What's With All The Fog Lately
- Rex Ballard
- Dec 12, 2025
- 3 min read
Updated: Dec 13, 2025
A massive blanket of dense fog has enveloped much of California's Central Valley, including the Redding area, for over two weeks, marking one of the most prolonged and intense episodes in recent years. This phenomenon, known as tule fog—a type of radiation fog common in the region during winter—has reduced visibility to near zero in spots, lowered temperatures significantly, and raised safety concerns on roadways.
The fog, stretches approximately 400 miles all along California's central valley. From Redding in northern California extending southward through the Sacramento and San Joaquin valleys. The fog forms when moist air near the ground cools rapidly under clear night skies, condensing into thick mist. The fog is then held in place with another layer of cooler air that comes in from the Pacific coast.
This year's event is fueled by an exceptionally wet autumn, with higher than average rainfall in September through November, ranking as the wettest on record in some areas. The saturated soil provides ample moisture. A stable high-pressure system has trapped cool, moist air in the valleys, creating stagnant conditions that prevent the fog from dissipating, even during daylight hours. As a result, cities like Sacramento and Fresno have seen unseasonably cold starts to December, with highs stuck in the upper 40s—about 10 degrees below average—and multiple days of persistent cloud/fog cover.
Compared to the past three years (2022-2024), the 2025 fog episode stands out for its duration and intensity. While those years experienced denser fog than the long-term average due to occasional early rainfall, episodes were typically shorter, lasting around a week or less. Overall, tule fog has declined by nearly half since the 1980s, attributed to warmer winters, drier soils from droughts, and reduced air pollution. However, this year's early heavy precipitation has reversed that trend temporarily, leading to nine dense fog advisories in November alone—the most since 2005.
The last time the Central Valley saw fog of this magnitude was in December 1985, when Fresno endured 16 consecutive days of dense fog and Sacramento logged 17, one of the worst on record. More recent notable events, such as multi-vehicle pile-ups in 1997, 2002, 2007, and 2024, were linked to shorter bursts, though still hazardous.
Safety officials, including the California Highway Patrol (CHP), have reported fog-related accidents during this latest stretch, with crashes on major routes like Interstate 5 attributed to low visibility and drivers failing to reduce speed. While comprehensive 2025 statistics are not yet available, CHP has issued repeated warnings, noting that about three-quarters of fog-related collisions stem from excessive speed. General traffic fatality trends in California show a decline earlier in the year, but the current fog's severity suggests heightened risks in affected areas.
Meteorologists from the National Weather Service predict the fog could linger for several more days, potentially through Christmas. Don't worry, the fog has nothing to do with "Ciimate Change" or rising CO2, its just California doing a California thing. Residents are advised to monitor forecasts closely and exercise caution on the roads. Slow down!
References
[0] An Unrelenting Tule Fog - NASA Science - https://science.nasa.gov/earth/earth-observatory/an-unrelenting-tule-fog/
[1] Why it's been so cold in the S.F. Bay Area — and how long it will last - https://www.sfchronicle.com/weather/article/california-cold-fog-21231083.php
[2] 'Extreme' cloud bank for 16 days and counting in Central Valley - https://www.latimes.com/california/story/2025-12-09/persistent-fog-clouds-central-california-could-bring-record-cool-december
[3] What Is The Radiation Fog Lingering Over California's Central Valley? - https://www.iflscience.com/radiation-fog-a-643-kilometer-belt-of-mist-lingers-over-californias-central-valley-81806
[4] 12/11/2025 NWS Los Angeles What a difference a 100 miles makes ... - https://www.facebook.com/groups/CaliforniaDisasterALERT/posts/1799450827371941/
[5] Fog at Its Worst in Years: Central Valley & California Break Down the ... - https://www.youtube.com/watch?v=TaYgiZVsDWM
[6] California's recent tule fog stretch was the longest in years - MSN - https://www.msn.com/en-us/weather/topstories/californias-recent-tule-fog-stretch-was-the-longest-in-years-but-it-was-far-from-historic/ar-AA1RA6Pe
[7] Why is it so cold? Here's a look at 'radiation fog' and how it's ... - https://abc7news.com/post/bay-area-weather-heres-look-radiation-fog-how-contributing-cold-december-temperatures/18272290/
[8] California drivers may be seeing dense fog — but research says it's ... - https://www.capradio.org/articles/2025/12/10/california-drivers-may-be-seeing-dense-fog-but-research-says-its-becoming-less-common/
[9] Dec. 5: Where's The Sun? A Meteorologist Breaks Down ... - YouTube - https://www.youtube.com/watch?v=U1QJTq5LXAY
