How California's Improving Air Quality Is Putting a Dent in the Region's Dangerous Fog

California's Central Valley is known for occasional bouts of tule fog — a heavy, pea soup-like haze that settles into the area from late fall to early spring. The dense fog sometimes covers the San Francisco Bay area, drifting under the Golden Gate Bridge.

The thick fog hovers on the ground instead of drifting in the air like most types of fog. It's named for a type of sedge grass found in California's wetlands. Though eerily picturesque, tule fog can be very dangerous. It's known for causing traffic problems and even closing schools.

Tule fog has been decreasing over the past few decades and researchers at the University of California, Berkeley, wanted to know why. They theorized the change was due to a drop in air pollution levels.

For the study, which was published in The Journal of Geophysical Research: Atmospheres, researchers analyzed Central Valley air pollution and meteorological data going back to 1930. They found fluctuations in fog frequency that coincided with yearly weather patterns. However, long-term fog trends matched the levels of pollution in the air.

"That increase and then decrease in fog frequency can't be explained by the rising temperatures due to climate change that we've seen in recent decades, and that's what really motivated our interest in looking at trends in air pollution," Ellyn Gray, a graduate student in environmental science, policy and management at UC Berkeley and first author on the paper, told Berkeley News.

"When we looked at the long-term trends, we found a strong correlation between the trend in fog frequency and the trend in air pollutant emissions."

Roller coaster fog days

The results help explain why the number of "fog days" in the region have gone up and down. They increased 85% between 1930 and 1970, then dropped 76% between 1980 and 2016. Researchers say this rise-and-fall pattern mirrors air pollution trends in the valley, which rose when the region was farmed and industrialized in the early part of the century, and then began to drop when air pollution regulations were put in place in the 1970s.

The research also explains why the fog is more prevalent in southern parts of the valley, where it should be less common because higher temperatures should suppress its formation.

"We have a lot more fog in the southern part of the valley, which is also where we have the highest air pollution concentrations," Gray said.

The researchers say they now plan to look at the link between air pollution, tule fog and traffic safety in the area.

According to the Federal Highway Administration, fog-related auto accidents average about 25,000 each year, injuring 9,000 and killing nearly 500. As the American Council on Science and Health (ACSH) points out, that mortality rate is more than deaths attributable to heat, floods, lightning and tornadoes combined.

"When I was growing up in California in the 1970s and early 1980s, tule fog was a major story that we would hear about on the nightly news," said Allen Goldstein, a professor in the Department of Environmental Science, Policy, and Management, and in the Department of Civil and Environmental Engineering at UC Berkeley and senior author on the paper.

"These tule fogs were associated with very damaging multi-vehicle accidents on freeways in the Central Valley resulting from the low visibility. Today, those kind of fog events and associated major accidents are comparatively rare."