Tule fog (pronounced too-lee) is a very thick fog that collects in parts of California during the rainy season of the late fall and winter. It is a type of radiation fog, caused by the combination of increased humidity due to the rain and rapid cooling due to the longer nights. This fog makes for very low visibility and is the cause of many accidents every year.
The main area where this fog gathers is in the Central Valley, from Bakersfield in the south to Chico in the north, and sometimes as far west as San Francisco. Tule fog is created because warm air rises. Cold mountain air descends into the valley during the night and becomes trapped due to low air drainage throughout the Central Valley. The cooler temperatures and reduced sunlight of the winter months make fog very slow to burn off, and it can persist for days. The air above is warmer, drier, and lighter, further serving to trap the heavy, humid fog within the valley.
Visibility in tule fog ranges from a high of 600 feet (183 meters) to under 1 foot (30.5 cm). Traffic in zero visibility fog has been the cause of fatal accidents, usually due to multiple vehicle pile-ups. Such disasters are often exacerbated by accompanying weather conditions, including freezing drizzle and black ice, which makes the road slippery but is invisible to drivers.
To stay safe during the tule fog season, people should avoid driving in the Central Valley as much as possible, opting for train service whenever feasible. Those who do have to drive should be aware that safe speeds are well below the posted speed limit when visibility is low. Drivers should use low-beam headlights, as high-beams can reflect back into the car, further reducing visibility. They should also listen carefully to traffic when it's hard to see and be especially wary at intersections, avoiding those with continuous cross traffic whenever possible. Drivers must also follow all instructions of the California Highway Patrol.