Cloud seeding

Cloud seeding is a form of weather modification, a way of changing 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, but hail and fog suppression are also widely practiced in airports.
Cloud seeding also occurs due to ice nucleators in nature, most of which are bacterial in origin.