If you’ve noticed California looking like it’s been swallowed by clouds lately, you’re not imagining it. A massive blanket of fog, known as radiation fog, has been hanging over the Central Valley for weeks, stretching nearly 400 miles from Redding to Bakersfield.
The fog began settling in late November and has lasted into December 2025, affecting more than 20 counties. Satellite imagery from NASA and NOAA showed a sea of white filling the valley day after day, occasionally spilling through the Carquinez Strait toward San Francisco Bay.
What is radiation fog?
Despite the name, radiation fog has nothing to do with nuclear radiation. It forms when moist ground cools rapidly after sunset, causing water vapor near the surface to condense into a dense, low-lying cloud. Plus, one of California’s wettest autumns on record created perfect conditions for the fog to settle in.
This fog is also known locally as tule fog, named after the tule reeds that thrive in California’s wetlands.
Why was this year different?
Precipitation levels from September through November 2025 ranked in the top 10 percent historically, according to climate scientist Daniel Swain. Then a strong high-pressure system acted like a lid, trapping moisture in the valley and keeping storms from clearing it out.
While the rest of California saw warmer-than-normal temperatures, the Central Valley stayed cooler beneath the fog. Overall air temperatures were still warmer than average. That warmth sometimes kept the fog higher than usual, more like low stratus clouds than thick ground-hugging fog.
Silver Linings in the Fog
There is a silver lining. Radiation fog helps the Central Valley’s fruit and nut trees enter dormancy and protects buds from too much sun. While commuters may be frustrated, the fog is a vital part of the valley’s agricultural cycle.
Driving Hazards
Dense radiation fog has long been a hazard for drivers, causing major traffic accidents in years past. This year’s fog hasn’t been as severe at street level everywhere, but visibility remains limited, and some locals have reported respiratory irritation. Breaking the fog will require a strong weather system capable of mixing the atmosphere.
For now, Los Angeles is literally living under a cloud. If you’re driving home for the holidays, slow down, use low-beam headlights, and give yourself extra time. At least you can enjoy a front-row seat to one of California’s most iconic weather phenomena.