Cloud seeding, a form of intentional weather modification, is the attempt to change the amount or type of precipitation that falls from clouds, by dispersing substances into the air that serve as cloud condensation or ice nuclei, which alter the microphysical processes within the cloud. The usual intent is to increase precipitation (rain or snow), but hail and fog suppression are also widely practiced in airports. This idea was first suggested in 2009 by David Mitchell and William Finnegan of the Desert Research Institute in Nevada. It relies on a rather counter intuitive effect: the warming influence of clouds could be reduced by adding tiny “seed” particles to the upper troposphere that actually encourage the formation of the ice crystals from which the clouds are made.
So how does that work? The ice crystals of clouds generally form spontaneously in moist, cold air. But seed particles, if present in the right concentration, could grab all the water vapour to produce a small number of large ice crystals, preventing the formation of lots of unseeded little ones. This would have two consequences. First, the resulting clouds would be more transparent – just as big blobs of water in oil create a more transparent mixture as salad dressing separates out, compared with the milky, opaque emulsion you get when you shake it into lots of tiny droplets. Thinner clouds absorb less radiation from the warm ground, allowing more to escape into space. Second, clouds made from larger ice particles have shorter lives, because the big crystals sink down through the atmosphere under gravity.