What is Tulle Fog?

Tulle fog accumulates in parts of California during the rainy season.

Tule fog (pronounced too-lee) is a very thick fog that accumulates in parts of California during the wet season of late fall and winter. It is a type of radiation fog, caused by the combination of increased humidity due to rain and rapid cooling due to longer nights. This fog makes visibility very low and is the cause of many accidents every year.

Zero visibility fog traffic has been the cause of fatal accidents.

The main area where this fog is concentrated is in the Central Valley, from Bakersfield in the south to Chico in the north and sometimes as far west as San Francisco. Tulle mist is created as warm air rises. Cold mountain air descends into the valley at night and is trapped due to poor air drainage throughout the Central Valley. The cooler temperatures and reduced sunlight of the winter months make fog very slow to dissipate and can persist for days. The air above is warmer, drier, and lighter, serving even more to trap the damp, heavy fog within the valley.

Visibility in tulle fog ranges from a height of 600 feet (183 meters) to less than 1 foot (30.5 cm). Zero visibility fog traffic has been the cause of fatal accidents, usually due to multiple vehicle pile-ups. These disasters are often exacerbated by weather conditions, including freezing drizzle and black ice, which make the road slippery but invisible to drivers.

To stay safe during the foggy season, people should avoid driving in the Central Valley as much as possible, opting for train service whenever possible. Those who must drive should be aware that safe speeds are well below the posted speed limit when visibility is low. Drivers should use dipped beams as the high beams can reflect back into the car, further reducing visibility. They should also listen carefully to traffic when it is difficult to see and be especially careful at intersections, avoiding those with continuous cross traffic whenever possible. Drivers must also follow all California Highway Patrol instructions.

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