Swallowed by a layer of chilly fog, many California cities are experiencing their coldest start to December in decades.
San Francisco is running nearly 4 degrees below normal so far this month, based on daily maximum temperatures. It’s the coldest start to December in the city since 2013. It was even chillier in the Central Valley, North Bay and East Bay. Some cities are experiencing their coldest December in half a century.
The cold pattern comes from a tule fog bank that has been sticking around since before Thanksgiving. It started in the Central Valley and subsequently spread westward to the Bay Area. If it weren’t for the fog, the Bay Area and Central Valley would be delightfully warm 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 on land. The Central Valley becomes a fog machine in the winter, especially after rain, like what California experienced last month. Clear, cool nights and calm winds are the perfect recipe.
Tule fog can linger for days or weeks at a time, keeping the areas below it cold.
The cold shows no signs of ending. There is no storm on the horizon to blow away the fog. It may ease around the edges by the end of the week, leading to afternoon sun and slight warming in the North Bay and East Bay as well as San Francisco, but Sacramento and Fresno will likely remain cloudy 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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Where there is no fog, it was sunny and pleasant. Days have been hotter in Tahoe than in Sacramento recently, where the skies are clear. Temperatures are forecast to reach the 60s in South Lake Tahoe this week, threatening December records.
This pattern is known as a temperature inversion, when cold, dense air is trapped beneath warmer air aloft. Without winds to mix the air around or bright sun to burn the fog away (since the angle of the winter sun is low and weak), the pattern holds. Tule fog is rare in the spring, when the sun is higher in the sky. Inversions also trap pollutants near the ground, leading to poor air quality.
While the temperature contrast between the Sierra is the most striking, the coastal locations are also warmer than the valley.
The fog avoided Big Sur, where the average high temperature this month is 65 degrees. Santa Cruz and Monterey are experiencing similar conditions. Even Half Moon Bay was hitting the 60s, warmer than any other Bay Area city to start December. Southern California was even hotter. Los Angeles was forecast to reach the low 80s this week, about 15 degrees above normal.
Tule fog is a staple of California winter weather, but the recent stretch is remarkable. It’s one of the Central Valley’s foggiest spells since January 2011, which ended up even colder with many nights nearing freezing. December 2021 also featured an extended period of tule fog following a wet fall.
This article originally published in Why it’s been so cold in the SF Bay Area – and how long will it last.