Cloud seeding is used primarily to increase rainfall and snowfall in areas that need more water. It’s also used to suppress hail, clear fog from airports, and reduce the severity of droughts. Programs in over a dozen countries, including the United States, China, Australia, India, and the UAE, use the technology to squeeze more precipitation out of existing clouds, with studies reporting increases of 10 to 30 percent in targeted areas.
How Cloud Seeding Works
Cloud seeding doesn’t create weather from nothing. It works by introducing particles into clouds that already contain moisture, encouraging that moisture to form into rain or snow faster than it would on its own. The energy involved in weather systems is so massive that no technology can conjure clouds, redirect wind patterns, or eliminate storms. What seeding can do is nudge clouds that are already primed to produce precipitation.
There are two main techniques. Glaciogenic seeding targets cold clouds by dispersing silver iodide particles or dry ice (solid carbon dioxide) into them. These particles act as tiny seeds around which supercooled water droplets freeze into ice crystals, which then grow heavy enough to fall as snow or rain. Hygroscopic seeding takes a different approach, releasing large salt particles into the base of warmer liquid clouds. The salt attracts water droplets, speeding up the process by which small droplets merge into larger ones that fall as rain.
Both methods can be delivered from aircraft flying through or above the cloud, or from ground-based generators positioned upwind so the particles rise naturally into the cloud layer.
Boosting Rainfall in Dry Regions
The most common use of cloud seeding worldwide is increasing rainfall in water-scarce areas. The UAE, which receives less than 120 millimeters (about 4.7 inches) of rain per year, launched an aircraft-based hygroscopic seeding program in 2002. Initially targeting summertime storms along its northeastern mountains, the program expanded to cover the entire country year-round by 2010. Analysis of long-term rain gauge records shows an average 23 percent increase in annual surface rainfall over seeded areas.
Operational programs across Australia, China, India, Israel, South Africa, Thailand, and the United States have recorded precipitation increases of 10 to 30 percent. A review by the U.S. Government Accountability Office found that estimates in the studies it examined ranged from 0 to 20 percent additional precipitation, reflecting the reality that results vary depending on weather conditions, geography, and timing. Seeding doesn’t guarantee rain every time. It improves the odds and volume when suitable clouds are already present.
Building Winter Snowpack
In the western United States, cloud seeding has been used for decades to increase snowpack in mountain ranges. More snow in the mountains means more water flowing into reservoirs during spring and summer melt, which is critical for agriculture, drinking water, and hydroelectric power. Randomized experiments in the Colorado Rockies during the 1960s and in Montana’s Bridger Range in the early 1970s both reported statistically significant increases in snowpack from ground-based seeding.
Wintertime glaciogenic seeding over mountains is the area where the science is strongest. The World Meteorological Organization has noted that recent research in this specific method has demonstrated an evidence-based causal relationship between seeding and increased precipitation, a level of confidence that hasn’t been reached for all types of cloud seeding. Researchers have also found that seeding holds considerable potential in seasons with well-below-normal snowfall, exactly when additional water supply matters most.
Reducing Hail Damage
Hailstorms cause billions of dollars in damage to crops and property each year. Cloud seeding is used in several countries to reduce hail size by introducing extra ice-forming particles into thunderstorms. The idea is that with more particles competing for the available moisture, individual hailstones grow smaller and cause less destruction on the ground.
The evidence here is more mixed. A study of cloud seeding operations in Kansas agriculture found that seeding did reduce hailstone size in target areas but did not significantly decrease overall crop damage from hail or drought. The WMO acknowledges that economic analyses suggest hail suppression programs could deliver significant benefits if successful, but the uncertainties involved make these investments risky. Several countries continue to operate hail suppression programs, treating them as one tool among many for protecting agricultural regions.
Clearing Fog at Airports
Fog at airports causes costly delays and cancellations. Cloud seeding can speed up fog dissipation, particularly in cold fog conditions where temperatures are below freezing. In these cases, dry ice is dispersed into the fog layer, causing supercooled droplets to freeze into ice crystals that settle to the ground, opening up visibility.
Experiments using dry ice seeding have shown that fog can clear roughly 30 minutes after treatment in some conditions. In controlled studies using different amounts of dry ice (500 grams, 1,000 grams, and 1,500 grams), researchers found that larger quantities cleared fog significantly faster. Natural fog dissipation took about 45 minutes, while 1,500 grams of dry ice cut that time to roughly 19 minutes, nearly halving the wait. Warm fog is harder to treat, and this application remains more limited than precipitation enhancement.
Cost Compared to Other Water Sources
One of cloud seeding’s biggest advantages is its price. According to California’s Department of Water Resources, the overall cost of cloud seeding ranges from $20 to $40 per acre-foot of water, depending on the watershed location and seeding method. Some ground-based programs have reported costs as low as $2 to $3 per acre-foot. For context, desalination typically costs $1,000 to $2,500 per acre-foot, and recycled water runs $300 to $1,300. Even at its most expensive, cloud seeding is a fraction of the cost of other water supply options.
That cost advantage explains why so many states and countries invest in cloud seeding despite the scientific uncertainties. When a program adds even a modest percentage of extra precipitation to a large watershed, the resulting water supply can be worth many times the investment.
Environmental Safety of Silver Iodide
Silver iodide is the most commonly used seeding agent, and questions about its environmental impact come up frequently. The key fact is that silver iodide is extremely insoluble in water. The maximum concentration of free silver that can dissolve from silver iodide particles under standard conditions is about 0.98 parts per billion. That’s well below the U.S. drinking water standard of 100 parts per billion and below the EPA’s freshwater acute toxicity guideline of 4.1 parts per billion.
Aquatic life is more sensitive than humans to silver. Some species of trout show adverse developmental effects at concentrations as low as 1.7 parts per billion, and sensitive aquatic plants can accumulate silver and grow poorly at 3.3 to 8.2 parts per billion. But the concentrations that result from cloud seeding fall below even these lower thresholds. Decades of cloud seeding in the western U.S. have not produced detectable environmental contamination in watersheds where programs operate.
What Cloud Seeding Cannot Do
The WMO is clear on this point: technologies that claim to create rain from clear skies, redirect weather patterns to bring moisture into a region, or eliminate severe weather do not have a sound scientific basis. Cloud seeding works within narrow conditions. You need clouds with sufficient moisture, the right temperature profile, and proper timing. It’s a tool for enhancing what nature is already doing, not for controlling the weather. The WMO also distinguishes cloud seeding from climate intervention, noting that weather modification operates on local to regional scales, not global ones.
Scientific proof for some alternative methods, including hail cannons (devices that shoot shockwaves into the air) and ionization towers, is still lacking. These technologies continue to be marketed despite having no demonstrated effect at the cloud scale.