From AMS Glossary
The addition of agents (aerosol, small ice particles) that will alter the phase and size distribution of cloud particles, with the intent of influencing precipitation.
The most frequently used agents are granulated solid carbon dioxide (dry ice), silver iodide aerosol for initiation of the ice phase, and salt (sodium chloride) for initiation of larger cloud droplets. Many other agents (e.g., organic materials, bacteria) have been tested and proposed for use. The intent of cloud seeding is to modify or alter the natural development of the cloud so as to enhance or redistribute precipitation, suppress hail formation, dissipate fog or stratus cloud, or suppress lightning. Cloud seeding may involve different techniques. Particles may be released from the ground, from aircraft, or from rockets. The goal of ice phase cloud seeding is to induce the phase transition from a supercooled water cloud to one composed partially or entirely of ice. The goal of dynamic cloud seeding is to stimulate or enhance vertical air motions in the cloud through increased buoyancy derived from the release of latent heat of freezing. Hygroscopic seeding utilizes hygroscopic salt aerosols that readily condense water and may grow large enough to become centers for coalescence growth of precipitation.