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Cloud seeding can influence precipitation in some clouds that are already suitable, but it cannot create a storm or make a flood. The strongest causal evidence is for a specific method used in winter mountain clouds. Whether seeding can reliably suppress hail is less settled, and the World Meteorological Organization says 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 effort to affect how precipitation forms. Hygroscopic seeding targets liquid droplets; glaciogenic seeding targets ice crystals. Both depend on suitable cloud conditions. Seeding does not provide the water vapor or large-scale energy needed to create a storm.
The World Meteorological Organization (WMO) explains that weather systems contain too much energy for seeding to create rain-producing cloud systems, redirect winds to bring moisture into a region, or eliminate severe weather. Its 2025 statement on weather modification uses evidence available through June 2024.
Can cloud seeding make it rain or snow?
In a specific setting, the answer is yes: WMO reports an evidence-based causal relationship for wintertime glaciogenic seeding of orographic clouds—clouds influenced by air flowing over mountains. The constrained dynamics of these clouds have made the method more amenable to evaluation. This finding does not establish that all seeding methods work in all clouds, places, or seasons.
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A 2024 U.S. Government Accountability Office (GAO) technology assessment reports estimates of additional precipitation ranging from 0 to 20 percent among the studies it reviewed. That is a spread of study estimates, not a promised outcome or a universal result. GAO also says effectiveness is difficult to evaluate, reliable information is limited, and seeding can enhance precipitation only when the right kind of clouds are present.
Why the effect can be hard to measure
A 2024 analysis of radar observations from the SNOWIE project found detectable seeding signatures during three intensive operation periods when background natural precipitation was weak. When natural snowfall was present, clear signatures generally were not detected. The authors cautioned that a missing radar signature cannot show whether there was no seeding effect or whether natural precipitation masked it. In complex clouds such as supercells, WMO likewise says natural variability can obscure the seeding signal.
Can cloud seeding make hail smaller?
Hail-suppression programs use glaciogenic seeding in an attempt to change how hailstones grow. Some project assessments have reported smaller hail for particular storm classifications, but WMO says the experiments have not convincingly established the underlying scientific concepts. Hailstorms have complex dynamics, high energy, and substantial natural variability, so hail reduction should be treated as an unsettled claim—not a proven or guaranteed result.
| Claim | What the evidence supports | Main limitation |
|---|---|---|
| Increase rain or snow | Causal evidence for wintertime glaciogenic seeding of orographic clouds; reviewed study estimates of additional precipitation ranged from 0 to 20 percent (GAO, 2024). | The finding is specific to suitable clouds and conditions. Estimates are not guaranteed operational outcomes, and measurement is difficult. |
| Reduce hail | Some project assessments report reductions for particular storm classifications. | WMO says the evidence does not convincingly establish the proposed scientific concepts; storm variability complicates attribution. |
| Prevent or modify floods | No demonstrated cloud-seeding method for modifying floods, according to WMO. | Seeding cannot create a storm system or redirect moisture into a region. |
Can cloud seeding cause a flood?
WMO says no methods have been demonstrated to modify floods by cloud seeding. Seeding aims to affect precipitation processes in suitable clouds; it is not a way to generate a storm system or steer moisture into a region. A flood that happens after a seeding operation is not, by timing alone, evidence that the operation caused it.
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Assessing a particular event requires suitable scientific analysis. Natural precipitation and storm variability can make it difficult to separate a seeding effect from the storm’s background conditions. Without event-specific evidence establishing that link, it is not sound to attribute a flood to cloud seeding.
What to check when a seeding operation is linked to a storm
For a specific claim, separate evidence that an operation took place from evidence that it changed the weather. In the United States, these checks can help:
- Look for suitable clouds. Seeding depends on existing clouds with conditions appropriate to the method; an operation alone does not show that those conditions existed.
- Check the exact claim. Evidence about local precipitation enhancement in winter mountain clouds does not establish that seeding created a storm, caused hail, or produced a flood.
- Look for event-specific attribution. Natural precipitation can mask a seeding signal, and storm variability makes simple before-and-after comparisons unreliable.
- Treat public operation reports as incomplete evidence. NOAA says the Weather Modification Reporting Act of 1972 requires anyone intending to conduct weather modification in the United States to report to NOAA at least 10 days beforehand. A 2026 GAO review found NOAA’s reporting database incomplete, inconsistent, and unreliable: reports were missing, and over half of sampled reports likely contained errors, including missing required information.
What is known about safety?
WMO reports that published studies found no significant human-health or environmental impact from silver iodide or other commonly used seeding agents in past operations. It calls for evaluation when substantially greater quantities or new agents are planned. GAO similarly says the limited recent research it reviewed suggests no concern at current levels, while noting that the effects of much wider use remain unknown. These findings do not establish that every agent or scale of use is risk-free.
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