification," 1975. p. 165.

98 Crutchfield, James A., "Weather Modification : The Economic Potential." Paper prepared
for U.S. Department of Commerce Weather Modification Advisory Board. University of
Washington, Seattle, May 1977, pp. 5-6.

34-857 O – 79 – 9


through the fog layer, collect the smaller fog droplets, increasing
visibility as the fog particles are removed in the fallout. 97 There is a
wide diversity of hygroscopic particles which can and have been used
for warm fog dissipation. Sodium chloride and urea are the most
common, but others have included polyelectrolyte chemicals, an ex-
ceedingly hygroscopic solution of ammonium-nitrate urea, and some
biodegradable chemicals. Seeding particle size is critical to the effec-
tiveness of a warm fog dispersal attempt ; it has been found that poly-
dispersed particles (i.e., material with a distribution of particle sizes)
are more effective in inducing fog modification than are extra fine
particles of uniform size, which were only thought to be optimum in
earlier experiments. Other problems which are the subject of con-
tinuing study relate to the seeding procedures, including the number
of flights, number of aircraft to be used, and flight patterns in
accordance with the local terrain and wind conditions. One of the
most difficult operational problems in the seeding of warm fog is that
of targeting. One solution to this problem, suggested by the Air Force,
is the implementation of wide-area seeding instead of single-line
seeding, which is so easily influenced by turbulence and wind shear. 98
Another technique for dissipation of warm fog makes use of heating.
The physical principle involved is the vaporization of the water drop-
lets through introduction of sufficient heat to vaporize the water and
also warm the air to such a temperature that it will hold the additional
moisture and prevent condensation. Knowing the amount of liquid
water in the atmosphere from physical measurements, the necessary
amount of heat energy to be injected can be determined. 99 The fea-
sibility of this approach was first demonstrated in England during
World War II, when it was necessary to fly aircraft in all kinds of
weather in spite of frequent fogbound conditions in the British Isles.
The acronym FIDO, standing for Fog Investigations Dispersal Of,
was applied to a simple system whereby fuel oil in containers placed
along the runways was ignited at times when it was necessary to land
a plane in the fog. Although burning as much as 6,000 gallons of oil
for a single airplane landing was expensive and inefficient, it was
justified as a necessary weather modification technique during war-
time. 99 *

Initial and subsequent attempts to disperse fog by burning liquid
fuel were found to be hazardous, uneconomical, and sometimes in-
effective, and, as a result, not much was done with this heating tech-
nique until the French revised it, developing the Turboclair method
for dissipating fog by heating with underground jet blowers. After 10
years of development and engineering testing, the system was tested
successfully by the Paris Airport Authority at Orly Airport. This
program has given a new interest and stimulated further research and
development of this technique both in the United States and elsewhere.
In the United States, the Air Force conducted Project Warm Fog
to test the effectiveness of heating to remove warm fog. It is clear that
this method is promising; however, further studies are needed. 1

97 Mosohnndreas. Demetrlos J., "Present Capabilities to Modify Warm Fog and Stratus,"
Geomet. Inc.. report No EF-300. Technical report for Office of Naval Research and Naval
Air Svstems Command, Rockvllle, Md., Jan, 18, 1974, p. 13.

88 Ibid., pp. 16-17.

" Ibid pp. 24. 30.

Halacy, Daniel S., Jr., "The Weather Changers," New York, Harper and Row. 1968,
pp. 105-107.

1 Moschandreas. "Present Capabilities to Modify Warm Fog and Stratus," 1974, pp.


Research and development on warm fog dispersal systems has con-
tinued under sponsorship of the U.S. Air Force, using both passive
heat systems, and thermokinetic systems which combine both heat and
mechanical thrust. A thermokinetic system, known as the Warm Fog
Dispersal System (WFDS), consists of three components: The com-
bustors, the controls, and the fuel storage and distribution hardware.
Testing of the WFDS by the Air Force is to be conducted during late
1978 and 1979 at Otis Air Force Base in Massachusetts, after which it
is to be installed and operational at an Air Force base by 1982. 2 Dis-
cussion of the Air Force development program and of the concurrent
studies and interest on the Federal Aviation Administration in this
thermokinetic fog dispersal system is found in chapter 5 of this report. 3

There have been attempts to evaporate warm fogs through mechani-
cal mixing of the fog layer with warmer, drier air from above. Such
attempts have been underway using the strong downwash from heli-
copters ; however, such a technique is very costly and would likely be
employed only at military installations where a number of helicopters
might be available.

The helicopters hover or move slowly in the dry air above the fog
layer. Clear dry air is moved downward into the fog by the circulation
of the helicopter rotors. The mixture of dry and cloudy air permits the
fog to evaporate, and in the fog layer there is created an opening whose
size and lifetime are determined by the meteorological conditions in
the area, by the flight pattern, and by the kind of helicopter.

Conclusions reached by scientists involved in a series of joint U.S.
Air Force- Army research projects using helicopters for fog dispersal
follow :

The downwash method by a single helicopter can clear zones
large enough for helicopter landing if the depth of the fog is less
than 300 feet (100 meters) .

Single or multiple helicopters with flight patterns properly
orchestrated can maintain continuous clearings appropriate for
aircraft takeoff and landing in fogs of less than 300 feet (100
meters) deep. 4

In addition to the more commonly applied experimental techniques,
such as seeding, heating, and mechanical mixing, other attempts have
been made to disperse warm fogs. These have included the injection of
ions or charged drops into the fog and the use of a laser beam to clear
the fog. Further research is needed before definitive results can be
cited using these methods. 5

Table 8 is a summary of research projects on warm fog dispersal
which had been conducted by various organizations in the United
States between 1967 and 1973. Note that, in addition to field experi-
ments, research included modeling, field measurements and observa-
tions of fog, chamber tests, statistical interpretation, model evaluation,
and operational assessment.

On the basis of his study of research projects through 1973 and
claims projected by the scientists involved in the various warm fog

8 Kunkel. Bruce A., "The Design of a Warm Fog Dispersal System." In preprints of the
Sixth Conference on Planned and Inadvertent Weather Modification. Champaign, 111..
Oct 10-13. 1977. Boston, American Meteorological Society, 1977, pp. 174-176.

3 See pp. 305 and 308.

4 Moschandreas, "Present Capabilities To Modify Warm Fog and Stratus," 1974, p. 45.
6 Ibid., p. 14.


modification programs, Demetrios Moschandreas formulated the fol-
lowing conclusions on warm fog dispersal :

Seeding with hygroscopic particles has been successful; how-
ever, targeting problems would require the wide-area approach to
seeding. Urea has also been projected as the agent which is most
effective and least harmful to the environment.

The heating technique is very promising and very efficient;
studies for further verification of its capabilities are in order.

The helicopter technique by itself has not been as promising as
the combination of its use with hygroscopic seeding.

Studies on the other less often used techniques have not reached
the stage of wide field application.

Numerical modeling has provided guidelines to the field experi-
ments and insights to the theoretical studies of fog conditions.

The laboratory experiments have given the scientists the con-
trolled conditions necessary to validate a number of theories. The
unique contribution of chamber tests to a better understanding of
the dynamics of fog formation has been widely recognized. 6


THROUGH 1973 «

[From Moschandreas, 1974]

Area of effort

Year of publication

1967 2







Modeling and numerical ex-





















Field measurements; fog ob-















Chamber tests






Field experiments












Statistical interpretation


Assessment of operational







i Research is listed by agency conducting the research, or sponsoring it, when reporting its contractor's efforts; or by
contractor's name when contractor's report is principal reference; individual researchers are not listed because these
change, even though the cont ; mjity of effort is maintained.

s Work reported prior to 1967 is not included here.

Key: CAL— Cornell Aeronautical Laboratory, Inc.; AFCRL— Air Force Cambridge Research Laboratories; GEOMET—
GEOMET, Inc.; MRI— Meteorology Research, Inc.; NWRF— U.S. Navy Weather Research Facility; EPRF— U.S. Navy En-
vironmental Research Facility; EG&G— EG&G Environmental Services Ooeration; FAA— Federal Aviation Administra-
tion: NCAR— National Center for Atomospheric Research; NWC— Naval Weapons Center; USNPGS— U.S. Naval Postgrad-
uate School.


At any given time over the whole Earth there are about 2,000 thun-
derstorms in progress, and within these storms about 1,000 cloud-to-
ground discharges are produced each second. 7 Lightning is essentially
a long electric spark, believed to be part of the process by which an
electric current is conducted from the Earth to the ipnosphere, though

– 1H1U., pp. W^— »0. I, XT

7 National Science Board. "Patterns and Perspectives In Environmental Science, Na-
tional Science Foundation, Washington, D.C.. 1972, p. 157.


the origin of the lightning discharge is still not fully understood. In
fair weather the atmosphere conducts a current from the positively
charged ionosphere to the ground, which has a negative charge.

The details of the charge-generating process within a thunderstorm
are not well understood, though theories have been proposed by cloud
physicists. Probably a number of mechanisms operate together to bring
about cloud electrification, though, essentially, the friction of the air
on the water droplets and ice crystals in the storm strips off electrons
which accumulate near the base of cumulonimbus clouds, while posi-
tive charge collects in the upper part. The negative charge near the
cloud base induces a local positive charge on the Earth's surface be-
neath, reversing the normal fair weather situation. When the electri-
cal potential between the cloud and ground becomes sufficiently large,
an electrical discharge occurs, in which electrons flow from the cloud
to the ground. In addition, there are discharges between clouds and
between oppositely charged portions of the same cloud.

In the rapid sequence of events which comprise a lightning stroke,
the initial, almost invisible, flow of electrons downward from cloud
to Earth, called the leader, is met by an upward-moving current of
positive charges, establishing a conducting path of charged particles.
A return stroke, much larger, then rushes from the ground to the
cloud. All of these events appear as a single flash since they occur in
about fifty microseconds; however, while most people perceive the
lightning stroke as travelling from cloud to ground, it is actually the
return stroke which provides the greatest flash. 8

In the United States, lightning kills about 200 people annually, a
larger toll than that caused by hurricanes. Since 1940, about 7,000
Americans have lost their lives from lightning and related fires. 9 These
casualties occur most often singly or occasionally two at a time, so that
they are not nearly so newsworthy as are the multiple deaths and
dramatic property damage associated with hurricanes, tornadoes, and
floods. On the other hand, a lightning problem affecting large areas
is the ignition of forest fires, some 10,000 of which are reported each
year in the United States, where the problem is most acute in the
Western States and Alaska. 10 Such fires inflict damage on commercial
timber, watersheds, scenic beauty, and other resources, causing an
estimated annual damage cost of $100 million. 11 Other examples in
which lightning can be especially dangerous and damaging include
discharges to aircraft and spacecraft and effects on such activities as
fuel transfer operations and the handling of explosives.

Because of the relative isolation of personal accidents due to light-
ning, the only feasible controls over loss of life are through implemen-
tation of safety measures which prevent exposure or by protection
of relatively small areas and structures with lightning arresters. For-
ested areas, however, require large area protection from lightning-
caused fires in order to promote sound forest management. It is hoped

8 Anthes. Richard A., Hans A. Panofsky, John C. CaMr, and Albert Rango, "The Atmos T
phere," Columbus. Ohio. Charles E. Merrill. 1975, p. 174.

9 U.S. Department of Commerce, "Peak Period for Lierhtniner Nears ; NOAA Lists Safety
Rules." News Release NOAA 77-156. Washington. DC. June 19. 1977, p. 1.

10 Fuquay. Donald M., "Lightning Damage and Lightning Modification Caused by Cloud
Seeding." In Wilmot N. Hess (ed.), "Weather and Climate Modification," New York, John
Wiley & Sons, 1974, p. 605.

"Ibid., p. 604.


that the widespread damage to forest resources resulting from the
lightning-fire problem can be alleviated through use of weather modi-
fication techniques.

Lightning modification

General approaches to lightning suppression through weather mod-
ification, which have been contemplated or have been attempted, in-
clude :

Dissipation of the cloud system within which the thunderstorm
originates or reduction of the convection within the clouds so that
vigorous updrafts and downdrafts are suppressed.

Reduction of the number of cloud-to-ground discharges, es-
pecially during critical fire periods.

Alteration of the characteristics of discharges which favor
forest fuel ignition.

Use of other weather modification techniques to produce rains
to extinguish fires or to decrease the probability of ignition
through increase of ambient relative humidity and fuel moisture.
Lightning is associated with convective clouds; hence, the most
direct suppression method would involve elimination of the clouds
themselves or of the convection within them. Removal of the clouds
would require changes to gross properties such as temperature insta-
bility and moisture content of the air ; thus, such modification is not
technically, energetically, or economically feasible. However, it might
be possible to reduce somewhat the convection within the clouds. 12

The formation of convective clouds depends on the upward motion
of moist air caused by thermal instability and the subsequent produc-
tion of water through cooling. This condensation releases more heat,
which, in turn, causes further buoyancy and rising of the cloud. At
these heights the temperature is low enough that the water can freeze,
releasing more latent heat and enabling the cloud particles to rise
even higher. As a result of the presence of nuclei which are naturally
present in the cloud, glaciation proceeds continuously. Through arti-
ficial nucleation, by seeding, natural glaciation may be reinforced and
development of the cloud assisted. Rapid, premature seeding, how-
ever, would still promote buoyancy but could also introduce so much
turbulence that the cloud is unable to develop, because colder air en-
tering the cloud by turbulent mixing would lower the changes of the
cloud reaching moderate altitudes. Since there is a high correlation
between cloud height, convective activity, and lightning, such early
nucleation of a cloud should reduce the likelihood of intense elec-
trical activity. Seeding would be accomplished by releasing silver
iodide into the cores of growing cumulus clouds ; it could be delivered
from ground dispensers or from aircraft into the updraft under the
cloud base. The amount of seeding material must be chosen carefully,
and, in order to increase the chances for cloud dissipation, overseed-
ing is probably most effective, though such overseeding will also tend
to reduce precipitation. On the other hand, rainfall may be advan-
tageous for other purposes, including its inhibiting lightning-caused
forest fires by providing moisture to the forest fuel. Consequently, the
advantages which might be achieved through reducing cloud con-

13 Stow, C. D.. "On the Prevention of Lightning," Bulletin of the American Meteorological
Society, vol. 50, No. 7, July 1969, p. 515.


vection and its attendant electrical activity must be weighed against
the possible advantages lost through reduced precipitation. 13

A more efficient lightning-suppression approach might involve in-
terference with the processes which bring about charge separation in
the cloud. At least five different mechanisms by which cloud electrifica-
tion is established have been theorized, and possibly all or most of these
mechanisms are active in any given situation, although on different
occasions it is likely that some are more effective than others, depend-
ing on meteorological conditions and geographical locations. 14 Data
are as yet insufficient for determining which mechanisms will predomi-
nate. It is not considered likely that a single treatment method would
suffice to suppress all lightning activity through prevention of charge
buildup, though it is conceivable that a given treatment may be capable
of suppressing more than one charge-generating process. 15 In addition
to glaciation of the cloud by overseeding (described above in connec-
tion with convection reduction), accumulation of charge can be in-
hibited through seeding with various chemicals which affect the
freezing of water. Another technique uses seeding with a conducting
chaff (very fine metalized nylon fibers), which increases conductivity
between oppositely charged regions of the- storm and keeps the electric
field from building up to the lightning-discharge level. The chaff fibers
are of the type that have been used for radar "jamming," which can be
dispensed underneath a thunderstorm from an aircraft. Experiments
have shown this attempt at lightning suppression to have some
promise. 16

Although reduction in the number of cloud-to-ground discharges
through cloud seeding would undoubtedly be instrumental in de-
creasing the total number of forest fires, ignition is also influenced by
such factors as the type of discharge, surface weather conditions, the
terrain-fuel complex, and the influence of preceding weather on fuel
moisture. The kind of discharge most frequently causing forest fires
has been observed and its characteristics have been measured. Observa-
tions indicate that ignition is most often caused by hybrid cloud-to-
ground discharges having long continuing current phases, whose
duration exceeds 40 milliseconds and that the probability of ignition is
proportional to the duration of the continuing current phase. 17

Evaluation of lightning suppression technology

Seeding experiments to date have yielded results which suggest that
both the characteristics and the frequency of lightning discharges have
been modified. The physical processes by which lightning is modified
are not understood ; however, basic physical charging processes have
been altered through massive overseeding with silver iodide freezing
nuclei. Direct measurements of lightning electricity have also shown
that lightning strokes which contain a long continuing current are
probably responsible for most lightning-ignited forest fires. Keduction
of the duration of the long continuing current discharge through wea-
ther modification techniques may, therefore, be more significant in

13 Ibid.

" Ibid., pp. 516-519.
16 Ibid , p 519

" Kasemir. Heinz W.. "Lightning Suppression by Chaff Seeding and Triggered Light-
ning." In Wilmot N. Hess (editor), "Weather and Climate Modification," New York, Wiley.
1974, N pp. 612-622. n a . „ . B „

"Fuquav, "Lightning Damage and Lightning Modification Caused by Cloud Seeding,
1974, p. 606.


reducing forest fires than reduction of the total amount of lightning
produced by storms.

From experiments in lightning suppression carried out under Proj-
ect Skyfire by the U.S. Forest Service of the Department of Agricul-
ture between 1965-67. Fuquay summarizes the following specific re-
sults, based on a total of 26 individual storms (12 seeded and 14
unseeded) : 18

Sixty-six percent fewer cloud-to-ground discharges, 50 percent
fewer intracloud discharges, and 54 percent less total storm light-
ning occurred during seeded storms than during the not-seeded

The maximum cloud-to-ground flash rate was less for seeded
storms : over a 5-minute interval, the maximum rate averaged 8.8
for not-seeded storms and 5 for seeded storms; for 15-minute in-
tervals, the maximum rate for not-seeded storms averaged 17.7
and 9.1 for seeded storms.

The mean duration of lightning activity for the not-seeded and
seeded storms was 101 and 64 minutes, respectively. Lightning
duration of the not-seeded storms ranged from 10 to 217 minutes,
while that of seeded storms ranged from 21 to 99 minutes.

There was no difference in the average number of return strokes
per discrete discharge (4.1 not-seeded versus 4 seeded) ; however,
a significant difference was found for hybrid discharges (5.6 not-
seeded versus 3.8 seeded) .

The average duration of discrete discharges (period between
first and last return stroke) decreased from 235 milliseconds for
not seeded storms to 182 milliseconds for seeded storms.

The average duration of continuing current in hybrid dis-
charges decreased from 187 milliseconds for not-seeded storms to
115 milliseconds for seeded storms.
In a recent Federal appraisal of weather modification technology
it was concluded that results of field experiments to suppress light-
ning through silver iodide seeding have been ambiguous. 19 Although
aim lysis of data previously obtained is continuing, the experimental
seeding program of the Forest Service has been terminated. In more
recent experiments, thunderstorms have been seeded from below
with chaff (very fine metalized nylon fibers). Based on an analysis of
10 chaff-seeded thunderstorms and 18 unseeded control storms, the
number of lightning occurrences during the seeded storms was about
25 percent of those observed in the control storms. This observed differ-
ence was statistically significant even though the experiments were
not strictly randomized. 20

Experiments in lightning modification through cloud seeding have
given results showing that, in some cases, lightning can be modified
in a beneficial manner. From these results and the measured charac-
teristics of lightning strokes, a hypothesis of lightning modification is
being developed. There has been progress in identifying significant cor-
relations between occurrence of lightning and such variables as storm

u Fuquav. "Lightning Damage and Lightning Modification Caused by Cloud Seeding,"
1974, p. 6li.

19 U.S. Domestic Council, Environmental Resources Committee, Subcommittee on Climate
Change, "The Federal Role in Weather Modification." Washington, D.C., December 1975.
p. 10.



size, updraft characteristics, precipitation rates, and hail occurrence.
According to Fuquay, such early successes ought not obscure the mag-
nitude of the research yet required in order to identify and quantify
the degree and applicability of lightning modification to the lightning-
fire problem. 21 He also warns that :

Until more is known about the adverse effects of seeding incipient thunder-
storms, unexpected and adverse effects must be considered, although improved
numerical models that accurately predict cloud development and the effects of
seeding should minimize the risk of unexpected events. 22


Severe storms have a greater immediate impact on human life and
property than most other weather phenomena. A major portion of
losses due to natural disasters results from two of the most destructive
kinds of severe storms — hurricanes and tornadoes. During an average
year the U.S. mainland is threatened by 8 tropical slorms and experi-
ences over 600 tornadoes. 23 Among the results of the annual devastation
from these storms are the loss of hundreds of lives and the accumula-
tion of hundreds of millions of dollars in property damage.

Perhaps the most important problems to be attacked in weather
modification are associated with the abatement of severe storms. While
rainfall augmentation promises borderline economic value at best, al-
ternatives which can contribute more significantly to severe water
shortages may prove more suitable. On the other hand, the annual
threat of tolls in damages and fatalities from hurricanes and tornadoes
will persist year after year, and research directed toward modification
of these severe phenomena requires continued support. There have been
dramatic attempts, with some successes, in demonstrating the potential
reduction of the hazards of hurricanes ; however, almost no research
has been directed toward tornado suppression.


A hurricane is an intense cyclone which forms over tropical seas,
smaller in size than middle-latitude cyclones, but much larger than a
tornado or a thunderstorm. With an average size of 500 miles (800
kilometers) in diameter, the hurricane consists of a doughnut-shaped
ring of strong winds in excess of 64 knots which surrounds an area of
extremely low pressure and calm at the storm's center, called the eye. 2 *
The generic name for all vortical circulations originating over tropi-
cal waters is "tropical cyclone." When fully developed with sufficiently
strong winds, such storms are called hurricanes in the Atlantic and the
eastern Pacific Oceans, typhoons in the northwest Pacific, baguios in
the Philippines, Bengal cyclones in the Indian Ocean, and willy-willies
near Australia. For a tropic cyclone whose winds are in the range of
33 to 64 knots, the official name' in the United States is a tropical storm.
The hurricane season is that portion of the year having a relatively

21 Fuquay, "Lightning Damage and Lightning Modification Caused by Cloud Seeding,"
1974. p. 612.

22 Ibid., p. 606.

23 Feieral Coordinator for Meteorological Services and Supporting Research. "Federal
Plan for Meteorological Services and Supporting Resenrch : Fiscal Year 1973." U.S. Depart-
ment of Commerce, National Oceanic and Atmospheric Administration, Washington, D.C.,
January 1972. p. 1.

24 Anthes, Richard A.. Hans A. Panofskv. -Tohn J. Cahir. and Albert Rango. "The Atmos-
phere." Columbus, Ohio, Charles E. Merrill. 1975. p. 150.


high incidence of hurricanes and usually is regarded as the period
between June and November in the Northern Hemisphere. 25

Owing to their duration, which exceeds that of earthquakes, and to
their violence, which approaches that of tornadoes, hurricanes are the
most destructive natural phenomena. Prior to Hurricane Agnes in
1972, whose total damage exceeded $3 billion, the annual hurricane
property losses in the United States amounted to about $450 million,
although two hurricanes in the 1960's, Betsy (1965) and Camille
(1969), each caused damage exceeding $1.4 billion. 26 Improved tech-
niques in hurricane detection and warning have dramatically reduced
the number of deaths caused by hurricanes ; however, property losses
have continued to grow, as a result of increased population and activi-
ties in vulnerable coastal areas, with the attendant concentration of
new houses, buildings, and other facilities of higher replacement value.
Figure 8 shows the simultaneous increase in property losses and de-
crease in deaths due to hurricanes in the United States in the 20th
century through 1969.

Devastation and fatalities occur essentially from three phenomena
associated with hurricanes : the force of the winds in the storm itself,
the storm surge on coastal areas, and flooding which can result from
excessive and widespread rainfall as the storm moves inland. Since
wind force varies with the square of the wind speed, a 50-mile-per-hour
wind exerts four times as much force as a 25-mile-per-hour wind. Ac-
cordingly, a 10-percent reduction in maximum windspeed yields a de-
crease in wind force of about 20 percent. 27 Attempts to modify hurri-
cane winds can thus be expected to reduce storm damage caused by
winds in approximate proportion to the corresponding reduction in
wind force.

25 Federal Coordinator for Meteorological Services and Supporting Research, U.S. Depart-
ment of Commerce, National Oceanic and Atmospheric Administration, "National Hurricane
Operations Plan," FCM 77- 2. Washington, D.C., May 1977, pp. 6-7.

20 Gentry, K. Cecil, "Hurricane Modification." In Wilmot N. Hess (ed.). "Weather and
Climate Modification," New York, John Wiley & Sons, 1974, p. 497.

27 Ibid., p. 498.


Figure 8. — Losses in the United States from hurricanes, 1915 through 1969, in
5-year periods (from National Oceanic and Atmospheric Administration).

_ As a hurricane moves across the coast from the sea. the strong winds
pile up water to extreme heights, causing storm surges. The resulting
onrushing water wreaks damage to shoreline and coastal structures.
The severity of the storm surge is increased by the hurricane-generated
wind waves which are superimposed on the surge. From Hurricane
Camille, the storm surge at Pass Christian, Miss., was 24.6 feet, higher
than any previous recorded tide. As a result, 135 people were killed,
63,000 families suffered personal losses, and Mississippi alone sustained
$1 billion in damage. 28 The height of the storm surge depends both on

Anthes, Panofsky, Cahir, and Rango, "The Atmosphere," 1975, p. 159.


the windspeed and the shape and slope of the sea bottom offshore. If
there is a sharp dropoff in depth not far off the beach, the rise of the
sea level will be small, for example. Nearshore attempts to modify a
hurricane could lead to uncertain results, depending upon local condi-
tions. If the windspeed is reduced without moving the position of
maximum winds along the coast, the overall effect would likely be a
reduction in storm surge. However, should the modification activity
result in developing a new windspeed maximum at a different location,
the surge might increase or decrease, depending on bathymetry and
bottom topography. 29 Solutions are not yet clear, and the storm surge
prediction problem is being studied intensely with the use of numerical

Major hurricane damage can often be attributed to heavy rains and
the massive and sudden flooding which can result as the storm move's
inland. In mountainous regions especially, the floods from such rain-
fall can be devastating in losses to both life and property. Such flood-
ing was a major contributor to the 118 deaths and $3.5 billion in prop-
erty destruction 30 which resulted in June 1972 from Hurricane Agnes,
which set the record of achieving the greatest damage toll of all U.S.
hurricanes. Ironically, Agnes caused almost no major damage as it
went ashore. Hurricane modification activities which have been at-
tempted or are contemplated are unfortunately not designed to reduce
the rains significantly, but are intended rather to reduce the maxi-
mum winds. 31

Generation and characteristics of hurricanes

A hurricane can be thought of as a simple heat engine driven by
temperature differences between the center of the storm and its mar-
gins. At each level the central column must be warmer than the
surrounding area to insure maintenance of the strong convection on
which the storm depends. 32 While the energy which forms extratropical
cyclones is provided by temperature differences between different air
masses, the energy which generates and maintains hurricanes and
other tropical cyclones is derived from a single air mass through
condensation of water vapor, and there are seldom present any of
the frontal activities which are characteristic of storms originating
in temperate latitudes. The moisture-laden winds continuously supply
water vapor to the tropical storm, and the condensation of each gram
of the vapor releases about 580 calories of latent heat. Within this
thermally driven heat engine tremendous quantities of energy are
converted from heat to mechanical motion in a short time, a fact
readily apparent from the fury of the winds. The daily power of the
energy liberated within a hurricane has been estimated to be about
ten thousand times the daily power consumption in the United States. 33
The importance of tin 1 ocean in providing moisture to a hurricane
is seen in the weakening and dissipation of the storms after they have
crossed coastlines and travel over land.

20 Gentrv. "Hurricane Modification," 1974. p. 499.

30 National Advisory Committee on Oceans and Atmosphere. "The Agnes Floods.: a Cost-
Audit of the Effectiveness of t^c Storm and Flood Warning System of the National Oceanic
and Atmosnheric Administration," a report for the Administrator of NOAA. Washington,
D.C., Nov. 22. 1972. p. 1.

:;1 Gentrv. "Hurricane-Modification." H>74. n. 490.

^Donn. William L. "Meteorology." 4th edition. New York. McGraw-Hill, 1975, p. 336.
"Ibid., p. 338.


Exactly how hurricanes form is not yet fully understood. They
are all generated in the doldrums (a region of equatorial calms),
though rarely if ever within latitudes closer than 5 degrees from the
Equator, over water whose temperature is at least 27° C. The relatively
high surface temperature is necessary for initiation of the convection.
Hurricanes are relatively rare features even of the tropics, and the
exact triggering mechanism is not yet known. 34 Their origin is usually
traced to a low pressure disturbance which originates on the equatorial
side of the trough of an easterly wave.

Such a tropical disturbance moves slowly westward and slightly
poleward under the direction of the tropical east winds. If conditions
are right, this cluster of thunderstorms intensifies as it reaches the
region near the boundary between the tropical easterlies and the
middle-latitude westerlies, at about 25° latitude. It may then follow
a path which reverses toward the east as it leaves the tropics. The
tracks of 13 major hurricanes in the Northwest Atlantic Ocean are
shown in figure 9.

The development of the intense storm which might result from the
conditions noted above is described in the following way by Anthes
et al. :

The increased inflow toward the center of falling pressure produces increased
lifting of air, so that the thunderstorms become more numerous and intense. The
feedback cycle is now established. The inflowing air fuels more intense thunder-
storm convection, which gradually warms and moistens the environment. The
warmer air in the disturbance weighs less, and so the surface pressure continues
to fall. The farther the pressure falls, the greater the inflow and the stronger
the convection. The limit to this process would occur when the environment is
completely saturated by cumulonimbus clouds. Further condensation heating
would not result in additional warming, because the heat released would exactly
compensate for the cooling due to the upward expansion of the rising air. 35

34 Ibid.

35 Anthes, Panofsky, Cahir, and Rango, "The Atmosphere," 1975, p. 154.


Figure 9. — Tracks of thirteen major hurricanes in the Xorth Atlantic from 1879
through 1955 (from U.S. Naval Oceanographic Office, Publication No. 21,
Sailing Directions for the West Indies, 1958).

As the storm forms, the winds begin to strengthen about the center,
increasing especially to the right of the direction in which the center
is moving, normally on the poleward side. The clouds organize them-
selves into a system and dense cirrus move forward in the direction
of the movement of the center. Suddenly, the pressure falls over a
small area and hurricane force winds form a tight band of 20 to 40


miles radius around the center. The well-organized clouds show a
spiraling structure, and the storm acquires an eye, a small nearly
circular area, coinciding with the region of lowest pressure. The winds
in the eye are light and variable and the clouds are scattered or
entirely absent. 36 As the storm matures, the pressure ceases to fall
and the maximum winds do not increase further. Now the storm ex-
pands horizontally and large amounts of air are drawn in. As the
storm expands to a radius of about 200 miles or more it becomes less
symmetrical. Figure 10 is a vertical cross-section of the structure of
a typical mature hurricane, showing the direction of flow and cloud
distribution. 37

In spite of the great damage and fatalities caused by hurricanes,
their effects are not completely destructive. In many areas of South-
east Asia and the west coast of Mexico, tropical storms are depended
upon for a large part of the water supply. Throughout the Southern
United States, hurricanes have also provided valuable drought relief. 38
– Hurricane and other tropical cyclones are always characterized by
high wind velocities and by torrential rains. Wind velocities of 60 to
70 knots and more are normal for such storms. The air rotates rapidly,
moving spirally toward the center. Maximum gusts exceed 100 knots
and may reach 200 knots, although such high speeds are unrecorded
since instruments are blown away or made inoperable at these wind
speeds. 39

Figure 10. — Vertical cross section through a hurricane, showing typical cloud
distribution and direction of flow, as functions of height and distance from
the eye. (From Anthes, Panofsky, Cahir, and Rango, 1975.)

Compared with extratropical storms, hurricanes are generally small,
circularly shaped zones of intense low pressure, with very steep pres-
sure gradients between the center and the periphery. The pressure
drop between the eye and the periphery is quite large, 20 to 70 milli-
bars being typical. The winds are in a constant circular cyclonic
motion (counterclockwise in the Northern Hemisphere and clockwise
in the Southern Hemisphere) ; however, the center of the storm is a

36 p P tterssen. Sverre. "Introduction to Meteorology," second edition, New York, McGraw-
Hill. 1958, pp. 242-243.

37 Anthes. Panofsky. Cahir. and Rango. "The Atmosphere," 1975. p. 157.

ssReihl, Herbert, "Introduction to the Atmosphere," New York, McGraw-Hill, 1965, pp.

39 Gentilli. J.. "Tropical Cyclones." In Rhodes W. Fairbridge fed.). "The Encyclopedia
of Atmospheric Sciences and Astrogeology." Reinhold, New York, 1967, p. 1028.

* Widely scattered
_ — — shallow cumulus


Distance from hurricane center (km)


calm region of low pressure, called the eye. which is about 10 miles
across on the average. The warm dry character of this region is due
to subsiding air, which is necessary for existence of the storm. Around
the eye is the wall, consisting of cumulonimbus clouds and the at-
tendant extreme instability and rising motion; in the wall area adja-
cent to the eye, heavy rains fall. Out from the central zone altostratus
and nimbostratus clouds mix to form a layer with a radius as great
as 200 miles. At higher altitudes and reaching to the outer regions
of the storm is a mixture of cirrus and cirrostratus clouds. 40

In a mature hurricane a state of relative equilibrium is reached
eventually, with a particular distribution of wind, temperature, and
pressure. Such distributions for a typical hurricane are shown sche-
matically in figure 11. Note that the greatest pressure change and the
maximum windspeeds are in the region of the wall clouds, near the
center of the storm. 41

Figtjbe 11.— Radial profiles of temperature, pressure, and windspeed for a mature
hurricane. The temperature profile applies to levels of 3 to 14 kilometers;
pressure and windspeed profiles apply to levels near the surface. (From
Gentry, 1974. )

Modification of hurricanes

Since the damage inflicted by hurricanes is primarily a result of the
high windspeeds, the principal goal of beneficial hurricane modifica-

40 Jerome Williams. John J. Hipsinson. and John D. Rohrhoujjh. "Sea and Air: The
Naval Environment," Annapolis. Md.. U.S. Naval Institute. 1968, pp. 262-263.

41 Gentry. "Hurricane Modification." 1974. pp. 502-503.


tion is the reduction of the severity of the storm's maximum winds.
The winds result from the pressure distribution, which, in turn, is
dependent on the temperature distribution. Thus, hurricane winds
might be reduced through reduction of temperature contrasts between
the core of the storm and the region outside.

Gentry notes that there are at least two important fundamentals of
hurricanes which have been established through recent studies, which
suggest possible approaches to modification of the severity of the
storms : 42

The transfer of sensible and latent heat from the sea surface to the
air inside the storm is necessary if the hurricane is to reach or retain
even moderate intensity.

The energy for the entire synoptic-scale hurricane is released by
moist convection in highly organized convective-scale circulations lo-
cated in and around the eye of the storm and in the major rain bands.
The first principle accounts for the fact that hurricanes form only
over warm tropical waters and begin to dissipate after moving over
land or cool water, since neither can provide sufficient energy flow to
the atmosphere to maintain the intensity of the storm. The second
principle explains why such a low percentage of tropical disturbances
grow to hurricane intensity. Possible field experiments for beneficial
modification of hurricanes follow from these principles. On the basis
of the first, techniques for inhibiting evaporation might be employed
to reduce energy flux from the sea surface to the atmosphere. Based
on the second principle, it might be possible to affect the rate of release
of latent heat in that small portion of the total storm which is occupied
by the active convective-scale motions in such a way that the storm is
weakened through redistribution of heating. 43

Gentry discusses a number of possible mechanisms which have been
suggested for bringing about changes to the temperature field in a
hurricane. 44 Since the warm core development is strongly influenced
by the quantity of latent heat available for release in air columns ris-
ing near the center of the storm, the temperature might be decreased
through reducing the water vapor in these columns, the water vapor
originating through evaporation from the sea surface inside the region
of high storm winds. It has been suggested that a film spread over the
ocean would thus reduce such evaporation. No such film is available,
however, which could serve this purpose and withstand rupturing and
disintegration by the winds and waves of the storm. Another sugges-
tion, tiiat the cooling of the sea surface might be achieved through
dropping cold material from ships or aircraft, is impractical, since
such great expenditure of energy is required. It has also been postu-
lated that the radiation mechanisms near the top of the hurricane might
be modified through distribution of materials of various radiation
properties at selected locations in the clouds, thus inducing changes to
the temperatures in the upper part of the storm. This latter suggestion
needs further evaluation both from the standpoint of its practicality
and from the effect such a change, if included, would theoretically have
on storm intensity.

The potential schemes for hurricane modification which seem to be
practical logistically and offer some hope for success involve attempts

42 Ibid., 1974. p. 503.
« Ibid., p. 504.
44 Ibid., p. 505.

34-857 O – 79 – 10


to modify the mechanism by which the convective processes in the eye-
wall and the rain bands distribute heat through the storm. Since water
vapor is condensed and latent heat released in the convective clouds, it
should be possible to influence the heat distribution in the storm
through changing the pattern of these clouds. 45 Recent success in
modifying cumulus clouds promises some hope of success in hurricane
modification through cloud seeding. By modifying the clouds in a hur-
ricane, the storm itself may be modified, since the storm's intensity will
be affected through changing the interactions between the convective
(cloud) scale and the synoptic (hurricane) scales. 46 Figure 12 shows
how the properties of a hurricane might be redistributed as a result
of changing the temperature structure through seeding the cumulus
cloud structure outside the wall. The solid curves in the figure repre-
sent distributions of temperature, pressure, and windspeed identical
with those shown in figure 11 without seeding; the dashed curves rep-
resent these properties as modified through seeding. 47

The first attempt at hurricane modification was undertaken by sci-
entists of the General Electric Co., on a hurricane east of Jacksonville,
Fla., on October 13, 1947. Clouds outside of the wall were seeded with
dry ice in order to cause freezing of supercooled water, so that the ac-
companying release of latent heat might alter the storm in some man-
ner. Results of the experiment could not be evaluated, however, owing
to the lack of adequate measuring equipment for recording cloud char-
acteristics. Furthermore, the penetration of the wall clouds to the eye
or to the area of intense convection in the storm's rain bands was pre-
vented by failure of navigation aids. Based on information acquired
from more recent seeding experiments and increased understanding of
hurricanes, it seems doubtful that the 1947 seeding could have been
effective. 48

« Ibid.

"Ibid., p. 504.
«Ibid., pp. 504-505.
48 Ibid., pp. 505-506.


Figure 12. — Radial profiles of temperature, pressure, and windspeed for a mature
hurricane before (solid curves) and possible changes after (dashed curves)
seeding. (The solid curves are the same as those in fig. 11.) (From Gentry,

Hurricane seeding experiments were undertaken by the Department
of Commerce and other agencies of the Federal Government in 1961,
initiating what came to be called Project Stormfury. To date only four
hurricanes have' actually been seeded under this project — all of them
between 1961 and 1971 ; however, Stormfury has also included inves-
tigation of fundamental properties of hurricanes and their possible
modification through computer modeling studies, through careful
measurements of hurricane properties with research probes, and
through improvements in seeding capabilities.

The goal of hurricane seeding is the reduction of the maximum winds
through dispersing the energy normally concentrated in the relatively
small band around the center of the storm. The basic rationale for seed-
ing a hurricane with silver iodide is to release latent heat through
seeding the clouds in the eye wall, thus attempting to change the tem-
perature distribution and consequently weaken the sea level pressure
gradient. It is assumed that the weakened pressure gradient will allow
outward expansion, with the result that the belt of maximum winds
will migrate away from the center of the storm and will therefore
weaken. Actually, stimulation of condensation releases much more
latent heat than 'first hypothesized in 1961, and theoretical hurricane
models show that a new eve wall of greater diameter can be developed
by encouraging growth of cumulus clouds through dynamic seeding. 49

» Ibid., pp. 510-511.


Following seeding of the four storms in Project Stormf ury, changes
were perceived, but all such changes fell within the range of natural
variability expected of hurricanes. In no case, however, did a seeded
storm appear to increase in strength. Hurricane Debbie, seeded first
on August 18, 1969, exhibited changes, however, which are rarely
observed in unseeded storms. Maximum winds decreased by about 30
percent, and radar showed that the eye wall had expanded to a larger
diameter shortly after seeding. After Debbie had regained her strength
on August 19, she was seeded again on August 20, following which
her maximum winds decreased by about 15 percent. 50 Unfortunately,
data are not adequate to determine conclusively that changes induced
in Debbie resulted from seeding or from natural forces. Observations
from Hurricane Debbie are partially supported by results from simu-
lated experiments with a theoretical hurricane model ; however, simu-
lation of modification experiments with other theoretical models have
yielded contrary results. 51

One of the problems in evaluating the results of hurricane modifi-
cation is related to the low frequency of occurrence of hurricanes
suitable for seeding experiments and the consequent small number of
such experiments upon which conclusions can be based. This fact re-
quires that hurricane seeding experiments must be even more carefully
planned, and monitoring measurements must be very comprehensive,
so that data acquired in the few relatively large and expensive experi-
ments can be put to maximum use. Meanwhile theoretical models must
be improved in order to show the sensitivity of hurricane characteris-
tics to changes which might be induced through seeding experiments.

Gentry has suggested that the following future activities should be
conducted under Stormf ury : 52

1. Increased efforts to improve theoretical models.

2. Collection of data to further identify natural variability in

3. Expanded research — both theoretical and experimental — on
physics of hurricane clouds and interactions between the cloud
and hurricane scales of motion.

4. More field experiments on tropical cyclones at every oppor-

5. Tests of other methods and material for seeding.

6. Further evaluation of other hypotheses for modifying

7. Development of the best procedures to maximize results of
field experiments.


The structure of tornadoes is similar to that of hurricanes, consist-
ing of strong cyclonic winds 53 blowing around a very low pressure
center. The size of a tornado, however, is much smaller than that of a
hurricane, and its wind force is often greater. The diameter of a tor-
so National Oceanic and Atmospheric Administration. "Stormfury— 1977 to Seed One
Atlantic Hurricane U.S. Department of Commerce News, NOAA 77-248, Washington.
D.C., Sept. 20. 1977, p. 3.

51 Gentry, "Hurricane Modification," 1974. p. 517.

^ Cyclonic > winds blow counterclockwise around a low pressure center in the Northern
Hemisphere ; in the Southern Hemisphere they blow clockwise.


nado is about one- fourth of a kilometer, and its maximum winds can
exceed 250 knots in extreme cases. 54 On a local scale, the tornado is the
most destructive of all atmospheric phenomena. They are extremely
variable, and their short lifetime and small size make them nearly
impossible to forecast with any precision.

Tornadoes occur in various parts of the world; however, in the
United States both the greatest number and the most severe tornadoes
are produced. In 1976. there were reported 832 tornadoes in this coun-
try, 55 where their origin can be traced to severe thunderstorms, formed
when warm, moisture-laden air sweeping in from the Gulf of Mexico
or the eastern Pacific strikes cooler air fronts over the land. Some of
these thunderstorms are characterised by the Auolent updrafts and
strong tangential winds which spawn tornadoes, although the details
of tornado generation are still not fully understood. Tornadoes are
most prevalent in the spring and occur over much of the Eastern two-
thirds of the United States; the highest frequency and greatest devas-
tation are experienced in the States of the middle South and middle
West. Figure 13 shows the distribution of 71,206 tornadoes which
touched the ground in the contiguous United States over a 40-year

Even in regions of the world favorable to severe thunderstorms, the
vast majority of such storms do not spawn tornadoes. Further-
more, relatively few tornadoes are actually responsible for deaths and
severe property damage. Between 1960 and 1970, 85 percent of tornado
fatalities were caused by only 1 to iy 2 percent of reported tornadoes. 56
Nevertheless, during the past 20 years an average of 113 persons have
been killed annually by tornadoes in the United States, and the annual
property damage from these storms has been about $75 million. 57

Modification of tornadoes

Alleviation from the devastations caused by tornadoes through
weather modification techniques has been a matter of considerable
interest. As with hurricanes, any such modification must be through
some kind of triggering mechanism, since the amount of energy pres-
ent in the thunderstorms which generate tornadoes is quite large. The
rate of energy production in a severe thunderstorm is roughly equal to
the total power-generating capacity in the United States in 1970. 58
The triggering mechanism must be directed at modifying the circula-
tion through injection of small quantities of energy.

^ Anthes, Panofsky, Cahir, and Rango, "The Atmosphere," pp. 150, 180.

50 NOAA news. "Skywarn 1977 — Defense Against Tornadoes," U.S. Department of Com-
merce, National Oceanic and Atmospheric Administration. Rockville, Md., Feb. 18, 1977,
vol. 2, No. 4, pp. 4-5.

56 Davies-Jones, Robert and Edwin Kessler, "Tornadoes." In Wilmot N. Hess (ed.),
"Weather and Climate Modification," New York, John Wiley & Sons, 1974, p. 552.
» Ibid.

58 Anthes, Panofsky, Cahir, and Rango, "The Atmosphere," 1975, p. 185.


Figure 13. — Tornado distribution in the United States, where contours enclose
areas receiving equal numbers of tornadoes over a 40-year period. Frequencies
are based on number of 2-degree squares experiencing first point of contact
with the ground for 71,206 tornadoes. (From Wilkins, 1967, in Encyclopedia
of Atmospheric Sciences and Astrology, Reinhold.)

Tornado modification has not been attempted in view of the pres-
ent insufficient knowledge about their nature and the lack of adequate
data on associated windspeeds. There are potential possibilities, how-
ever, which can be considered for future research in tornado modifica-
tion. One proposal is to trigger competing meteorological events at
strategic locations in order to deprive a tornadic storm of needed in-
flow. This technique, suggested by the presence of cumulus clouds over
forest fires, volcanoes, and atomic bomb blasts could use arrays of
large jet engines or oil burning devices. Another approach for dis-
persal of convective clouds which give rise to thunderstorms might
involve the use of downrush created by flying jet aircraft through
the clouds. A further possibility would depend on changing the char-
acteristics of the Earth's surface such as the albedo or the availability
of water for evaporation. 59

Tornadoes tend to weaken over rougher surfaces due to reduction
of net low-level inflow. Upon meeting a cliff, tornadoes and water-
spouts often retreat into the clouds, and buildings also tend to reduce
ground level damage. Thus, forests or artificial mounds or ridges
might offer some protection from tornadoes, although very severe
tornadoes have even left swaths of uprooted trees behind. 60

Modification of tornadoes by cloud seeding would likely bo the cheap-
est and easiest method. Sodium iodide seeding could possibly shorten
the life of a tornado if the storm's cold air outflow became stronger and
overtook the vortex sooner, thus cutting off the inflow. Seeding a
neighboring cell upstream of the low-level inflow might also be bene –

09 Davies-Jones and Kessler, "Tornadoes," 1974, p. 590.
» Ibid.


ficial, if the rapidly developing seeded cloud, competing for warm,
moist air, reduces the inflow and weakens the rotating updraft. It is
also possible that seeding would increase low-level convergence, lead-
ing to intensification of a tornado. 61
Davies- Jones and Kessler conclude that :

Any efforts to modify a severe storm with potential or actual tornadoes
obviously will have to be carried out with extreme caution * * *. Actual modifica-
tion attempts on menacing tornadoes are probably several years away. In the
meantime, we should seek improved building codes and construction practices
and continue research into the actual morphology of convective vortices. 62

In spite of the speculations on how tornadoes might be modified, no
tests have yet been conducted. The small size and brief lifetime of tor-
nadoes make them difficult and expensive to investigate. However, in
view of their destructiveness, they must be given more attention by
meteorologists, who should seek ways to mitigate their effects. Only
further research into the character of tornadoes, followed by careful
investigation of means of suppressing them, can lead to this desired
reduction in the effects of tornadoes.

Technical Problem Areas in Planned Weather Modification

In this section a number of major problem areas associated with the
development of weather modification technology will be addressed.
These topics are not necessarily confined to the modification of any one
of the weather phenomena discussed in the previous section but apply
in general to a number of these categories of phenomena. Some of the
problem areas have implications which extend beyond the purely
technical aspects of planned weather modification, bearing also on
social, economic, and legal aspects as well. Included are discussions on
the problems of seeding technology, evaluation of results of weather
modification projects, extended area and extended time effects from
advertent weather modification, and potential approaches to weather
and climate modification which involve techniques other than seeding.
The problems of inadvertent weather modification and of potential
ecological effects from planned weather modification could also prop-
erly be included in this section ; however, these topics are addressed in
chapter 4 and 13, respectively, in view of their special significance.

seeding techonology

In recent years there has been progress in developing a variety of
ice-nucleating agents available for cloud seeding, although silver iodide
continues to be the principal material used. Other seeding agents which
have been studied include lead iodide, metaldehyde, urea, and copper
sulfide. Nucleants have been dispensed into the clouds from both
ground-based generators or from aircraft. In some foreign countries,
such as the Soviet. Union, rockets or artillery have been used to place
the seeding material into selected regions of the clouds; however, this
means of delivery does not seem to be acceptable in the United States.

There have been both difficulties and conflicting claims regarding the
targeting of seeding materials, particularly from groimd generators,
ever since the earliest days of cloud seeding. It is always hoped that

ft Ibid., pp. 590-591.
«a Ibid., p. 591.


the nucleant will be transported from the generator site by advection,
convection, and diffusion to parts of the clouds which have been iden-
tified for modification. Difficulties have been observed under unstable
conditions, where the plume of nucleants was disrupted and wide angle
turbulent diffusion was severe. Valley locations in mountainous areas
are often subjected also to inversions and to local channeling so that
trajectory determinations are extremely difficult. Even plumes of seed-
ing material from aircraft have shown an erratic pattern. The prob-
lems of irregular plume goemetry appear to increase as distortion
occurs near fronts in mountain terrain, that is, under just the circum-
stances where cloud seeding is often attempted. 63

In view of the limited vertical transport of silver iodide observed
in some studies (that is, up to 450 meters above the terrain at distances
of several kilometers from the generators), some have concluded
that, under conditions of the tests, ground-based generators are
probably not effective. However, other studies have shown that one
cannot generalize that ground generators are not always effective.
Thus, more desirable effects can be achieved with generators at high
altitudes where there is little chance of inversion trapping of the
silver iodide as in other tests. 64

Much of the ambiguity associated with ground-based generators is
reduced when the nucleant material is placed into the cloud directly
by an aircraft using flares or rockets. However, airborne seeding also
presents important targeting problems. Of course, targeting difficul-
ties are reduced in the case of single cloud seeding, where the aircraft
is flying directly beneath the cloud in the active updraft area. How-
ever, questions of proper vortical ascent persist when the objective is
to lay down from the aircraft an elevated layer of nucleant-rich air
that is intended to drift over the target area. 65

In conclusion, the 1973 National Academy of Sciences study says :

To summarize the results of the past few years' work on targeting, it can he said
that earlier dobuts about the inevitability of nuclei reaching effective altitudes
from ground generators tend to be supported by a number of recent observational
studies. Some of these merely confirm the rather obvious prediction that stable
lapse rates will be unfavorable to the efficacy of ground generators ; others indi-
cate surprising lack of vertical ascent under conditions that one might have
expected to favor substantial vertical transport. The recent work also tends to
support the view that plumes from ground generators in mountainous terrain
must be expected to exhibit exceedingly complex behavior ; and each site must
be expected to have its own peculiarities with respect to plume transport. Tracking
experiments become an almost indispensable feature of seeding trials or operations
in such cases. 66

There are three types of airborne seeding agent delivery systems in
common use — burners, flares, and hoppers. Burners are used mainly
for horizontal seeding, often at the cloud base as discussed above. Poly-
technic flares are of two types — those used in vertical drops, similar to
a shotgun shell or flare-pistol cartridge, and the end-burning type,
similar to warning flares. The flares contain silver iodide with or with-
out an auxiliary oxydizer, such as potassium nitrate, together with
aluminum, magnesium, and synthetic resin binder. Dropping flares are

68 National Academy of Sciences, National Research Council, Committee on Atmospheric
Sciences, "Weather and Climate Modification : Problems and Progress," Washington, D.C..
1973. pp. 115-16.

61 Ibid., p. 117.

85 Ibid., pp. 118, 120.

M Ibid., pp. 119-120.


intended to be dropped into updrafts and to seed the cloud over a verti-
cal depth as great as a kilometer, while burner seeding is intended to be
more controlled and gradual. Hoppers dispense materials in solid form,
such as the particles of dry ice crushed and dropped into clouds and
cold fogs. For warm fog and cloud modification hoppers are used to
dispense dry salt or urea. Sometimes these materials are pumped in a
solution to nozzles in the wings, where the wingtip vortices help mix
the agent into the air. 67

On the ground there are a number of seeding modes which are fre-
quently used, and types of nucleants used with ground-based genera-
tors are commonly of two types — a complex of silver iodide and sodium
iodide or of silver iodide and ammonium iodide. Outputs from the gen-
erator are usually from 6 to 20 grams per hour, although generators
with much greater outputs are used sometimes. One seeding mode in-
volves dispensing continuously into the airstream from a ground gen-
erator at a fixed point, the approach used most commonly in mountain-
ous terrain. If the generator is located in flat country at temperatures
above freezing, the nucleation level is reached through entrainment of
the material into the convection. 68

The nucleating effectiveness of silver iodide smoke is dependent upon
the cloud temperature, where the colder the temperature the greater is
the number of ice crystals formed per gram of silver iodide. Tests of
nucleating effectiveness are made in the Colorado State University
cloud simulation facility, where the nucleant is burned in a vertical
wind tunnel and a sample of the aerosol is collected in a syringe and
nucleant density calculated from the pyrotechnic burn rate and the
tunnel flow rate. The syringe sample is diluted with clean, dry air and
injected into a precooled isothermal cold chamber containing cloud
droplets atomized from distilled water. Ice crystals which grow and
settle out are collected on microscopic slides, so that nucleating effec-
tiveness can be calculated as the ratio of concentrated crystals detected
to the mass of nucleating material in the air sample. 69

As part of the preparations for the 1976 seeding operations in the
Florida area cumulus experiment (FACE) of the National Oceanic
and Atmospheric Administration (NOAA), Sax et al., carefully
evaluated the silver iodide effectiveness of different flares used in
FACE. The results of these effectiveness studies, conducted with the
Colorado State University facility, are shown in figure 14. It was dis-
covered that a newly acquired airborne flare, denoted as NEI TB-1
in the figure, was considerably more effective than both the Navy
flares used earlier and another commercially available flare (Olin
WM-105). The superiority of the NEI TB-1 material at warmer
temperatures is particularly noteworthy. 70 In another paper, Sax,
Thomas, and Bonebrake observe that crystalline ice concentrations in
clouds seeded in FACE during 1976 with the NEI flares greatly
exceeded those found in clouds seeded during 1975 with Navy flares.

67 Ruskin, R. E. and W. D. Scott, "Weather Modification Instruments and Their Use."
In Wilmot N. Hess (ed.), "Weather and Climate Modification," New York, Wiley, 1974, pp.

68 Elliott, Robert D., "Experience of the Private Sector." In Wilmot N. Hess (ed.),
"Weather and Climate Modification," New York, Wilev, 1974, p. 57.

09 Sax, Robert I.. Dennis M. Garvey, Farn P. Parungo, and Tom W. Slusher, "Characteris-
tics of the Agl Nucleant Used in NOAA's Florida Area Cumulus Experiment." In preprints
of the "Sixth Conference on Planned and Inadvertent Weather Modification," Champaign,
111., Oct. 10-13. 1977. American Meteorological Society, Boston, 1977, p. 198.

70 Ibid., pp. 198-201.


They conclude that, if differences in sampling time intervals and effects
of instrumentation housing can be ignored, there is indicated a much
greater nucleation effectiveness for the XEI flares which were used
predominantly after July 1975. 71 The implications of this result are
very far reaching, since the borderline and/or slightly negative results
of many previous experiments and operational projects 1 can possibly
be laid to the ineffectiveness of the silver iodide flares previously

-5 -10 -15 -20

Figure 14. — Effectiveness of various silver iodide flares in providing artificial
nuclei as a function of cloud temperature. The principal comparison is between
the XEI TB-1 and the Navy TB-1 flares (see text) ; the curve of mean data for
the Olin WM-105 flares is included for comparison. The curves show that the
XEI flares, used In FACE in late 1975 and 1976 were significantly more effec-
tive in producing nuclei at warmer temperatures just below freezing. ( From
Sax, Garvey, Parungo, and Slusher, 1977.)


There has been much emphasis on evaluation methodology on the
part of weather modification meteorologists and statisticians, partic-
ularly with regard to precipitation modification. Progress in this

71 Sax. Robert I.. Jack Thomas. Marilyn Bonebrake. "Differences in Evolution of Ice
Within Seeded and Nonseeded Florida Cumuli as a Function of Nucleating Agent." In pre-
prints of the "Sixth Conference on Planned and Inadvertent Weather Modification. " Cham-
paign, 111., Oct. 10-13, 1977. Boston, American Meteorological Society, 1977," pp. 203-205.


area has been slow, owing to the complexity of verification problems
and to inadequate understanding of cloud physics and dynamics.

Having reviewed previous considerations of evaluation attempts,
Changnon discovered a wide variety of results and interpretations,
noting that "a certain degree of this confusion has occurred because
the methods being used were addressed to different purposes and
audiences, and because there has been no widely accepted method of
verification among investigators." 72 He continues :

For instance, if one considers identification of changes in the precipitation
processes most important to verification of modification efforts, then he will
often undertake evaluation using a physical-dynamic meteorological approach.
If he considers statistical proof of surface precipitation changes the best method,
he may concentrate verification solely on a statistical approach or make in-
adequate use of the physical modeling concepts. On the other hand, if the evalua-
tion is to satisfy the public, the consumer, or the governmental decision-maker,
it must be economic-oriented also. Hence, a review of the subject of previous
evaluation methodology must be constantly viewed with these different goals
and concepts in mind. 73

Evaluation methodology for weather modification must deal with
three fundamental problems which Changnon has identified : 74

1. There are many degrees of interaction among atmospheric forces
that result in enormous variability in natural precipitation, greatly
restricting attempts for controlled experiments that are attainable
in other physical and engineering sciences.

2. There is an absolute need to evaluate weather modification with
statistical procedures; this requirement- will exist until all underlying
physical principles of weather modification can be explained.

3. The data used in the evaluation must be sufficiently adequate in
space and time over an experimental region to overcome and describe
the natural variability factors, so that a significant statistical signal
may be obtained within the noise of the variability.

It is further recognized that analysis of weather modification ex-
periments is closely akin to the weather prediction problem, since
evaluation of weather modification efforts is dependent on a com-
parison of a given weather parameter with an estimate of what would
have happened to the parameter naturally. Thus, the better the pre-
diction of natural events, the better can a weather modification proj-
ect be designed and evaluated, at the same time reducing the verifica-
tion time required by a purely statistical approach. 75

Initially, weather modification evaluation techniques used only the
observational or "look and see" approach, improved upon subsequently
by the "percent of normal" approach, in which precipitation during
seeding was compared with normals of the pre-experimental period.
Later, using fixed target and control area data comparisons, regres-
sion techniques were attempted, but the high variability of precipita-
tion in time and space made such approaches inapplicable. In the
mid-1960's there was a shift in sophisticated experiments toward
use of randomization. In a randomized experiment, seeding events
are selected according to some objective criteria, and the seeding
agent is applied or withheld in sequential events or adjacent areas

72 Changnon. Stanley A.. Jr.. "A Review of Methods to Evaluate Precipitation Modifica-
tion in North America." Proceedings of the WMO/IAMAP Scientific Conference on Weather
Modification. Tashkent. U.S.S.R.. Oct. 1-7, 1973, World Meteorological Organization.
WMO— No. 399. Geneva, 1974, p. 397.

73 Ibid., p. 398.

74 Ibid.

75 Ibid.


in accordance with a random selection scheme. An inherent problem
with randomization is the length of experimental time required;
consequently, the approach is not often satisfying to those who wish
to obtain maximum precipitation from all possible rain events or
those who want to achieve results in what appears to be the most
economical manner. As a result, commercial projects seldom make
use of randomization for evaluation, and such techniques are gen-
erally reserved for research experiments. 76

In very recent years the randomization approach, which to many
appeared to be too "statistical" and not sufficiently meteorological
in character, has been improved on through a better understanding
of atmospheric processes, so that a physical-statistical approach has
been adopted. 77

Changnon reviewed approximately 100 precipitation modification
projects in North America and found essentiallv 6 basic methods
that have been employed in project evaluations. He identified these
as (1) direct observation (usually for single element seeding trials),
(2) one-area continuous with no randomization (involving historical
and/or spatial evaluation), (3) one-area randomization, (4) target-
control area comparisons, (5) cross-over with randomization, and
(6) miscellaneous. 78 These methods, along with the kinds of data
which have been used with each, are listed in table 9.



(From Changnon, "A Review of Methods to Evaluate Precipitation Modification in North America," 1974]



precipitation data

elements data

economic data

Direct observation Change in type; duration

of precioitation; areal
distribution (vs. model)

One-area continu- Historical Area-rain regressions;

ous (nonrandom). weekend-weekday

rainfall differences;
frequency of rain

Spatial Area-rain regressions;

pattern recognition;
trend surfaces; rain
rates; raindrop sizes;
frequency of rain
days; rain cell differ-
ences; precipitation
type change; areal
extent of rain.

Target control Area rainfall (day,

month, season) repres-
sions; area snowfall
(day, month, season).
One-area ran- Basically Area precipitation;

domized (hours statistical. plume area precipi-

pulsed). tation: change in pre-

cipitation type. Period
Physical plus precipitation; echo
statistical. area; rain rates; echo
reflectivity; rain

Crossover ran- Area rainfall; zonal

dnmized. rainfall.

Miscellaneous (post

hoc stratifica-

Cloud parameters; echo
parameters; seed and

Frequency of severe Added runoff; crop
weather; frequency yields; ecological,
of smoke days.

Synoptic weather con- Runoff increases; crop
ditions; cloud parame- yields; ecological,
ters; echo parameters;
Agl plums; nuclei
sources; airflow-
plume behaviors;
tracers in rain; atmos-
pheric electrical

Echo parameters Runoff regressions.

Synoptic weather con-
ditions; cloud parame-
ters; seed material in
plumes. Fcho parame-
ters; Agl in rain; cloud
numerical models;
storm behavior;
cloud base rain rate.

Synoptic types and
upper air conditions.

Upper air:

1. Temperature.

2. Winds.

3. Moisture stability

Synoptic weather types.

Water yield; runoff;
ecosystem (plant and
animals) and erosion;
avalanche— disbene-

76 Ibid., p. 399.

77 Ibid., p. 400.

78 Ibid., p. 407.


The direct observation technique was the first major approach to
evaluation and is still used occasionally. In addition to direct observa-
tion of the change and type of precipitation at the surface, the time of
precipitation initiation, and areal distribution following treatment of
a cloud or cloud group, other meteorological elements have been ob-
served ; these include radar echo characteristics, plume of the seeding
material, and cloud parameters (microphysical properties and dynam-
ical and dimensional properties such as updrafts, cloud size, and rate
of growth.). 79

The one-area continuous (nonrandomized) techniques have been
employed to evaluate many of the commercially funded projects in
North America, recent efforts to investigate inadvertent precipitation
modification by large urban-industrial areas, and the statewide South
Dakota seeding program. This category includes the largest number
of projects, and control data for these nonrandomized projects have
included both historical data and data from surrounding areas. The
uncertainty of the control data as a predictor of target data is the basic
problem in using this approach. 80

* Most federally sponsored weather modification projects have used
the one-area randomization method, which involves the use of a variety
of precipitation elements, including duration, number of storms, and
storm days and months. Projects evaluated with this method fall into
two categories, including, as shown in table 9, those using the basic
statistical approach and the more recent physical plus statistical tech-
niques. The latter group of projects have been based on a greater
knowledge of cloud and storm elements, using this information in
defining seedable events and combining it with statistical tests to detect
effects. Surface data, including rainfall rates and area mean rainfall
differences, are used to evaluate such one-area randomized projects. 81

The target-control method involves a single area that is seeded on
a randomized basis and one or more nearby control areas that are never
seeded and, presumably, are not affected by the seeding. 82 The method
had been used in about 10 North American projects through 1974.
Evaluation data have been mostly area rainfall or snowfall regres-
sions, runoff differences, and radar echo parameter changes. 83

The crossover (with randomization) method has been considered
by many to be the most sophisticated of the statistical evaluation
methods. The crossover design includes two areas, only one of which
is seeded at a time, with the area for seeding selected randomly for
each time period. As with the target-control method, a problem arises
in this method in that there is the possibility of contamination of the
control areas from the seeded area. 84 In the single project to which the
method had been applied up to 1974, the evaluation procedure involved
classification of potential treatment events according to meteorological
conditions, followed by area and subarea rainfall comparisons. 85 The

so Ibid., pp. 408-409.

81 Ibid., p. 409. „ . „ T

82 Brier. Glenn W. "Design and Evaluation of Weather Modification Experiments. In
Wilroot N. Hess (editor), "Weather and Climate Modification," New York. Wiley, iy74.

P ' safhangnon. "A Review of Methods To Evaluate Precipitaiton Modification in North
America." 1974. p. 409. , . „' Wil 01A

84 Brier. "Desiern and Evaluation of Weather Modification Experiments. 1974. p. 210.

ssChangnon. "A Review of Methods To Evaluate Precipitation Modification in Nortn
America," 1974, p. 409.


miscellaneous methods in table 9 refer basically to evaluation efforts
that have occurred after but generally within the context of the five
methods mentioned above, and have been largely post-hoc stratifica-
tions of results classified according to various meteorological subdivi-
sions, followed by re-analysis of the surface rainfall data based on
these stratifications. 86

IFrom Changnon "A Review of Methods to Evaluate Precipitation Modification in North America," 1974]


Surface hail data

Meteorological elements Geophysical-economic

Direct observation Cessation of hail; hail Echo parameters; cloud

pattern; hail sizes parameters; Agl in hail.

change; hailstone


One-area continuous Historical Number of hail days


Spatial Number of hail-produc- Radar echo character-
ing clouds/unit time; istics.
hailstreak frequencies;
number of hail days;
rainfall characteristics;
impact energy; loca-
tion of hail vs. total
precipitation area.

Target-control Energy; hail day frequen- Radar echo characteris-

cy. tics.

One-area random- Impact energy; hail day Radar echo characteris-

ization. frequency; hailf all tics; Agl in hail-rain,


Cross-over random- Energy; area of hail; vol- Agl in hail,

ized. ume of hail.

Crop-hail loss (insurance);
insurance ratej.
Crop-hail loss (insurance)

Hail loss (insurance).

Ecosystem (Agl); crop-
loss data.

About 20 projects concerned with hail modification were also ana-
lyzed by Changnon with regard to the' evaluation techniques used. The
five methods used, shown in table 10, include the first five methods
listed in table 9 and discussed above for precipitation modification
evaluation. A comparison of tables 9 and 10 reveals that the evaluation
of rain and snow modification projects uses much less variety of kinds
of data, especially the meteorological elements. The evaluation of hail
projects is largely statistical, owing to the lack of sophistication in the
physical modelling of hailstorms. There has been greater use of eco-
nomic data in hail evaluation, however, than in evaluation of rainfall
projects, due to some extent to the lack of surface hail data in weather
records and the consequent need to make use of crop insurance data. 87

In hail evaluation, the direct observation method has been used to
look at physical effects from seeding individual storms and storm
systems, involving analysis of time changes in surface hail parameters,
radar echo characteristics, and cloud properties. The one-area contin-
uous (non-random) method has been the principal one used in com-
mercial hail projects and in studies of inadvertent urban-industrial
effects on hail, using historical and/or spatial data in the evaluation.
One major data form in these evaluations is the crop-hail loss from
insurance data. The target-control method has made use of hail fall
enerjry, hail-day frequencies, and crop-hail loss as evaluation data. 88

» Ibid.

87 IMd., pp. 412-413.

88 Ibid., p. 413.


The one-area randomization method is the method used in the Na-
tional Hail Research Experiment. 89 Various degrees of randomization
have been used, ranging from 50-50 to 80-20 ; however, the evaluation
data have been similar to those used in other methods. Silver concen-
trations in samples of rain and hail and elsewhere in the ecosystem
have been used as evaluation criteria. The crossover randomized
method of evaluation has also been applied to hail projects, using such
data as areal comparisons of impact energy, area extent of hail, and
total hail volume, noting also the concentrations of seeding material
in the hailstones. 90

A necessary part of any evaluation scheme involves the measurement
or estimation of the amounts of precipitation fallen over a given area
following seeded or control storm events. Such measurement is part of
a more general requirement as well in collecting data for validation
of weather predictions, development of prediction models, compilation
of climatic records, and forecasting of streamrlow T and water resources.
Although the customary approach to precipitation measurement has
been to use an array of rain gages, weather radars have proven to be
useful tools for studying generally the spatial structure of precipita-
tion. Depending on the quality of the onsite radar system calibration,
there have been varying degrees of success, however, in use of this
tool. Often radar and rain gage data are combined in order to obtain
the best estimate of precipitation over a given area. In this arrange-
ment, the radar is used to specify the spatial distribution and the
gauges are used to determine the magnitude of the precipitation. 91
. Exclusive use of rain gauges in a target area in evaluation of con-
nective precipitation modification projects requires a high gauge den-
sity to insure adequate spatial resolution. For a large target area, such
an array would be prohibitively expensive, however, so that weather
radars are often used in such experiments. The radar echos, which
provide estimates of precipitation, are calibrated against a relatively
smaller number of rain gages, located judiciously in the target area
to permit this calibration.

It has been shown that adjusted radar estimates are sometimes
superior to either the radar or the gages alone. Furthermore, the best
areal estimates are obtained using a calibration factor which varies
spatially over the precipitation field rather than a single average
adjustment. Erroneous adjustment factors may be obtained, however,
if precipitation in the vicinity of the calibration gage is so highly
variable that the gage value does not represent the' precipitation
being sampled by the radar. The technique for calculating the adjust-
ment factor typically involves dividing the gage measurement by the
summed rainfall estimates inferred from the radar, to obtain the
ratio, G/E, used subsequently to adjust radar estimates over a greater
area. 92

89 The National Hail Research Experiment is discussed as part of the weather modifica-
tion program of the Natonal Science Foundation, ch. 5, p. 274ff.

90 Changnon, "A Review of Methods To Evaluate Precipitation Modification in North
America," 1974, p. 413.

91 Crane, Robert K., "Radar Calibration and Radar-rain Gauge Comparisons." In pre-
prints of the "Sixth Conference on Planned and Inadvertent Weather Modification," Cham-
paign, 111., Oct. 10-13, 1977. Boston, American Meteorological Society, 1977, p. 369.

92 Klazura, Gerald E., "Changes in Gage/radar Ratios in High Rain Gradients by Varying
the Location and Size of Radar Comparison Area." In preprints of the "Sixth Conference
on Planned and Inadvertent Weather Modification," Champaign, 111., Oct. 10-13, 1977.
Boston, American Meterological Society, 1977, p. 376.


In the evaluation of hail suppression experiments, or measurements
of hailfall in general, there must be some means of determining the
extent and the magnitude of the hail. One technique is to use a net-
work of surface instruments called hailpads. Since single storms can
lay down hail swaths up to 100 kilometers long and tens of kilometers
wide, made up of smaller patches called "hailstreaks," the spacings of
hailpads must be reduced to a few hundred meters to collect quantita-
tive data over small areas. Even over small distances of the order of
1 kilometer, it has been discovered that total numbers of hailstones,
hail mass, and hail kinetic energy can vary by over a factor of 10. 93
Another means of estimating hailfall is through use of crop- damage
studies. Such results are obtained through crop-loss insurance data,
aerial photography of damaged fields, and combinations of these data
with hailpad measurements. 94


The term "extended area effects" refers to those unplanned changes
to weather phenomena which occur outside a target area as a result of
activities intended to modify the weather within the specified target
area. Such effects have also been called by a variety of other names
such as "downwind effects," "large-scale effects," "extra-area effects,"
"off-target effects," and "total-area effects." When the time dimen-
sion is considered, those changes which occur, or are thought to have
occurred, either within the spatial bounds of the target area or in
the extended area after the intended effects of the seeding should
have taken place are referred to as "extended time effects." These
inadvertent consequences are usually attributed either to the transport
of seeding material beyond the area intended to be seeded or the
lingering of such material beyond the time during which it was to be

In a number of experiments there have been indications that an
extended area effect occurred. The present state of understanding does
not permit an explanation of the nature of these effects nor have the
experimental designs provided sufficient information to describe their
extent adequately. The subject is in need of additional study, with
experiments designed to provide more specific data over pertinent
areal and time scales. In recent years two conferences on extended
area effects of cloud seeding have been convened. The first conference,
attended by 18 atmospheric scientists, was held in Santa Barbara,
Calif., in 1971 and was organized by Prof. L. O. Grant of Colorado
State University and by Kobert D. Elliott and Keith J. Brown of
North American Weather Consultants. Attendees at the 1971 seminar
discussed existing evidence of extended area effects, considered the
possible means of examining detailed mechanisms responsible for
the effects, and debated the implications for atmospheric water re-
sources management.

A second workshop was held, under the sponsorship of the National

63 Morgan, Griffith M. and Nell G. Towery. "Surface Hall Studies for Weather Modifica-
tion." In preprints of the "Sixth Conference on Planned and Inadvertent Weather Modi-
fication," Champaign, 111., Oct. 10-13, 1977, p. 384.

»* Ibid.


Science Foundation, at Colorado State University, Fort Collins, Colo.,
Aug. 8-12, 1977. 95 The Fort Collins meeting was attended by 44 partici-
pants, composed of social scientists, observationists, physical scientists,
modellers, statisticians, and evaluators. The group was exposed to a
mass of data from various weather modification projects from all over
the world and proposed to accomplish the following objectives through
presentations, workshop sessions, and general discussions :

Renew the deliberations of the Santa Barbara seminar.

Expand the scope of participation so as to integrate and inter-
pret subsequent research.

Better define the importance of extended spatial, temporal, and
societal effects of weather modification.

Prepare guidelines and priorities for future research direction. 96
Extended area effects have special importance to the nontechnical
aspects of weather modification. From deliberations at the 1977
extended area effects workshop it was concluded that :

The total-area of effect concept adds a new dimension to an already complex
analysis of the potential benefits and disbenefits of weather modification. A speci-
fied target area may have a commonality of interests such as a homogeneous crop
in a farm area or a mountain watershed largely controlled by reservoirs built for
irrigation and/or hydroelectric power generation. Socioeconomic analysis of this
situation is much more direct than the consideration of the total-area of effect
which may well extend into areas completely dissimilar in their need or desire for
additional water. The spatial expansion of the area of effect may increase or de-
crease the economic and societal justification for a weather modification program.
The political and legal consideration may also be complicated by this expansion in
scope since effects will frequently extend across state or national borders. 81

The strongest evidence of extended area effects is provided by data
from projects which involved the seeding of wintertime storm systems.
Statistical analyses of precipitation measurements from these projects
suggest an increase in precipitation during seeded events of 10 to 50
percent over an area of several thousand square kilometers. Some of the
evidence for these effects, based mostly on post hoc analyses of project
data, appears fairly strong, though it remains somewhat suggestive and
speculative in general. 98

Based upon two general kinds of evidence: (1) observational evi-
dence of a chemical or physical nature and (2) the results of large
scale/long-term analyses ; a workshop group examining the extended
area effects from winter orographic cloud-seeding projects assembled
the information in table 11. It should be noted that the quality of the
evidence, indicated in the last column of the table, varies from "well
documented" and "good evidence" to "unknown" and "no documenta-
tion available;" however, the general kinds of extended area and
extended time effects from a number of winter projects are illustrated. 99

95 Brown. Keith J., Robert D. Elliott, and Max Edelstein, "Transactions of Workshop on
Extended Space and Time Effect of Weather Modification," Aug. 8-12, 1977, Fort Collins,
Coio North American Weather Consultants, Goleta, Calif., February 1978. 279 pp.

«* Ibid., pp. 7-9.

67 Ibid., p. 13.

68 Ibid., p. 10.

"Warburton, Joseph A.. "Extended Area Effects From Winter-orographic Cloud Seeding
Projects," report of workshop panel. In Keith J. Brown, et al. "Transactions of Workshop
on Extended Space and Time Effects of Weather Modification," Aug. 8-12, 1977, Fort Col-
lins, Colo. North American Weather Consultants, Goleta, Calif., February 1978, pp. 137-164.



[From Warburton, 19781




Type of effect of effect Area of effect Mechanism

Quality of

Ice crystal anvil production Spatial and
from dry ice seeding of time,
cumulus clouds, Blu3
Mountains, Australia.


Persistence of ice nuclei at
Climax— probably Agl for
days after seeding.

Transport of Agl from Climax Spatial,
generators to 30 km down-

Silver in snow.Sierra Nevada do.

and Rockies— up to 100 km
from generators.

Produced rain
6-12 mm
over 18-hour

nuclei con-

30 N/liter
(-20° C).

4 to 100X

1500 km 2 Cirrus seeding Documentation

and transport needed (is

of crystals available),
from seeding
with C02.

Unknown Unknown Well documented

(is available).

~40 km 2 Transport of Few aircraft

nuclei. observations.

Pressure reductions in seeded
band periods, Santa Bar-

Cirrus shield produced by
airborne seeding, Warra-
gamba, Australia.

Time Max. —2 mb.


Up to 25 per-
cent of
seeded days.

Continuum from

Continuum from


sites < — 1000 km 2 ). 2000 km 2 (l aircraft). Physical trans- port of Agl on hydro- meter's con- taining Agl. Dynamic heat ing. Ice crystal seeding of lower clouds. 5 yr of observa- tions. Fair to moderate documenta- tion. Documentation needed (is available). B. RESULTS OF LARGE-SCALE/LONG-TERM ANALYSES Projection description Type of effect Magnitude of effect Area of effect Quality of evidence Spatial 30 percent > 40-

yr, average, 3
successive yr.

Time; long-term 10 to 40 percent.

Spatial +25 percent.

Victoria, Australia, drought
relief— non-randomized.

Warragamba and other large-
scale experiments — Aus-
tralia decrease in S/NS
ratio wth years of experi-
ment. 1

Israel I— randomized north
and central seeded.

Santa Barbara band seed- do +25 percent (+50

ing— randomized. percent in bands).

Santa Barbara storm seeding do Unknown

of multiple bands.

Time Seed/no seed ratios

of 1.5 to 4 mean
50 percent-in-

Spatial Unknown analysis


35,000 km 2 ; conti-
nuum from seed-
ing sites.

Artifact of analysis..

6,000 km 2 ; conti-
nuum from seed-
ing sites.

3,000 km 2 ; conti-
nuum from seed-
ing sites.


Santa Barbara duration of
seeded/nonseeded bands.

Climax and east to plains of
Colorado using "homo-
geneous" data base deter-
mined by new synoptic

3,000 km 2 ; conti-
nuum from seed-
ing sites.

600 km*; 130 km
east of Climax,
30 to 50 km
south of Denver.

No documentation

Reanalysis needed
avoiding ratios
and double ratios.

Reliable records for

Moderately well


Good evidence.


'Tasmania experiment may confirm artifact.

Examination of data from summertime convective cloud-seeding
projects reveals "more mixed"' results by comparison with data from
wintertime projects, when extended area effects are considered. This
general conclusion accords with the mixed results from evaluations
of convective cloud seeding within the target area. It was concluded
by participants on a panel at the 1977 Fort Collins workshop that,
for summertime convective cloud seeding, there are statistical evi-
dences of both increases and decreases in the extended area, though
there are a large number of nonstatistically significant indications.
Table 12 was assembled by the panel to summarize the characteristics
of these effects for each of the projects examined. 1

1 Smith. T. B.. "Report of Panel on Rummer Weather Mortification." In Keith J. Brown
et al., "Transactions of Workshop on Extended Spare and Time Effects of Weather Modi-
fication." Aug. 8-12. 1077. Eort Collins, Colo. North American Weather Consultants. Goleta.
Calif.. February 1978. pp. 228-326.




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o < 128 It was the general consensus of the 1977 workshop participants that seeding can effect precipitation changes over relatively large areas which extend beyond the typical target area. Such changes can be positive or negative and may be of the same sign as the effect in the designated target area or of opposite sign. For example, among summertime projects considered the Israeli experiment provided sub- stantial evidence for positive effects in the target and in the extended areas (see table 12). Project Whitetop and the Arizona experiment, on the other hand, showed strong evidence of precipitation decreases in the target areas, downwind, and in surrounding areas. The Florida area cumulus experiment (FACE) revealed significant rainfall in- creases in the target area, but seemed to show decreases in surround- ing areas, and the 1969-1972 South Dakota project demonstrated negative seeding effects in the target area and positive effects in ex- tended areas. Of all projects reviewed, however, and in view of all the differing results suggested, the combination of target- and extended- area effects which appears to have the least support is that combina- tion most likely to occur to many lay people, i.e., increases in the tar- get area with compensating decreases in some area "downwind" — the "robbing Peter to pay Paul" analogy. 2 Statistical evidence of extended area and time effects seems to be reasonably common; however, the mechanics causing these effects are not understood. It appears that there may be a number of mech- anisms which come into play, the dominating ones operating under various storm types and seeding techniques. In some projects there is evidence that seeding intensified the storm dynamically through release of latent heat of sublimation. In other cases silver iodide has been transported for distances of 100 kilometers downwind of the seeding area and has persisted for several days in the atmosphere after seeding. Also ice crystals produced from seeding may, in turn, seed lower clouds downwind. 3 With particular regard to extended area or time effects in cumulus seeding experiments, Simpson and Dennis have identified the follow- ing list of possible causes : 1. Physical transport of the seeding agent. 2. Physical transport of ice crystals produced by a seeding agent. 3. Changes in radiation and thermal balance, as for example, from cloud shadows or wetting of the ground. 4. Evaporation of water produced. 5. Changes in the air-earth boundary, such as vegetation changes over land or changes in the structure of the ocean boundary layer following cloud modification. 6. Dynamic effects: (a) Intensified subsidence surrounding the seeded clouds, com- pensating for invigorated updrafts. (b) Advection or propagation of intensified cloud systems which subsequently interact with orography or natural circulations. (c) Cold thunderstorm downdrafts, either killing local convec- tion or sotting off new convection cells elsewhere. sp.rnwn. et nl., "Trnnsnotions of the Workshop on Extended Space and Time Effects of Weather Mortification." 1978, p. 11. ' Ihid.. p. 12. 129 (d) Extended space-time consequences of enhancement or sup- pression of severe weather owing to cumulus modification. (e) Alteration, via altered convection, of wind circulation pat- terns and/or their transports which could interact with other cir- culations, perhaps at great distances. 4 Kecommended research activities to further explore and develop understanding of extended area and extended time effects of weather modification are summarized in the final section of this chapter, along with other research recommendations. 5 APPROACHES TO WEATHER MODIFICATION OTHER THAN SEEDING Nearly all of the techniques discussed earlier for modifying the weather involve some kind of "cloud seeding." The exception is in the case of warm fog dispersal, where attempts to dissipate have also included mechanical mixing or application of heat. While most cloud- seeding techniques involve the use of artificial ice nuclei such as those provided by silver iodide particles, other "seeding" substances, such as dry ice, sodium chloride, urea, propane, and water spray, have been used in certain applications. Clouds have also been seeded with metal- ized plastic chaff in order to dissipate electrical charge build-up and reduce the incidence of lightning. There may also be some promise in future years of beneficially changing the weather, over both large and small scales of time and space, using technologies that are not in the general category of cloud seeding. Indeed, some such schemes have been proposed and there has been research conducted on a number of these possibilities. In the following chapter the effects of man's activities and. some nat- ural phenomena in changing the weather unintentionally will be dis- cussed. While these inadvertent effects may be of general concern and should be studied in view of potential dangers, they should also be understood inasmuch as they may provide valuable clues on how the atmosphere can be more efficiently modified for beneficial purposes. For example, major heat sources judiciously located might be used to affect weather in ways useful to man. Solution of problems which overlap considerations of both weather and energy could be investigated and solved in common by scientists and engineers working in both fields. Such research should be under- way and some practical applications could be forthcoming during the 1980's. Dissipation of supercooled clouds and fog over large and medium-sized cities, which now appears to be technically feasible, may become desirable when solar energy collectors are more common. Ee- duction of radiative losses to space could be facilitated by allowing the clouds to reform at night. It is speculated that this diurnal cycle of operation would tend to weaken inversions that are often associated with fog and low stratus and so tend to alleviate problems of air pollution, though there might be some increase of photochemical effects in the daytime with additional sunlight. 6 Excess heat and moisture from nuclear and other powerplants and from their cooling towers could be usefully employed for generating 4 Simpson and Dennis, "Cumulus Clouds and Their Modification," 19,74, pp. 274-277. 5 See p. 143. 6 Dennis and Gagln, "Recommendations for Future Research In Weather Modification," 1977, p. 79. 130 clouds if the plants are optimally located with regard to water sources and meteorological conditions. The clouds so formed might be used for protection to crops during periods of intense heat or as a shield over a city at night to prevent re-radiation of heat back to space. The clouds might also be seeded subsequently somewhere downwind of the power- plant to enhance precipitation. Recently, Simpson reviewed and summarized the state of research and development of a number of the nonseeding approaches to weather modification which have been proposed. 7 She discusses effects of changes to radiation and to sea-air interface processes : Some expensive, brute force successes have been obtained by burning fuels to clear fogs or even to create clouds. A more ingenious approach is to use solar heat to alter part of the air-surface boundary or a portion of the free atmosphere. Black and Tarmy (1963) proposed ten by ten kilometer asphalt ground coatings to create a "heat mountain"' to enhance rain, or to reduce pollution by breaking through an inversion. Recently Gray, et al. (1975) have suggested tapping solar energy with carbon dust over 100-1,000 times larger areas for numerous weather modification objectives ranging from rain enhancement to snow melt, cirrus pro- duction, and storm modification. The physical hypotheses have undergone pre- liminary modelling with promising results, while the logistics appear marginally feasible. Drawbacks are the unknown and uncontrollable transport of the dust and its environmental unattractiveness. A cleaner way of differentially heating the air appears to be a possible future byproduct of the space program. A Space Solar Power Laboratory is in the plan- ning stages at NASA. Its main purpose is to provide electric power, which will be sent by the space laboratory to the earth's surface. The microwave power will be converted to DC by means of groups of rectifying antennas, which dissi- pate a fraction of the power into heat. Preliminary calculations * * * indicate that the atmospheric effect of the estimated heating would be comparable to that by a suburban area and thus could impact mesoscale processes. Future systems could dissipate much more heat and could conceivably be a clean way to modify weather processes. It is not too soon to begin numerical simulation of atmospheric modifications that later generation systems of this type might be able to achieve. Radiation alteration appears to be a hopeful weather modification approach still lacking a developed technology. A cirrus cover has long been welcomed as natural frost protection when it restricts the nocturnal loss of long-wave radia- tion. More recently, the effect of cirrus in cutting off short-wave daytime radia- tion has been modelled and measured. * * * Artificial simulation of cirrus effects by minute plastic bubbles impregnated with substances to absorb selected wave- lengths received preliminary attention . . . but, to my knowledge has not been pursued. Alteration of the sea-air interface is also a potentially promising weather modification technique, particularly to suppress convection or to mitigate the de- struction by tropical hurricanes. However, the technology in this area may be farther from actual field trials than that in radiation. If methods could be de- veloped to restrict sea-air latent and sensible heat flux, the development from tropical storm to hurricane might be inhibited, while not losing rainfall or other benefits of the system. Presently the monomolecular films which cut down the evaporation from reservoirs do not stay intact in oceanic storm conditions, even if the logistics of their delivery over wide areas ahead of the storm were solved. Logistic obstacles have also impeded implementation of the promising idea of cooling the waters ahead of the hurricane by mixing up the ocean layer above the thermocline. 8 One possible means of achieving the mixing of ocean layers to cool the sea surface, suggested above by Simpson, might be accomplished, 7 Simpson. Joanne, "What Weather Modification Needs." 1977, unpublished, pp. 13–1.". (Most of the needs of weather modification identified In this unpublished paper, but not including her summary of nonseeding approaches, were published in another paper with the same title by Dr. Simpson : preprints of "Sixth Conference on Planned and Inadvertent Weather Modification." Champaign, 111., Oct. 10-13. 1977. Boston, American Meteorological Society. 1977, pp. 304-307. 8 Ibid. 131 at least in part, as a beneficial byproduct of another power source under development — the ocean thermal energy conversion (OTEC) concept. The OTEC plants, located in tropical waters where hurri- canes are spawned and grow, can provide surface cooling and so assist, at least in localized areas, in the abatement of tropical storms and their attendant damages. This is another area of overlap between energy and weather interests where cooperative research and development ought to be explored. Research Needs for the Development of Weather Modification In previous sections of this chapter the rationale and the status of development of the various techniques used to modify several kinds of weather phenomena were summarized and discussed in some detail. Applications of these techniques in both operational and research proj- ects were considered and some measures of the current effectiveness were presented. Among these discussions were a variety of statements, some explicit and some implied, on further research necessary to ad- vance weather modification technology. This section addresses re- search needs more generally and in a more sysf'matic manner. Included are specific requirements and recommendations identified by individual experts and organizations. Recommendations of a policy nature on weather modification research, such as the role of the Federal Government and the organizational structure for managing research, are discussed in chapter 6, which summarizes the recommendations of major policy studies. Current research programs of Federal agencies are discussed in some detail in chapter 5. Research recommendations summarized in this section are primarily concerned with advancing the technology of advertent weather modi- fication intended for beneficial purposes. Research needs in support of other aspects of planned weather modification and on inadvertent modification are included in other chapters on those subjects. In some cases, however, in the following sets of recommendations, research efforts in these other areas are included with those dealing with tech- nology improvement in order to preserve the completeness of the par- ticular set of recommendations. general considerations Peter Hobbs identifies four main phases through which most devel- oping technologies such as weather modification must pass — the estab- lishment of scientific feasibility, engineering development, demonstra- tion projects, and full-scale plant operation. 9 He illustrates these phases in terms of relative expenditures and elapsed time for each in figure 15 and discusses the probable stage of development for weather modification. Noting that some would optimistically place develop- ment of the technology as far along as the dashed line YY, he himself would more cautiously place the progress of weather modification in the vicinity of XX, so that the major task ahead remains as the testing of the scientific feasibility to produce significant artificial modification to the weather. 10 9 Hobbs, Peter V., "Weather Modification ; a Brief Review of the Current Status and Sug- gestion for Future Research." Background paper prepared for the U.S. Department of Com- merce Weather Modification Advisory Board, March 1977, p. 10. 10 Ibid….

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