Tule fog is the thick winter fog that settles over California's Central Valley, and it is one of the most dangerous weather hazards on the state's roads. It can cut visibility to a few feet across hundreds of miles of highway, and it has caused some of the worst chain-reaction crashes in California history. In the Bay Area, tule fog is a smaller concern. It sometimes drifts west into the Delta and the inland valleys on cold winter mornings, but it rarely reaches the coast. Knowing how this fog forms, and how it differs from the Bay Area's own fog, helps explain why winter drives into the valley can turn dangerous so quickly.
For today's conditions, our Bay Area fog forecast lists the dense fog advisories active in the Central Valley, the Delta and the inland Bay Area valleys.
What Makes Central Valley Fog Different
The Central Valley runs more than 400 miles from Redding in the north to Bakersfield in the south. It is broad and flat, with the Sierra Nevada on one side and the Coast Ranges on the other, so it works like a giant bowl. After a winter storm passes, the skies often clear and the wind dies down. On those long, clear nights, the valley floor loses heat quickly, and cold, heavy air pools across the bottom of the bowl.
If recent rain has left the ground wet and the air moist, that cooling chills the air until its water vapor condenses into fog. Because the fog forms right where it sits, it is a type of radiation fog, unlike the coastal fog that blows in from the ocean. The name comes from the tule reeds that once covered the valley's marshes.
The valley's flatness is what makes the fog so dangerous. In hilly country, fog collects in low spots, and a driver can often climb out of it within a few miles. In the Central Valley, the fog can be equally thick for a hundred miles or more, with no high ground to escape to. Interstate 5 and Highway 99 run straight through the heart of it. On November 3, 2007, more than 100 vehicles crashed in dense fog on Highway 99 near Fresno. It was one of many pileups that have struck these highways over the years.
When the fog is at its worst, the California Highway Patrol sometimes runs a pacing program on valley highways. Officers lead lines of traffic through the densest stretches at a steady, slow speed. That gives drivers a set of taillights to follow and keeps anyone from racing ahead into the fog. Schools in the valley also use fog delay schedules, starting classes later on mornings when buses cannot run safely.
How Tule Fog Reaches the Bay Area
The Bay Area connects to the Central Valley through the low, flat Delta and the Carquinez Strait. When a deep layer of tule fog fills the valley, the cold air at its base can slide west through that opening. It spreads across Suisun Bay and into eastern Contra Costa County, so towns such as Antioch, Pittsburg and Fairfield often see the fog first. On some mornings it pushes as far as the northern edge of San Pablo Bay.
The inland valleys of the Bay Area also make fog of their own. The Livermore Valley, Pleasanton and the rest of the Tri-Valley form radiation fog on clear, calm winter nights, much as the Central Valley does on a larger scale. When the valley fog is especially deep, it can also spill west over the low gap at the Altamont Pass and join the local fog.
The North Bay has similar pockets of fog. The Napa Valley floor, the Carneros district and the low ground around Petaluma can sit in thick fog on winter mornings while the surrounding hills are in sunshine. The same clear, calm nights that bring this fog also bring frost. Grape growers in these valleys watch the overnight forecast closely in early spring, when young vine shoots are easily damaged.
How Long Tule Fog Lasts
Bay Area marine layer fog often clears within a few hours of sunrise on summer days, when the sun is high and strong. Tule fog behaves differently, because the midwinter sun is low and the days are short. The bright top of the fog also bounces much of the sunlight back to space. The ground below barely warms, so the fog may thin a little by afternoon but never fully clear before it thickens again that night.
The weather pattern behind the fog also tends to hold steady. A strong area of high pressure keeps the wind calm and traps cold air in the valley beneath a layer of warmer air above. Until a new storm arrives to stir up the air, the pattern can repeat day after day. Fresno, Bakersfield and other valley cities sometimes go a week or more under fog or low gray cloud. On those cold, damp days, the temperature may barely climb out of the 40s.
The fog is less common than it used to be. Studies of Central Valley weather records have found far fewer fog days in recent decades than in the 1980s. Cleaner air with fewer particles for droplets to form on and warmer winter nights are both likely reasons. The fog has not disappeared, though, and dense fog still closes schools and snarls traffic in the valley most winters.
In the Bay Area's inland valleys, fog spells tend to be shorter, often lasting a morning or two before a breeze or a new storm breaks them up. For anyone driving over the Altamont Pass or through the Delta into the Central Valley in winter, the change can be sudden. A clear road can drop into a wall of fog within a mile. Slow down well before you reach it, use your low beams and leave far more room than usual. Those steps are what prevent the chain-reaction crashes the valley is known for.
