Tule fog is a thick, ground-hugging radiation fog that forms in California's Central Valley during the cooler months. It is one of the densest fogs in the United States, often cutting visibility to less than 200 feet and sometimes to near zero. For Bay Area residents, it is a familiar hazard on inland commutes and a key part of California's winter weather. To see whether tule fog is out this morning, check the current dense fog advisories and fog map on our Bay Area fog forecast.
What Makes Tule Fog Different?
Unlike the marine layer fog that rolls through the Golden Gate and blankets San Francisco, tule fog is a type of radiation fog. It forms over land when the ground cools rapidly on clear, calm nights, chilling the air just above the surface until moisture condenses into fog. Marine fog, by contrast, forms over the cold Pacific Ocean and is pushed inland by onshore winds.
The distinction matters because the two fog types behave very differently. Marine fog typically sits a few hundred feet above the ground and burns off by midday as the sun warms the air. Tule fog hugs the ground, can persist for days or even weeks during strong inversions, and is far denser at its worst. Visibility in tule fog can drop below 10 feet in extreme cases, making it significantly more dangerous for drivers.
Why Is It Called “Tule” Fog?
The name comes from the tule reeds (Schoenoplectus acutus) that once grew abundantly in the marshes and wetlands of California's Central Valley. Before large-scale agriculture transformed the valley, vast tule marshes stretched from present-day Sacramento south to Bakersfield. The fog that settled into these low-lying wetlands became known as tule fog, and the name stuck even as most of the marshes were drained for farmland.
Ironically, the farm irrigation that replaced those marshes now helps feed tule fog. Moist soil from irrigated fields supplies the surface moisture the fog needs. Human activity has helped keep the fog going even as the marshes it was named for have largely disappeared.
How Tule Fog Forms
Tule fog needs a specific set of conditions to develop, which is why it appears almost exclusively between November and March:
- Clear skies at night: Cloud cover acts like a blanket, trapping heat near the ground. On clear nights, heat radiates freely into space, cooling the surface rapidly.
- Light or calm winds: Strong winds mix the air and keep the cold, moist layer from settling near the ground, so tule fog needs still air to form.
- Moist soil: Recent rain or irrigation supplies moisture that evaporates into the cooling air, and the Central Valley's farmland provides plenty of it through the winter. The first clear, calm nights after rain, with the ground still wet, are prime time for fog, as our guide to why fog follows a storm explains.
- Temperature inversion: A layer of warmer air aloft traps the cold, foggy air near the surface. The Central Valley's bowl-like geography between the Coast Ranges and the Sierra Nevada naturally creates inversions that can persist for days.
Once these conditions align, fog forms in the lowest-lying areas first, often along river channels and old lake beds, then spreads and deepens overnight. By morning, a continuous fog bank can stretch across the entire valley floor, an area roughly 50 miles wide and 400 miles long.
Tule Fog and the Bay Area
Tule fog is mainly a Central Valley fog, but it reaches parts of the Bay Area too. The Carquinez Strait is the narrow water gap where the Sacramento and San Joaquin rivers flow toward San Francisco Bay. It acts as a corridor that lets tule fog spill west into the East Bay.
Communities in the eastern reaches of the Bay Area are most affected. Livermore, Brentwood, and Antioch can experience tule fog conditions during strong events, while Concord and Walnut Creek may see reduced visibility as fog pushes through the strait.
The effect is most noticeable on morning commutes, when tule fog has spilled over Altamont Pass into the valley. Drivers heading east on Interstate 580 can go from clear skies in Dublin to near-zero visibility in Livermore within a few miles. In the evening, the fog can start forming again in the Central Valley and creep back west.
Tule Fog vs. Bay Area Marine Layer
Bay Area residents are used to fog, but the marine layer they usually see behaves nothing like tule fog. The main differences come down to where each forms, when it appears and how long it lasts.
- Origin: Marine fog forms over the cold Pacific Ocean. Tule fog forms over land in the Central Valley.
- Season: Marine fog peaks in summer (June through August). Tule fog is a winter phenomenon (November through March).
- Density: Marine fog usually allows visibility of a quarter mile or more. Tule fog regularly drops visibility below 500 feet and can reach near-zero.
- Duration: Marine fog often burns off by midday, while tule fog can last for days under strong inversions, sometimes for a week or more.
- Altitude: Marine fog often sits above ground level as a stratus layer. Tule fog is ground-level, which makes it more dangerous for travel.
Deep Dive: Why Can Tule Fog Last for Weeks?
Tule fog lasts because winter sunlight is too weak to break it, while every long night rebuilds it. Under a stable high pressure system, warm air aloft keeps a lid on the valley. Until a storm stirs the air, the fog can stay in place day after day.
Why Can't the Winter Sun Clear It?
At Sacramento, the noon sun stands about 28 degrees above the horizon in late December, compared with about 75 degrees in late June. By our calculation, low winter sun delivers only about half as much energy to each square foot of ground at midday. December days are also about five hours shorter than June days.
The fog itself makes matters worse. Its bright white top reflects much of the weak sunlight back to space. What does get through warms the air under the fog only a little. On many afternoons, the fog lifts into a low gray cloud deck instead of clearing, which Weather West, a California weather blog, calls “tule stratus.”
After dark, the fog top radiates heat to the clear sky and cools. That cooling makes the fog deeper and denser by morning. Over a long stretch of calm weather, the nightly gain outweighs the daily loss, and the fog persists.
What Holds the Fog in Place?
The Central Valley is a long trough, walled in by the Coast Ranges on the west and the Sierra Nevada on the east. Under winter high pressure, air sinking from above warms and forms an inversion over the valley. The cold, foggy air below is heavier than the warm air above, so it stays trapped between the mountains.
NASA's Earth Observatory documented an unusually long episode from November 24 to December 9, 2025. Satellite images showed fog filling most of the valley from Redding to Bakersfield, a stretch of about 400 miles. A very stable high pressure system acted as a lid, and soils were wet after a rainy autumn. No major storm came through to stir the air, so the fog kept returning for more than two weeks.
These episodes usually end when a storm arrives. Stronger winds mix the cold air with the warmer air above it, and clouds slow the nighttime cooling. Drier air behind a front can also evaporate the fog. The fog often returns a few days after the storm, once the skies clear and the wet ground cools again.
Safety and Driving in Tule Fog
Tule fog is one of the leading causes of weather-related traffic accidents in California. Multi-vehicle pileups on Central Valley highways are tragically common during fog season. The California Highway Patrol issues fog advisories when visibility drops significantly, and several sections of Interstate 5 and Highway 99 have fog-detection systems that trigger warning signs.
If you run into tule fog while driving, the most important rule is to slow down dramatically. Use low-beam headlights, because high beams reflect off the fog and make it harder to see. Leave extra following distance and be ready for sudden stops. If visibility drops to near zero, pull completely off the road if you can do so safely, because stopping in a travel lane is extremely dangerous.
Is Tule Fog Declining?
Tule fog has become much less common over the past few decades. In a 2014 study in Geophysical Research Letters, UC Berkeley scientists Dennis Baldocchi and Eric Waller analyzed 33 years of satellite images. They found that winter fog events in the Central Valley fell by 46 percent on average over 32 winters. The cause is not settled, and the authors pointed to warmer winters, less farm burning and changes in air pollution as possible factors.
While fewer fog days may seem like good news for drivers, tule fog plays an important role in Central Valley agriculture. Many fruit and nut trees, including almonds, walnuts, and cherries, require winter chill hours to produce well, and fog helps maintain the cold temperatures these crops need. Declining tule fog is one of several climate-related challenges facing California agriculture.
