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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11Cloud seeding does not make clouds or storms from nothing. It disperses particles into suitable clouds that already contain the right conditions, in an attempt to make precipitation form or fall more efficiently. In glaciogenic seeding, silver iodide can help ice crystals form in supercooled cloud water; in hygroscopic seeding, particles are added to liquid clouds to encourage droplets to collide and merge. Whether either approach produces measurable extra rain or snow depends on the cloud and is difficult to establish.
How does cloud seeding make it rain?
Cloud seeding introduces particles into an existing cloud to influence the physical processes that can produce precipitation. The particles may be released from aircraft or ground-based generators, depending on the operation. Seeding cannot supply the water vapor or energy needed to create a storm; it is an attempt to affect what happens inside a cloud that is already suitable.
The two broad approaches target different parts of cloud microphysics: glaciogenic seeding works with ice formation in supercooled or mixed-phase clouds, while hygroscopic seeding targets liquid droplets. Neither guarantees that precipitation will reach the ground.
What silver iodide does in a cloud
Glaciogenic seeding: encouraging ice formation
Some clouds contain supercooled liquid water: droplets that remain liquid below freezing. Silver iodide has a crystalline structure similar to ice, so particles of it can act as ice-nucleating material under suitable conditions. Once ice particles form, they can grow by taking up water from surrounding liquid droplets. If they become large enough, they fall as precipitation. Depending on the cloud and the air below it, that precipitation may reach the ground as snow or melt into rain.
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NOAA’s Atlantic Oceanographic and Meteorological Laboratory describes the process as ice particles growing at the expense of liquid droplets until they become heavy enough to fall. That mechanism explains the intention of seeding, not a promise that a treated cloud would otherwise have produced no rain or that treatment will make it rain.
Hygroscopic seeding: changing liquid-droplet growth
Hygroscopic seeding adds particles to the liquid portion of clouds with the aim of changing droplet sizes and encouraging collision and coalescence. As droplets collide and merge, some may grow large enough to fall. This approach is often associated with convective clouds, but those clouds have complex dynamics and substantial natural variability, making it hard to separate a seeding effect from what would have happened anyway.
Does cloud seeding create clouds or control storms?
No. Seeding acts on suitable clouds that already exist; it cannot create a rain-bearing cloud system or move water vapor into a region. As the World Meteorological Organization (WMO) explains in its 14 June 2025 statement on weather modification, the energy in weather systems is too large for cloud seeding to create rain systems, redirect winds to bring moisture, or eliminate severe weather.
Cloud seeding is a local-to-regional weather-modification technique, not a way to control hurricanes, tornadoes, floods, or severe weather. The WMO says there is no generally accepted evidence supporting tropical-cyclone modification and no demonstrated cloud-seeding method for modifying tornadoes, lightning danger, or floods. An operation taking place before a storm or flood does not, on its own, show that it caused the event; that claim would require evidence that accounts for the broader weather system and natural variability.
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How much extra precipitation can cloud seeding produce?
The U.S. Government Accountability Office (GAO) reported in December 2024 that estimates in the studies it reviewed ranged from 0 to 20 percent additional precipitation. That is a range across studies, not a guaranteed yield, a universal result, or a forecast for a particular operation. GAO also notes that seeding works only when the right kind of clouds are present, limiting opportunities for success.
The most persuasive evidence summarized by the WMO is for wintertime glaciogenic seeding of orographic clouds—clouds formed as air rises over mountains—when supercooled liquid water or mixed-phase conditions are present. The WMO reports both statistical and observational physical evidence of enhancement in these settings, and in some frontal systems containing supercooled liquid water. It says recent research on this specific method has demonstrated an evidence-based causal relationship.
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That evidence does not establish the same effect for every cloud type, climate, or seeding program. The WMO cautions that results depend on natural cloud characteristics and that results from one trial cannot be directly applied to a different environment. In complex convective clouds, natural variation can overwhelm the relatively weak signal; some trials have reported enhancement, while some historical trials did not match their original physical hypothesis.
How to tell whether a seeding program worked
A credible evaluation must distinguish the effect of seeding from natural precipitation and weather variability. The WMO recommends an evaluation design that combines a fair statistical comparison with evidence that the proposed physical process actually occurred.
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- Define suitable events in advance. Set objective criteria for which clouds or weather events qualify, rather than choosing cases after seeing the results.
- Randomize treatment. Treat only some qualifying events, leaving others unseeded as controls, so researchers can compare the two groups.
- Estimate the change and uncertainty. Report the precipitation difference along with its confidence interval, rather than presenting a percentage without its uncertainty.
- Check the physical mechanism. Use observations and physically based secondary analyses to test whether the intended cloud process occurred.
- Constrain models with observations. If a model is used, quantify its uncertainty and use observations to limit the range of plausible simulations.
When comparing programs or claims, look at the cloud type and season, whether supercooled liquid water was present, where and how particles were released, the control comparison, the precipitation outcome and its uncertainty, and any local environmental monitoring. A result reported for one setting is not automatically transferable to another.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Health and environmental considerations
The WMO says published studies have found no significant human-health or environmental impact from silver iodide and other commonly used agents in past operations. GAO likewise reports that the limited studies it reviewed suggest no concern at current levels. These findings are not proof that all quantities or uses are risk-free: the WMO says significantly greater quantities or a new agent should prompt a health and environmental assessment, and downwind, environmental, and ecological effects need further investigation. GAO notes that effects from wider use remain unknown.
Who runs cloud seeding in the United States?
NOAA says it does not modify the weather, fund or participate in cloud-seeding operations, or oversee them. Under the U.S. Weather Modification Reporting Act, people intending to undertake covered weather-modification activity must report it to NOAA at least 10 days beforehand. NOAA tracks those reports but says it has no authority to regulate the activities.
NOAA describes common U.S. operational settings as western mountain basins in winter, where programs aim to support snowpack, and the desert Southwest in summer, where programs aim to replenish reservoirs. The WMO reported operational weather-modification programs in more than 50 countries, including work on fog dispersion, rain and snow enhancement, or hail suppression. That figure reflects information available only through June 2024; the WMO notes that program goals are often difficult to establish confidently.
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