In late November and early December 2025, California’s Central Valley experienced an extended period of tule fog. This dense, low-lying fog is common in the region during colder months. Satellite images from NASA’s Terra satellite and NOAA’s Suomi NPP and NOAA-20 satellites showed fog stretching about 400 miles (640 kilometers) from Redding to Bakersfield. The fog lasted for more than two weeks, from November 24 to December 9, and was visible in daily satellite images.
Tule fog is named after a sedge plant found in the valley’s marshes. It forms when moist air near the ground cools quickly under calm winds, causing water vapor to condense. The Central Valley’s geography, bordered by the Coastal Range and Sierra Nevada, helps trap the fog. Sometimes, the fog moves through the Carquinez Strait toward the San Francisco Bay. In 2025, the fog event followed a wet autumn, with rainfall totals from September through November among the highest on record, according to climate scientist Daniel Swain.
A stable high-pressure system developed over California in late November. This system trapped moist air and allowed the fog to persist. With no major storms to change the weather pattern, the fog stayed in place. This led to cooler temperatures under the fog in the valley, while other parts of the state were warmer than normal. Swain said the overall air mass was warmer than usual, possibly due to warm offshore ocean waters and a low Sierra Nevada snowpack, which reduced cold air flow…