An atmospheric phenomenon occurring over much of California was unmistakable in satellite imagery in late autumn 2025. Fog stretching some 400 miles (640 kilometers) across the state’s Central Valley appeared day after day for more than two weeks in late November and early December. Known as tule (TOO-lee) fog, named after a sedge that grows in the area’s marshes, these low clouds tend to form in the valley in colder months when winds are light and soils are moist.
This animation shows a sprawling blanket of white fog filling most or all of the valley from Redding to Bakersfield between November 24 and December 9, 2025. While the fog mostly remained hemmed in by the Coastal Range and the Sierra Nevada, it sometimes spilled through the Carquinez Strait toward San Francisco Bay. These images were acquired with the MODIS (Moderate Resolution Imaging Spectroradiometer) instrument on NASA’s Terra satellite and the VIIRS (Visible Infrared Imaging Radiometer Suite) on the NOAA-20 and Suomi NPP satellites.
The Central Valley is fertile ground for the formation of tule fog, a persistent radiation fog, in late autumn and winter. It occurs when air near the surface, laden with moisture from evaporation, cools and the water saturates the air. If winds are calm, water droplets accumulate into fog clouds near the ground.
Plenty of water was present in the valley’s soils following a very wet autumn. Across nearly all of central and southern California, precipitation totals from September through November 2025 were among the top 10 percent on record, California Institute for Water Resources climate scientist Daniel Swain noted on his Weather West blog. In late November, a very stable high-pressure system developed over the state, which acted like a lid …