Tule fog is a thick ground fog that settles in the San Joaquin Valley and Sacramento Valley areas of California's Great Central Valley during the state's rainy season. It forms when there is a high relative humidity (typically after a heavy rain), calm winds, and rapid cooling during the night, and is typically confined to the Great Central Valley region by the surrounding mountain ranges. Tule fog, a low cloud usually below 2,000 feet (610 m) in altitude, is highly immobile and can last for weeks if undisturbed. This fog is the leading cause of weather-related deaths in California.Photograph: Jeff Schmaltz/NASA