Weather Words: Tule Fog

If you live in California’s Central Valley, the phrase “tule fog” is something you most likely dread. Tule fog is a dense, ground-hugging fog special to this region that occurs during the late fall and winter.

It develops when the perfect recipe comes together: cool nights, moist air from recent rain and a strong temperature inversion that traps that moisture near the surface.

Because the Central Valley is bowl-shaped and surrounded by mountains, the air can’t mix or escape, so the fog settles in and can linger for days at a time. The result is an incredibly thick, almost creamy fog that can drop visibility to just a few feet.

What makes tule fog especially dangerous is how suddenly it forms and how dramatically it limits visibility. It clings to the ground and can vary in thickness from mile to mile, leading to surprise whiteout conditions on highways that drivers can’t see coming. Some of the worst multivehicle pileups in California have happened during tule fog events.

Jennifer Gray is a weather and climate writer for weather.com. She has been covering some of the world's biggest weather and climate stories for the last two decades.