CLOUD SEEDING

The process in which stratus clouds or fog made of supercooled liquid water are seeded with small particles of dry ice or with smoke containing silver iodide. The dry ice particles (which are -78.5° C) act to freeze the water droplets into ice crystals by direct contact. Silver iodide provides an artificial ice nucleus owing to its ice-like crystal structure, which encourages freezing at higher supercooled temperatures. Either substance causes a large number of ice crystals to be produced. These ice crystals then grow via vapor deposition while the surrounding cloud droplets evaporate to maintain saturation. The ice crystals grow because the saturation vapor pressure over ice is less than that over water (the Bergeron process).