What is Tule Fog?

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.

Tule fog is a dense ground fog that primarily occurs in the Central Valley of California during the late fall and winter months. It forms when there is a high relative humidity, typically after the first significant rainfall of the season, and subsequent clear nights allow the ground to cool rapidly, condensing the moisture in the air.

Tule fog can drastically reduce visibility, sometimes to less than 100 feet, making driving conditions extremely hazardous. The California Highway Patrol often issues fog advisories or even closes roads to prevent accidents. Motorists are advised to drive with lights on, reduce speed, and maintain a greater following distance to ensure safety.

The main dangers of Tule Fog include transportation accidents due to reduced visibility. It's responsible for some of the most massive and deadly multi-vehicle pileups in California's history. Additionally, it can exacerbate respiratory problems for some individuals and impact agricultural practices by delaying crop ripening and increasing disease pressure.

Tule fog events can last for several days or even weeks. The fog usually forms overnight and can persist until the next afternoon, potentially re-forming again during the following night. The duration is often dependent on weather patterns, with the fog dissipating when a storm system brings wind or warmer temperatures.

Meteorologists can predict the likelihood of Tule fog formation by analyzing weather conditions such as humidity levels, temperature, and wind speed. While precise prediction of its occurrence and density can be challenging, weather models and satellite observations provide valuable tools for forecasting these fog events, allowing for advanced warnings.

To mitigate the impact of Tule Fog, transportation departments use fog detection systems, dynamic message signs, and pace cars to manage traffic flow. In agriculture, farmers may use wind machines to mix warmer air with the cold air near the ground, reducing the conditions that allow for fog formation and protecting crops.