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Tule fog (pronounced Too-lee) is a thick ground fog that settles in the San Joaquin Valley and Sacramento Valley areas of California's Great Central Valley. Tule fog forms during the late fall and winter (California's rainy season) after the first significant rainfall. The official time frame for Tule Fog to form is from Nov. 1st to March 31st. This phenomenon is named after the tule grass wetlands (tulares) of the Central Valley. Accidents caused by the tule fog are the leading cause of weather-related casualties in California.
Formation
Tule fog is a radiation fog, which condenses when there is a high relative humidity---typically after a heavy rain---calm winds, and rapid cooling during the night. The nights are longer in the winter months, which creates rapid ground cooling, and thereby a pronounced temperature inversion at a low altitude.
In California, tule fog can extend from Bakersfield to Chico. Tule fog occasionally drifts as far west as the San Francisco Bay Area, even drifting westward out the Golden Gate, opposite to the usual course of summertime ocean fog.
Tule fog is a low cloud, usually below one thousand feet in altitude and can be seen from above by driving up into the foothills of the Sierra Nevada to the east or the Coast Ranges to the west. Above the cold, foggy layer, the air is typically warm, dry and clear. Once tule fog is formed, turbulent air is necessary to break through the temperature inversion layer. Daytime heating sometimes evaporates the fog in some areas, although the air remains chilly and hazy below the inversion. Tule fog usually sticks around the southern and eastern parts of the Central Valley longer.
Visibility
There is not much one can see in tule fog. Its visibility is usually less than an eighth of a mile (about 600 feet), but can be less than 10 feet. Visibility can vary rapidly; in only a few feet visibility can go from 10 feet to near zero. Satellite and overhead photos of the San Joaquin Valley show the fog where agriculture and cities like Fresno or Sacramento can be seen.
These variable conditions with sometimes very dense fog is the cause of many chain reaction pileups on roads and freeways. In one such accident on Interstate 5 near Elk Grove south of Sacramento, 25 cars and 12 big rig trucks collided inside a fog bank on December 1997. Five people died and 28 were injured. Also in February 2002, two people were killed in a 80-plus car pile up on California State Route 99 in Selma. The visibility at the time of the accident was zero.
Safety tips
Tule fog can make driving very difficult, especially along the Interstate 5 freeway and the freeway/divided highway California State Route 99, two of the main roads through the valley. Interstate 5 can be considered a speedway, where many of the drivers exceed the speed limit. With the greatly reduced visibility resulting from the tule fog, safe driving speeds are often much less the posted speed limit. When possible, drivers should try to postpone their trip until the fog has lifted.
A driver must remember that horizontal visibility is limited just as much as vertical visibility. A driver should use his or her car's headlights to see ahead. Using low beam headlights is highly recommended, as the fog reflects high beam headlights back at the driver, reducing visibility. A driver should drive at a safe speed, which is usually below the speed limit. If a deadline is to be met, a driver should leave earlier to allow time to safely get to his or her destination.
Because of the low visibility, a driver should listen for traffic that cannot be seen. A driver must stay patient. If possible, a driver should avoid intersections where cross traffic does not stop (there are many of those intersections in the Central Valley). If there is an emergency, and a driver needs to pull his or her vehicle over, then he or she must be careful to pull the vehicle completely off the road and use flares to alert other drivers.
When pulling off the road at night and in extremely poor visibility, leaving the lights or parking lights on once the vehicle has been pulled over is dangerous as drivers often use the tail lights of the car they are following to help them determine where the road is and they may steer off the road in an attempt to stay lined up behind your car. For obvious reasons, it is also dangerous to remain in or in front of your car.
As always, one should be sure to heed instructions of the California Highway Patrol, especially if they are providing escorts. One must always read and obey the overhead traffic and weather information on signs along California highways.
If feasible, one should consider riding Amtrak, as trains have a signaling system that is not affected by the fog.
External links
- Page on tule fog from the National Oceanic and Atmospheric Administration (NOAA)
- Forecasting Radiation Fog University Corporation for Atmospheric Research
"Tule fog"