In California, tule fog can extend from Bakersfield to Red Bluff. 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.
It is formed when cold mountain air flows downslope into the valley during the night, pooling in the low areas until it fills the valley to the "brim" formed by the Coast Ranges and the Sierra Nevada. This occurs because most areas in the Great Central Valley have little or no air drainage below the level of mountain passes. Because of the density of the cold air in the winter, winds are not able to dislodge the fog and the high pressure of the warmer air above the mountaintops presses down on the cold air trapped in the valley, resulting in a dense, immobile fog that can last for days undisturbed.
Tule fog is a low cloud, usually below 1,000 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 remains longer in the southern and eastern parts of the Central Valley.
Visibility in tule fog 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 may show the fog where agriculture and cities like Sacramento can be seen.
The variability in visibility is the cause of many chain-reaction pile-ups 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 in December 1997. Five people died and 28 were injured. In February 2002, two people were killed in an 80-plus car pile-up on State Route 99 between Kingsburg and Selma. The visibility at the time of the accident was zero. On the morning of November 3, 2007, heavy tule fog caused a massive pile-up that included 108 passenger vehicles and 18 big rig trucks on Northbound State Route 99 between Fowler and Fresno. Visibility was cut to about 200 feet at the time of the accident. There were two fatalities and 39 injuries in the crash. .
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 a disabled car. For reasons which may be obvious to some, but not to others, it is also dangerous to remain in or in front of a 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.