What is Tule Fog?

Niki Foster
Niki Foster

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.

Tule fog collects in parts of California during the rainy season.
Tule fog collects in parts of California during the rainy season.

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.

Traffic in zero visibility fog has been the cause of fatal accidents.
Traffic in zero visibility fog has been the cause of fatal accidents.

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.

Niki Foster
Niki Foster

In addition to her role as a wiseGEEK editor, Niki enjoys educating herself about interesting and unusual topics in order to get ideas for her own articles. She is a graduate of UCLA, where she majored in Linguistics and Anthropology.

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Discussion Comments


According to my dad, tule fog used to be much worse when he was a kid. He always tells me of the time when there was a fog for nearly a month in December one year.

People are pretty much used to the fog now. Yes, there are times when extreme caution is required. I sometimes have to pull over to the side and wait for a while because I fear pile-ups. There have been 100 car pile ups in the past years because of tule fogs.

On the other hand, my uncle is a farmer and he claims that his produce turns out better when there is a lot of fog that year. So there might be benefits to the fog but the disadvantages are still more.


@ZipLine-- What time of day will you be driving through?

I visit that area once a year to see my aunt. From my experience, the fog is very bad in the morning but not so bad at other times of the day. So I think that if you plan in such a way that you are driving through later in the day, it won't be very difficult.

Like the article said though, it also depends on a few other factors. I have had to rely on the headlights of the car in front me and drive really very slowly to avoid accidents. When the fog is that thick, yea, you might have to drive ten miles an hour or less. But that kind of thick fog doesn't last forever. Eventually it get better and you can speed up.


I need to drive near Central Valley next week. I've never been in that area before and I've not come across tule fog before either. How bad is it really? Does it get as bad as the article implies? I don't have to drive ten miles an hour do I?

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