Rainmaking, also known as artificial precipitation or artificial rainfall, is the act of attempting to artificially induce or increase precipitation, usually to stave off drought. According to the clouds’ different physical properties, this can be done using airplanes or rockets to sow to the clouds with catalysts such as dry ice, silver iodide and salt powder, to make clouds rain or increase precipitation, to remove or mitigate farmland drought, to increase reservoir irrigation water or water supply capacity, or to increase water levels for power generation.
Clouds are made of millions of tiny water droplets, but those drops don’t automatically fall as rain or snow. They need something to stick to, like tiny dust particles. If a cloud doesn’t have enough dust then you have these very static, dead clouds that don’t precipitate, don’t produce any water and just keep on moving right through.”
That’s where the silver iodide comes in. It’s the right size and shape to help snowflakes form.
Cloud seeding only works in certain conditions — you have to have clouds already, and it has to be colder than 20 degrees — but over a season, it can make a difference.
Cloud seeding, the main type of intentional weather modification, began in the late 1940s. The basic idea was to use aircraft or rockets to inject silver iodide or another substance into the atmosphere to mimic ice nuclei. The amount of rain or snow a cloud can produce depends on a balance between the number of ice nuclei inside it and the amount of water available to grow around those nuclei. Clouds often lack naturally occurring ice nuclei, so injecting them with silver iodide particles (which are very similar in structure to ice) increases the number of nuclei. This makes the clouds more efficient at generating ice crystals that either fall as snowflakes or melt to produce raindrops, depending on temperatures in and beneath the cloud. Cloud seeding is also used to disperse fog banks near some airports.
A more recent cloud-seeding technique is to use hygroscopic (water-attracting) particles such as potassium/sodium chloride to provide “seeds” for large droplets that fall more quickly, colliding with smaller droplets on the way and accelerating rainfall development.
In 1946. A chemist for General Electric, Schaefer made the first snowstorm in a laboratory freezer. The media predicted that cloud seeding could perform miracles, from dousing forest fires to ensuring white Christmases. But doubts quickly arose about the impact of meddling with nature. Concerns that cloud seeding might “steal” water from an area a cloud is traveling toward—robbing Peter to water Paul, as it were—have been dispelled. Storm clouds continually regenerate and release only a portion of their moisture when they rain, which means you can’t “wring out” all the moisture from one cloud.
Cold-weather seeding is done at the core of snow clouds that can reach altitudes as high as 60,000 feet: Flares filled with tiny flakes of silver iodide are ejected into the clouds’ centers. Silver iodide has a molecular structure similar to that of ice. As the silver particles drift down through the clouds, water gloms onto them as it would to ice, and snowflakes grow.
This method is also routinely used for mitigating hail storms, especially in Canada: When silver iodide particles are injected into a hail-producing storm cloud, there are suddenly more nuclei for the ice to cling to. Smaller ice pellets, or “graupel,” form rather than large hail stones.
Silver iodide in large concentrations can be harmful, but the concentrations found in snowpack after cloud seeding are often so low as to be undetectable. Breed’s NCAR study in Wyoming found that there was less silver iodide in snow and soil samples in areas where clouds had been seeded than there had been before the campaigns—either due to fluctuations in naturally occurring levels of silver iodide or because the extra water released by the seeding flushed the system.
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