Why it’s been so cold in the S.F. Bay Area
Engulfed in a frigid fog, many California cities are experiencing their coldest start to December in decades.
San Francisco is running about 4 degrees below normal so far this month, based on maximum daily temperatures. This is the coldest start to December in the city since 2013. It was even colder in the Central Valley, North Bay and East Bay. Some cities are experiencing the coldest December in the last half century.
The cold pattern is due to a cluster of tulle fog that has been around since Thanksgiving. It started in the Central Valley and then spread westward to the Bay Area. If it weren’t for the fog, the Bay Area and Central Valley would be very hot right now.
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While San Francisco is most famous for its summer fog that blows in from the ocean, tule fog is different: It forms over land. The Central Valley turns into a smog machine during the winter months, especially after rains like those experienced in California last month. Clear, cold nights and calm winds are the perfect recipe.
Tule fog can linger for days or weeks, keeping areas beneath it cold.
There’s no sign of the cold weather ending. There are no storms on the horizon to disperse the fog. Lightening weather toward the weekend could bring afternoon sunshine and a slight warm-up in the North Bay and East Bay as well as San Francisco, but Sacramento and Fresno will likely remain overcast until at least mid-December. It is difficult to predict whether the fog will continue until the end of the month.
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The weather was sunny and pleasant where there was no fog. Days have been warmer in Tahoe lately than in Sacramento, where skies have been clear. Temperatures in South Lake Tahoe are expected to reach the 60s this week, threatening December records.
This pattern is known as temperature inversion, where cold, dense air becomes trapped beneath warmer air above. When there are no winds to stir the surrounding air or strong sunlight to burn off the fog (because the winter sun angle is low and weak), the pattern remains in place. Tule fog is rare in spring when the sun is higher in the sky. Inversions also trap pollutants close to the ground, resulting in poor air quality.
Although the temperature difference between the Sierras is most noticeable, the coastal areas are also warmer than the valley.
Fog has been avoided in Big Sur, where the average high temperature this month is 65 degrees. Santa Cruz and Monterey experience similar conditions. Even Half Moon Bay was reaching the 60s by December, warmer than any other Bay Area city. Southern California was even hotter. Los Angeles was expected to reach the low 80s this week, about 15 degrees above normal.
Tule fog is a staple of winter weather in California, but a recent strain is notable. It’s been one of the Central Valley’s foggiest spells ever since. January 2011It got even colder with many nights approaching freezing. December 2021 also experienced prolonged tulle fog following a wet fall.
This article was first published at: Why is the SF Bay Area so cold and how long will it last?.
