Precipitation enhancement, commonly called “cloud seeding,” artificially stimulates clouds to produce more rainfall or snowfall than they would produce naturally. Cloud seeding injects substances into the clouds that enable snowflakes and raindrops to form more easily. Precipitation enhancement is the one form of weather modification done in California. Forms conducted in other states include hail suppression (reducing the formation of large, damaging hailstones) and fog dispersal (when fog is below freezing temperature). (There are some unconfirmed reports of hail suppression attempts in the San Joaquin Valley, using hail cannons, but the scientific basis for this method is dubious.)
Winter orographic cloud seeding (cloud seeding where wind blows over a mountain range, thereby causing clouds and rain or snow by lifting the air) has been practiced in California since the early 1950s. Most of the projects are along the central and southern Sierra Nevada, with some in the Coast Ranges. The projects generally use silver iodide as the active seeding agent, supplemented by dry ice if aerial seeding is done. Silver iodide can be applied from ground generators or from airplanes. Occasionally, other agents, such as liquid propane, have been used. In recent years, some projects have been trying hygroscopic materials (substances that take up water from the air) as supplemental seeding agents.
Most rain and snow enhancement projects are long-term projects that operate in all or most years. A few, such as Monterey County’s project, only ran for one or two seasons. Historically, the number of operating projects has increased during droughts, up to 20 projects in 1991, but has leveled off at about a dozen in wet or normal water years. Most of the agencies or districts doing precipitation enhancement projects suspend operations during very wet years once enough snow has accumulated to meet their water needs.