[NASA] An Unrelenting Tule Fog

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3. An Unrelenting Tule Fog

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November 24 – December 9, 2025

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

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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

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 (Moderate Resolution Imaging Spectroradiometer) instrument on NASA’s

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satellite and the

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 (Visible Infrared Imaging Radiometer Suite) on the

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and

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satellites.

The Central Valley is fertile ground for the formation of tule fog, a persistent

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, 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

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on record, California Institute for Water Resources climate scientist Daniel Swain noted on his

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blog. In late November, a very stable high-pressure system developed over the state, which acted like a lid that trapped moist air and confined the fog layer to the valley. With no major storms moving through to disrupt the stratification, the tule fog endured.

Temperatures have been notably

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under the fog layer, in sharp contrast to the rest of the state, which was mostly warmer than normal. Despite the contrast, however, the ambient air mass has been warmer overall, Swain wrote. This may be due in part to warm ocean water offshore and a low Sierra Nevada snowpack sending less cold air downslope, he added.

The warmer overall temperatures could explain why fog has lingered at a slightly higher level—more like

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clouds—at certain times and locations, said Swain. Colder temperatures would be necessary to produce the densest fog near the surface. The somewhat higher cloud in 2025 has differed from past events, when low visibility at ground level has caused

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.

Central California has seen long stretches of cold, socked-in days in the past. In 1985, for example, Fresno experienced

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of dense fog, and

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, according to news reports. Researchers have found, however, that tule fog has been

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in California in recent decades. Foggy days are beneficial for the valley’s fruit and nut trees, which need sufficient rest between growing seasons to be most productive. The fog typically comes with chilly weather that brings on a dormant *******; it also shields trees from direct sunlight that would otherwise warm the plant buds.

NASA Earth Observatory images by Lauren Dauphin, using MODIS data from NASA EOSDIS

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and

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, and VIIRS data from NASA EOSDIS

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,

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, the

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, and the

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(JPSS). Story by Lindsey Doermann.

References & Resources

• Baldocchi, D., and Waller, E. (2014) 
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  . Geophysical Research Letters, 41, 3251–3256.
• NASA Earth Observatory (2020, December 22)
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  . Accessed December 10, 2025.
• National Weather Service
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  . Accessed December 10, 2025.
• The Washington Post (2025, November 29)
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  . Accessed December 10, 2025.
• Weather West (2025, December 6)
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  . Accessed December 10, 2025.
• Weather West, via
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  (2025, December 2)
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  . Accessed December 10, 2025.

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