Cloudseeding is the technique of inducing rain from a cloud, usually by dropping suitable particles into clouds containing
supercooled water in an attempt to cause them to dissipate, modify their structure, or alter the intensity of associated phenomena, such as wind speed or hail.
Cloudseeding got its start in 1946 when Dr. Vincent J. Schaefer, working at the General Electric Laboratory in New York, was involved with research to create artificial clouds in a chilled chamber. During one experiment, Schaefer thought the chamber was too warm and placed dry ice inside to cool it. Water vapor in the chamber formed a cloud around the dry ice. The ice crystals in the dry ice had provided a nucleus around which droplets of water could form inside the chamber.
To encourage the warm rain process, calcium chloride is usually used to provide the nucleus for raindrop formation. For the cold rain process, silver-iodide (introduced from the air or ground) can be used as a nuclei because its structure is very similar to ice crystals. Dry ice introduced (at -80°C) from the air into cloud lowers the air temperature so that (particularly at temperatures below -40°C) some of the supercooled water droplets are converted into ice crystals which then grow by collisions with further droplets
Seeding of tropical cumuli sought to exploit the latent heat released by freezing as well. This strategy of dynamic seeding assumed that the additional latent heat would add buoyancy, strengthen the updrafts, ensure more low-level convergence, and ultimately cause explosive growth of properly selected cumuli.
Other substances, e.g. common salt or fine water droplets, may also be used to encourage coalescence. Natural seeding may be significant in cases where ice crystals from a high 'releaser' cloud (e.g. altostratus or cirrostratus) fall into a supercooled water 'spender' cloud (e.g. nimbostratus) and encourage ice-crystal growth. Most methods of limiting the development of hail rely on cloud seeding using ice nucleants, or with silver oxide. In theory, hail damage can be reduced by 25% through cloud seeding. However, despite many claims no quantifiable results have been produced in attempts to minimize damaging hail.