Tule fog

Tule fog /ˈtuːliː/ is a thick ground fog that settles in the San Joaquin Valley and Sacramento Valley areas of California's Great Central Valley. Tule fog forms from late fall through early spring (California's rainy season) after the first significant rainfall. The official time frame for tule fog to form is from November 1 to March 31. This phenomenon is named after the tule grass wetlands (tulares) of the Central Valley. Tule fog is the leading cause of weather-related accidents in California.

Tule fog is a radiation fog, which condenses when there is a high relative humidity (typically after a heavy rain), calm winds, and rapid cooling during the night. The nights are longer in the winter months, which allows an extended period of ground cooling, and thereby a pronounced temperature inversion at a low altitude.

In California, tule fog can extend from Bakersfield to Red Bluff, over 650 kilometres (400 mi) distant. Tule fog occasionally drifts as far west as the San Francisco Bay Area via the Carquinez Strait, and can even drift westward out through the Golden Gate, opposite to the usual course of the coastal fog.

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