Ice crystals in the cloud can be artificially formed if particles of solid carbon dioxide are introduced into it or some “ice-forming” aerosols are dispersed within the cloud.
Solid carbon dioxide (the dry ice) and ice-forming aerosols are called agents for the purpose of weather modification practices. Introduction of such agents into the cloud is called seeding.
The action of such a freezing agent, such as carbon dioxide, for example, consists in sharply decreasing the local air temperature during evaporation of its particles in a super cooled water droplet cloud (solid carbon dioxide evaporates at -79 °C), so that the air becomes heavily supersaturated with water vapor around such particles.
Thus, an enormous number of fine ice crystals is produced, and they stimulate water vapor pumping from water droplets to ice particles.