Come to California for the sunshine? Unfortunately, there are several types of fog that can blanket the state and block out the sun all year round.

In the spring and summer, we’ve got “May gray,” “June Gloom,” “no-sky-July” or “Fogust” – all of which are basically marine layers creating ground fog on California’s coast.

But in the fall and winter, California, and especially the Central Valley, sees tule fog.

According to KGET’s chief weather forecaster Kevin Charette, tule fog forms when ground moisture, usually after a heavy rain, is present and the air begins to cool, saturating the moisture and creating fog from the ground up. Overnight, you see the air cool, saturate and form fog.

This weather phenomenon is characteristic of the Central Valley because… it’s a valley. Charette said the surrounding mountain ranges and high-pressure descending air between them trap the fog in the valley. There is nowhere for the fog to escape, so it lingers.

Tule fog is extremely dense and very difficult for drivers to see through.

“Back in the day [early 2000s] this tule fog would hang around for a week and you couldn’t see more than 500 feet in front of you,” Charette said.

Robert Rodriguez with the California Highway Patrol said it caused several fatal crashes in Bakersfield over the 2021-2022 winter.

While it’s the bane of drivers’ existence, it benefits California crops and farmers in the agriculture-dense Central Valley.

But Charette said over the past 20 years, the tule fog has actually been decreasing for two reasons.

First, there’s a reduction in pollution over the past several decades. A 2019 study from the University of California Berkeley found a strong correlation between tule fog frequency and pollution.

“Changing air pollution is the main driver of the long-term trends in fog frequency because it provides water-attracting particles on which fog droplets form,” according to the study.

Second, there’s population growth and a reduction in farmland.

“We are cutting back farmland and building houses instead,” Charette said.

According to the American Farmland Trust, California loses about 50,000 acres of farmland every year to development. The less open space there is, the less moisture there is to condense, Charette said.

Satellite view of tule fog over the Central Valley.

Charette said Central Valley residents can expect tule fog to begin forming around December or after the season’s first heavy rain.

CHP gave some tips for driving through the fog should you encounter it:

  • Reduce your speed
  • Increase your following distance
  • Drive with lights on LOW beam (if during the day, turn on your headlights, don’t rely on daytime running lights)
  • Avoid crossing traffic lanes
  • Don’t stop on highways except in emergencies
  • Move away from stalled or disabled vehicle
  • Consider postponing your trip until the fog clears