Cloud seeding has been around for nearly 80 years. The first successful experiment took place on November 13, 1946, when a researcher named Vincent Schaefer scattered three pounds of dry ice into clouds over Schenectady, New York, and watched ice crystals form within five minutes. What started as a freezer experiment at General Electric has since grown into a global industry, with programs running on every inhabited continent.
The 1946 Experiment That Started It All
Schaefer’s breakthrough actually began in a laboratory, not in the sky. Working with a small freezer lined with black velvet, he noticed that when he breathed into the cold chamber, his moisture condensed into cloud-like particles. One day, wanting to drop the temperature further, he placed dry ice inside. The air in the freezer instantly filled with ice crystals. He had accidentally discovered that introducing extremely cold material into supercooled water droplets could trigger ice formation on demand.
The leap from freezer to atmosphere came quickly. On that November flight, pilot Curtis Talbot flew a small plane while Schaefer dropped dry ice into a three-mile line of supercooled stratus clouds. The cloud drops converted to ice crystals within five minutes, producing snowfall. It was the first time humans had deliberately made precipitation fall from the sky.
Shortly after, the General Electric team (which included Bernard Vonnegut, brother of novelist Kurt Vonnegut) began experimenting with silver iodide as a seeding agent. Silver iodide has a crystal structure remarkably similar to natural ice, which makes it effective at tricking water droplets into freezing around it. Unlike dry ice, silver iodide could be dispersed from ground-based generators rather than requiring an aircraft, making it far more practical. It remains the most widely used seeding material today.
Cold War Era and Government Programs
By the early 1960s, governments were investing seriously in weather modification. In 1961, the U.S. Congress funded Project Skywater, run by the Bureau of Reclamation, to explore whether cloud seeding could increase water supplies in the arid West. The program ran for roughly three decades and produced some encouraging numbers: rainfall increases of 10 to 15 percent in Texas and Arizona, and an estimated 95,000 acre-feet of additional water in Oklahoma. The Bureau’s overall estimate was that seeding had the potential to increase surface water by 10 to 20 percent in favorable areas. Despite those results, no permanent large-scale program was established for the Colorado River basin, and Project Skywater quietly lost its funding around 1990.
The military saw different potential. In October 1966, the U.S. launched a test phase of Operation Popeye over the Laotian panhandle during the Vietnam War. The goal was to extend the rainy season and keep roads muddy enough to disrupt truck traffic supplying North Vietnamese forces. Crews seeded clouds along supply routes to keep the ground near its saturation point through what would normally be the dry season. The program’s existence, once revealed, contributed to international backlash. In 1977, the United Nations adopted a treaty banning the hostile use of weather modification techniques.
Two Main Seeding Methods
Modern cloud seeding falls into two broad categories, depending on the type of cloud being targeted. Glaciogenic seeding is the older and more common method. It works on clouds that contain supercooled water, meaning liquid droplets that exist below freezing but haven’t turned to ice yet. Dispersing silver iodide or dry ice into these clouds gives the droplets something to freeze onto, forming ice crystals that grow heavy enough to fall as precipitation. This approach works best in winter storm clouds, particularly over mountains, where cloud-top temperatures sit around minus 10 to minus 12 degrees Celsius.
Hygroscopic seeding takes a different approach, targeting warmer clouds by introducing salt-based particles that attract water. These particles encourage small droplets to merge into larger, heavier ones that eventually fall as rain. This method works best on continental clouds with high concentrations of tiny droplets, like those found over South Africa and Mexico. Clouds with more maritime characteristics, which already have larger droplets and produce rain efficiently on their own, don’t respond well to this technique.
Where Cloud Seeding Operates Today
China runs the world’s largest weather modification program, spending a reported $2 billion between 2014 and 2021. In December 2020, the Chinese government announced plans to develop the capacity to modify weather over half the country’s land area by 2025. The United Arab Emirates has operated rain enhancement programs since 1990, working with partners including the National Center for Atmospheric Research and NASA. Current UAE research focuses on optimized seeding materials, drone-based delivery systems, and advanced weather modeling.
In the United States, cloud seeding programs operate across more than a dozen western states, primarily to boost winter snowpack in mountain watersheds. Idaho, Utah, Wyoming, Colorado, and California all run active programs. These operations typically target orographic clouds (those formed when air is forced upward over mountain ranges) and aim to add incremental snowfall that feeds reservoirs and rivers during spring melt.
Regulation in the United States
The Weather Modification Reporting Act of 1972 requires anyone conducting cloud seeding in the United States to file reports with the Secretary of Commerce. These reports must be submitted before, during, and after any seeding activity. The Secretary of Commerce maintains a public record of all weather modification activities and publishes periodic summaries. Individual states often have their own permitting requirements on top of the federal reporting rules.
Environmental Safety of Silver Iodide
Because silver iodide has been dispersed into the atmosphere for decades, its environmental accumulation has been studied extensively. The results are reassuring. Silver iodide is extremely insoluble in water, meaning it doesn’t easily release silver ions into the environment. The maximum concentration of free silver that can dissolve from silver iodide particles under standard conditions is about 0.984 parts per billion, which is far below any safety threshold.
Monitoring at cloud seeding target areas in Utah illustrates the point. At Salt Springs Reservoir, which collects runoff from a seeded watershed, the average silver concentration in water samples was less than 0.0005 parts per billion. That’s well within natural background levels and thousands of times below the U.S. drinking water standard of 100 parts per billion. Even the EPA’s more conservative freshwater guideline of 4.1 parts per billion is orders of magnitude higher than what accumulates from seeding operations.
How Effective Cloud Seeding Actually Is
After nearly eight decades of practice, the honest answer is that cloud seeding works, but modestly and inconsistently. The most reliable results come from winter orographic seeding, where silver iodide is dispersed into mountain storm clouds. Historical data from U.S. programs suggests precipitation increases of roughly 10 to 20 percent under favorable conditions. But “favorable conditions” is the key qualifier: you need the right cloud type, the right temperature range, and enough natural moisture to work with. Cloud seeding cannot create rain from a clear sky.
Proving effectiveness has always been the field’s biggest challenge. Weather is inherently variable, making it difficult to say with certainty that a given storm produced more precipitation because of seeding rather than natural fluctuation. Project Skywater found that 20 to 40 percent of winter storms could be successfully seeded and that over half of those yielded higher precipitation, but the program still couldn’t build enough statistical confidence to justify a permanent large-scale operation. That tension between promising results and rigorous proof has defined cloud seeding since Schaefer’s first flight in 1946.