(FOX40.COM) — For the last few weeks, the Sacramento Valley has experienced extremely dense fog, leaving residents wanting to know what’s causing it and when it will end.
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According to FOX40 chief meteorologist Adam Epstein, the heavy fog, particularly at night, is caused by air pollution – and it might be around for a while.
Dense fog fills the atmosphere in the Sacramento Valley on Jan. 19, 2026./FOX40
“Pollutants have been combining with fog, sinking into the Valley each night, then getting trapped by a dome of high pressure,” Epstein said. “The air quality has been less than ideal because of it.”
Foggy conditions are prominent in the Central Valley. However, it’s particularly thick around the Sacramento area.
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“So what’s happening is, every night, the cold air is filling the valley and we’re getting that fog to form — and also pollutants are getting trapped at the surface under what’s called an inversion,” Epstein said.
The inversion keeps the colder air below the warmer air. Under those conditions, high pressure pushes down on everything, and because there’s no wind or movement in the atmosphere, it remains still, except for the accumulation of more pollutants in the air.
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“The pollutants accumulate each day, and each day, you have bad air quality,” Epstein said.
‘Mixing’ could get rid of the fog. Mixing is a vertical stream of wind that would essentially clean out the air and improve air quality. A rainstorm could also clear the fog.
Currently, the air quality in the Sacramento Valley is poor.
“Anytime the AQI (air quality index) gets over 150, I think twice about spending too much time outside,” Epstein said. “You might not feel like yourself, but it affects everyone differently.”
The fog is expected to leave once the region gets another storm, which isn’t forecast to happen until the end of January or early February.
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