AS 3 Lecture Screens Cloud seeding is usually attempted in order to increase the amount of precipitation falling. Given that there is a lot of supercooled water droplets in middle latitude clouds, and relatively few naturally occurring ice crystals in these clouds that will grow first by the ice crystal process then by accretion, we try to increase the precip by increasing the number of ice crystals. This is accomplished with a seeding agent, which is a material that acts as ice-forming nuclei . A seeding agent must cause water to freeze into ice.
Silver iodide has a crystalline structure that looks like ice, so it can help freeze water.
The newly formed crystals first grow by the ice crystal process , then they accrete supercooled liquid water and grow into graupel , which may later melt into rain . Excessive numbers of ice crystals deplete the available supercooled water droplets, and the resulting ice particles are too small to fall out of the cloud and survive the trip to the ground without evaporating. It would appear that overseeding clouds causes them to dissipate. We could use that to our advantage if we were trying to suppress the formation of heavy precipitation, such as hail.