How Do We ‘Seed’ Clouds To Control The Weather?

Cloud seeding is a process where humans disperse particles (most commonly silver iodide, potassium iodide, dry ice, or salt) into clouds to change how much precipitation falls from them. It is used to boost rainfall and snowpack, reduce hail damage, and disperse fog. Silver iodide concentrations from seeding are very low and studies have generally found minimal environmental or health impact, but its overall effectiveness in adding rainfall is still debated.

Water evaporates from water bodies to form water vapor, a process that has been happening since water first appeared on Earth. These vapors then condense and form a large water droplet. This droplet precipitates when it becomes too heavy due to the effect of gravity. Thus occurs the phenomenon of rainfall. Humans have a history of messing with natural processes, so unsurprisingly, we have discovered a way to manipulate the amount of rain that falls, and can even alter some other weather events, such as hail and fog.

What Is Cloud Seeding?

Cloud seeding is a form of weather modification that changes the amount or type of precipitation that falls from clouds by dispersing substances into the air that serve as nuclei around which ice crystals are formed. The most common use of this technique is to increase precipitation (rain or snow), but hail and fog suppression are also within the domain of possible uses.

The most common chemicals used for cloud seeding include Silver Iodide (AgI), Potassium Iodide (KI) and dry ice (solid Carbon Dioxide (CO2). Liquid Propane (C3H8) expands into a gas at low pressures and has also been used with promising results. After rigorous research, the use of materials that absorb moisture from the air (hygroscopic), such as table salt, is becoming more popular.

How Does Cloud Seeding Actually Work?

Rain and snow do not start from pure water vapor alone. Inside a cloud, water droplets and ice crystals form around tiny floating specks of dust, soot, sea salt, or pollen called condensation nuclei (for droplets) and ice nuclei (for ice crystals). Many clouds are supercooled, meaning they hold liquid droplets at temperatures well below 0 °C (32 °F) because there are not enough good ice nuclei to kick-start freezing. Cloud seeding simply supplies the missing nuclei so the cloud can convert more of its water into precipitation that is heavy enough to fall.

This is why silver iodide (AgI) is the workhorse of cloud seeding. Its crystal lattice has the same hexagonal symmetry and almost the same atomic spacing as ice, so a water molecule cannot easily tell the difference and readily freezes onto its surface. A 2025 study from TU Wien, published in Science Advances, pinned down why: the silver-terminated face of the crystal keeps a hexagonal structure that matches ice almost perfectly, giving water molecules an ideal template to build on. In a supercooled cloud, a seeded crystal grows by pulling water vapor away from the surrounding liquid droplets until it is heavy enough to drop out as snow, often melting into rain on the way down.

Scientists use three broad approaches. Static (glaciogenic) seeding adds silver iodide or dry ice to cold, supercooled clouds (roughly −20 to −7 °C, or −4 to 19 °F) to turn liquid droplets into snow. Hygroscopic seeding works in warmer clouds by spraying salt particles, which soak up moisture and grow into large droplets that collide and merge until they fall as rain. Dynamic seeding goes further, using large amounts of material to release latent heat and pump up the cloud's vertical air currents so it processes more moisture. The seeding agent itself is delivered either from aircraft flying through the cloud or from ground-based generators that burn a silver iodide solution and let the rising air carry the particles up.

How Are Clouds And Fog Dispersed?

The same physics can be run in reverse to clear a cloud rather than wring rain from it. The most common target is fog, which is simply a cloud sitting at ground level, because it grounds flights and closes airports. How you disperse it depends on the temperature of the fog.

Supercooled (cold) fog, made of liquid droplets below freezing, is the easy case. Seeding it with dry ice, liquid propane, or silver iodide converts the droplets into ice crystals that grow, fall out, and leave a cleared hole in the fog. Operators follow tables that set the dose of dry ice against wind speed, fog thickness, and temperature. France's Turboclair system, installed at Paris Orly Airport in 1970, was the first purpose-built airport setup of this kind.

Warm fog, the more common type, sits above 0 °C and cannot be frozen out, so it is harder to budge. Engineers attack it with brute force instead: heating the air to evaporate the droplets, mixing in drier air from above, or spraying hygroscopic particles such as sodium chloride to sweep up the small droplets. The most dramatic example was Britain's wartime FIDO (Fog Investigation and Dispersal Operation), which burned petrol in pipes along runway edges so the heat boiled away the fog long enough for bombers to land. It was credited with helping thousands of aircraft return safely, though at a famously enormous fuel cost.

Does It Really Work Like It’s Supposed To?

Cloud seeding, in spite of being thoroughly explained theoretically, has had some difficulty in being implemented. According to some scientists, you can squeeze out a little more snow or rain in some places under certain conditions, but that’s quite different from a program claiming to reliably increase precipitation on a larger scale. It’s hard to tell for sure that this technique is efficient, because experiments related to weather and climate are very difficult to carry out.

So, if precipitation increases in some areas, we cannot say for certain that the precipitation would have been lower had the seeding not been done. In the 80 years following the first cloud-seeding demonstrations by Vincent Schaefer and Bernard Vonnegut at General Electric in 1946, substantial progress has been made in understanding the natural processes that account for our daily weather. Direct evidence has been slow to come, though. The first unambiguous observation of cloud seeding actually producing snowfall came from the SNOWIE field campaign over Idaho in 2017, published in PNAS in 2020.

Today, dozens of countries run active programs. China runs the world's largest, the United Arab Emirates seeds clouds routinely to boost rainfall in the desert, and several US western states (notably Colorado, Wyoming, Utah, Nevada, and Idaho) seed winter snowpack over the Rockies and Sierra Nevada to top up rivers and reservoirs that supply downstream cities.