Turns Out, California’s Famous Winter Fog Was Mostly Thanks to Air Pollution

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The fog was responsible for the death of three people back in December 2001 in Elk Grove, California.
The fog was responsible for the death of three people back in December 2001 in Elk Grove, California.
Photo: AP

Google “California tule fog accident,” and you’ll discover how the Golden State’s notoriously thick, ground-level fog has caused a flurry of deadly car accidents in the Central Valley. The silent killer in all this, however, appears to be poor air quality, which can fuel the fog’s creation.

A study recently published in the Journal of Geophysical Research: Atmospheres shows that air pollution is a key contributor to this seasonal phenomenon—and thanks to the passage of the Clean Air Act in 1970, tule fog in this region has declined by about 75 percent since 1980.

That’s great news for the residents of cities like Bakersfield and Fresno who are subject to this air pollution and its accompanying wintertime fog, but, as the study points out, this fog reduction could impact the region’s agriculture industry. Some fruit and nut trees benefit from heavy fog because it keeps the crops cool.


The research team from the University of California at Berkeley looked at different data sets from 1930 to 2016 to reach its conclusions, including fog frequency data from the National Oceanic and Atmospheric Administration, climate data from the National Climatic Data Center, and pollution data from the Environmental Protection Agency. The authors needed to factor in the role of weather and climate fluctuations, so historical records of the region’s temperature, dew point, and wind speeds were key.

As it turns out, these factors can cause short-term fog variability and definitely help the fog to form naturally—but air pollution exacerbates it. The researchers looked at nitrogen oxides, a class of pollutants that can lead to respiratory issues. This pollution can also interact with ammonia from agricultural sources to form particles that water can condense onto. The authors found over 36 years, five days of fog a year were lost for every 10 parts per billion decline of nitrogen oxide throughout the region.

“In order to get fog to form, not only do you need the temperature to go down, but there has to be some sort of seed for water to condense around, similar to how you would have a cloud seed in the atmosphere,” said author Ellyn Gray, a graduate student at Berkeley, in a statement.

Central Valley cities like Bakersfield, Fresno, and Porterville consistently rank among the most-polluted cities in the U.S, according to the American Lung Association. The region’s intense agriculture and fossil fuel industries result in a lot of diesel truck traffic and tractors spewing pollutants into the air. Because the valley is tucked between the California Coastal Range and the Sierra Nevada Mountains, pollution can get trapped. So does the fog it forms.


This study shows that the Clean Air Act has helped clean up our air. Still, while the fog is diminishing, this part of California continues to suffer from dangerous air quality, contributing to some of the highest asthma rates in the state among its heavily black and brown counties.

Car accidents might not be as a big a problem anymore, but the air pollution persists.