What is the mysterious ‘radiation’ fog blanketing California – and is it dangerous?

The fog pattern is a common one in California’s Central Valley and does not pose any extra risk to human health, researchers say

A massive fog bank that has been blanketing much of California’s Central Valley with low-lying clouds since Thanksgiving time has prompted fears online of a mysterious and harmful “radiation fog,” but scientists say this is a misunderstanding of basic scientific terms and common weather patterns in the region.

“There’s something in the fog that I can’t explain,” a California man said in a recent video as he wiped soot from his truck bumper, in a post by Wall Street Apes, a popular X account.

There is indeed a “radiation fog” over the region, but that term refers to the general radiation of energy, not nuclear radiation. During radiation fog events, or “tule fog” as it’s known in California, named for a native marsh plant, fog forms when the moist ground cools rapidly at night, causing water vapor in the air to condense into thick fog.

A rainy autumn and winter in California, as well as a late November high-pressure system over the state, has further exacerbated this effect, helping create a fog bank that often stretched 400 miles up the center of the state.

Residents described the fog, which may actually be getting less common in the region compared to historical trends, as cold and eerie.

Low-lying fog has blanketed central California for hundreds of miles between late November and December, an example of the region’s regular ‘radiation’ or ‘tule fog’ (NASA)

“It’s like going into a dream stage where you can’t see anything around you,” David Mas Masumoto, a peach farmer in the San Joaquin Valley, told The New York Times. “You feel like you’re in this twilight zone.”

Masumoto added that he can’t remember another time with such thick fog in the last 50 years.

As for the particles that some residents were seeing in the fog, there’s a standard explanation for those too.

“Fog is highly susceptible to pollutants,” Peter Weiss-Penzias, a fog researcher at UC Santa Cruz, told The Los Angeles Times.

The air above California’s heavily agricultural Central Valley can mingle with fog and trap pollutants, which could explain the particles some residents are seeing in heavy fog in recent weeks (AFP via Getty Images)

The Central Valley, home to the state’s key north-south highway and miles of agricultural land, is known for its poor air quality.

“It could be a whole alphabet soup of different things,” Weiss-Penzias added.

The fog, which continued through late this week, is expected to thin out as heavy rains disrupt weather patterns in the state.