Precipitation enhancement, commonly called “cloud seeding” or “weather
modification” is a scientific method to artificially stimulate clouds to produce more
rainfall and snowfall than would be produced naturally. The primary mechanism is to
inject substances, such as silver iodide or ice crystals (cloud seeding agents), into the
clouds, enabling snowflakes and raindrops to form more easily. Various cloud
seeding programs have been operational since the late 1950s. The majority of
programs continue for a number of years during winter months and as conditions
warrant. The following is a summary on previous cloud seeding programs in
California.