Cloud seeding can influence precipitation in some suitable, existing clouds, but it cannot create a storm or control a flood. The clearest causal evidence is for a specific method used in winter clouds over mountains. Evidence that seeding suppresses hail is less conclusive, and no method has been demonstrated to modify floods through cloud seeding.
What cloud seeding can—and cannot—do
Cloud seeding disperses particles into existing clouds in an attempt to affect how precipitation forms. Hygroscopic seeding targets liquid droplets; glaciogenic seeding targets ice crystals. Both depend on having the right cloud conditions already in place. Seeding does not supply the water vapor or large-scale energy that creates a storm.
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The World Meteorological Organization (WMO) explains that the energy in weather systems is too large for seeding to create rain-producing cloud systems, redirect winds to bring moisture into a region, or eliminate severe weather. Its 2025 statement, based on evidence available through June 2024, also says that no demonstrated cloud-seeding methods modify floods, tornadoes, or lightning danger. Read the WMO Statement on Weather Modification.
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Sometimes it may increase precipitation from clouds that are already suitable, but the evidence does not support a universal effect across all clouds, locations, seasons, or operations. WMO identifies the clearest causal evidence in wintertime glaciogenic seeding of orographic clouds—clouds shaped by airflow over mountains. The constrained cloud dynamics in this setting make it more amenable to evaluation than many other weather-modification claims.
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A 2024 U.S. Government Accountability Office (GAO) technology assessment found estimates of additional precipitation ranging from 0% to 20% among the studies it reviewed. That is a spread of study estimates, not a guaranteed increase or a forecast for any particular operation. GAO cautions that effectiveness is difficult to evaluate, reliable information is limited, and seeding can enhance precipitation only when the right kind of clouds are present. See GAO’s 2024 assessment.
Why measuring an effect is difficult
Natural precipitation can obscure the signal that seeding is intended to produce. A 2024 study analyzing radar data from the SNOWIE project found detectable seeding signatures during three intensive operation periods when background natural precipitation was weak. Where natural snowfall was present, clear signatures generally were not detected. The researchers caution that a missing radar signal cannot show whether seeding had no effect or whether natural precipitation masked it. Read the SNOWIE radar-signature study.
Can cloud seeding make hail smaller?
Hail-suppression programs use glaciogenic seeding with the aim of changing how hailstones grow, but the evidence is not settled. WMO notes that some project assessments have reported smaller hail for particular storm classifications. It also says the experiments are not sufficient to convincingly establish the underlying scientific concepts: hailstorms have complex, high-energy dynamics and substantial natural variability. It is therefore not accurate to describe hail reduction as proven or guaranteed.
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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstall| Claim | What the evidence supports | Important limit |
|---|---|---|
| Increase rain or snow | Causal evidence is clearest for wintertime glaciogenic seeding of orographic clouds. | Results in that setting do not establish the effect of every method or operation; GAO’s 2024 review found study estimates ranging from 0% to 20%, not a guaranteed outcome. |
| Reduce hail | Some project assessments reported reductions for particular storm classifications. | WMO says the evidence does not convincingly establish hail suppression as a scientific method. |
| Prevent or cause floods | No cloud-seeding method has been demonstrated to modify floods. | A flood after a seeding operation does not, by itself, show that the operation caused it. |
Can cloud seeding cause a flash flood or a major storm?
Current evidence does not establish that cloud seeding can create a storm system, bring moisture into a region, or cause or prevent flooding. Seeding seeks to influence precipitation processes inside suitable clouds; it does not generate the storm’s large-scale energy. A severe flood occurring after a seeding operation is not enough to establish causation.
Attributing a particular storm or flood to seeding requires event-specific scientific analysis. In complex clouds such as supercells, WMO says the seeding signature is largely masked by natural variability, making it especially difficult to distinguish a seeding effect from what the storm would have produced anyway.
What to check when someone claims seeding caused a storm
- Was there a suitable cloud? Seeding requires appropriate existing clouds; it cannot create the storm or moisture supply.
- What method and target were involved? Evidence from winter mountain clouds should not be treated as proof about hailstorms or other cloud types.
- Was an effect measured, or merely inferred from timing? A storm occurring after an operation does not establish a causal link.
- Could natural variability hide or mimic the signal? Background precipitation and complex storm dynamics can make attribution difficult.
- Are the operation records complete? In the United States, NOAA says the Weather Modification Reporting Act of 1972 requires anyone intending to conduct weather modification, including cloud seeding, to report to NOAA at least 10 days before activity. A 2026 GAO review found the agency’s reporting database incomplete, inconsistent, and unreliable: it identified missing reports and found that over half of sampled reports likely contained errors, including missing required information. Records can help establish what was reported, but they are imperfect evidence of operations. Read GAO’s 2026 review.
What is known about environmental and health effects?
WMO reports that published studies found no significant human-health or environmental impacts from silver iodide and other commonly used seeding agents in past operations. It calls for evaluation if substantially greater quantities or new agents are planned. GAO likewise says the limited recent research it reviewed suggests no concern at current levels, while noting that effects of much wider use remain unknown. These findings do not establish that every agent or any scale of use is risk-free.
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