Title page

Contents

Introduction 9

1. Background 11

1.1. What is cloud seeding? 11

1.2. Cloud seeding in the U.S. 14

1.3. Cloud seeding in other countries 15

1.4. Current and emerging cloud seeding technologies 15

1.5. Selected agency activities and relevant laws 16

2. Cloud Seeding May Have Benefits, but Estimating Effectiveness Is Challenging 19

2.1. Effective cloud seeding may result in multiple benefits 19

2.2. Estimates of effects and limitations to evaluating cloud seeding 20

3. Challenges to the Development and Use of Cloud Seeding 24

3.1. Lack of reliable scientific information 24

3.2. Uncertainty around unintended effects 26

3.3. Data availability, perception, and other challenges 27

4. Policy Options to Help Enhance Benefits or Address Challenges 29

5. Agency and Expert Comments 33

Appendix I: Objectives, Scope, and Methodology 34

Appendix II: Expert Participation 38

Appendix III: Related GAO Products 39

Appendix IV: International Cloud Seeding Activities 40

Appendix V: GAO Contact and Staff Acknowledgments 43

Table 1. Examples of the categories of cloud seeding technologies 16

Table 2. Weather modification requirements identified in states with active cloud seeding programs 18

Table 3. Policy option - Maintain status quo 29

Table 4. Policy option - Encourage targeted research to reduce uncertainty 30

Table 5. Policy option - Support more evidence-based operations 30

Table 6. Policy option - Improve monitoring and oversight 31

Table 7. Policy option - Expand education and science-based outreach 32

Table 8. International cloud seeding activities by country, 2020-2024 40

Figure 1. Cold season cloud seeding over mountains 12

Figure 2. Warm season cloud seeding 12

Figure 3. Map of states with active cloud seeding programs in 2024 14

Figure 4. Selected potential benefits of cloud seeding 20

Figure 5. Gaps in NOAA weather radar coverage at or below 4,000 feet above ground level, in the western U.S., and key causes 25