CLOUD SEEDING

deliberate introduction into clouds of various substances that act as condensation nuclei in an attempt to induce precipitation. The first experiments with cloud seeding were conducted in 1946; since then seeding has been done from aircraft, rockets, cannons, and ground generators. Many substances have been used, but solid carbon dioxide and silver iodide have been the most effective; when used in supercooled clouds (composed of water droplets at temperatures below freezing), they form nuclei around which raindrops coalesce. Attempts have been made to use these substances to prevent hail.