Why a 400-mile long fog bank lingered over California for a week

A fog bank hundreds of miles long in California’s Central Valley reappeared every day for a week and was likely to linger into the weekend.

Nicknamed “Tule fog” after the tule reeds that grow in the damp foothills, the fog is a staple of late autumn/early winter and blankets the San Joaquin and Sacramento valleys. At nearly 400 miles long, the fog bank fills the entire Central Valley from Bakersfield to north of Sacramento, near Red Bluff and Redding.

Tule fog is a type of radiation fog; at night, heat radiates into space, cooling the air. Temperatures drop to the dew point, or the point at which the air is saturated. The result? Moisture in the air condenses, causing a thick blanket of fog.

Consider Calaveras County Airport in the Central Valley. Each day around 8 p.m., fog materializes like clockwork. On Tuesday evening, visibilities dropped from 10 miles to one mile between 8:35 p.m. and 9:35 p.m.; on Wednesday, visibility dropped from 10 miles to a quarter-mile between 7:55 p.m. and 8:55 p.m.; and on Thursday, visibility plummeted from seven miles at 7:15 p.m. to below a quarter-mile at 7:55 p.m. Similar reductions in visibility were expected Friday night.

Heavy rains fell across the Central Valley on Nov. 16-17, totaling 1.16 inches in Hanford, 1.38 inches in Visalia and 1.47 inches in Redding. Much of that moisture has lingered since, trapped in the valley with no large-scale weather systems to dry out the region.

Unless dry air is blown through the valley to erode the moisture, or days of warmth and sunshine emerge to evaporate it, nightly fog will continue.

The fog also remains entrenched in place by an inversion, or an increase in air temperature with height. Chilly, dense air slides down the mountains and into the valley’s bowl-like depression at night. That cold air settles near the valley floor, causing fog. But warmer temperatures are present about 2,500 to 3,000 feet above. That acts as a ceiling of sorts to the lower atmosphere, trapping fog and pollutants below. (If the air stagnates too long, it can also bring about air quality issues.)

The National Weather Service has been plastering the Central Valley with dense fog advisories, warning of a “high transportation risk” on Interstate 5, as well as on Routes 99, 33, 41, 43, 46, 59, 152, 165, 168, 180 and 198.

“If driving, slow down, use your headlights, and leave plenty of distance ahead of you,” the agency wrote.

Fog in California’s Central Valley forms in a different way than the coastal fog that’s well-known in San Francisco. That’s more common in the warm season, and is caused by cool Pacific water chilling the air above.