The thick, dense "tule fog" that descends on the Central Valley – and sometimes drifts over to the San Francisco Bay Area – has been decreasing over the last few decades. And this decline in this type of fog matches the long-term downward trend in air pollution, according to a new study from scientists at UC Berkeley.

Tule fog, which hugs the ground instead of floating higher up in the sky, forms when there are clear skies and cooler temperatures, usually after rain storms. It's named after the sedge grass that is found in California's wetlands.

During the study, the results of which were published in The Journal of Geophysical Research: Atmospheres in late March, scientists analyzed Central Valley meteorological and air pollution data going back to 1930. Although yearly fluctuations in fog frequency in the Central Valley followed changes in seasonal weather patterns, the long-term trends matched the levels of pollutants 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 the Department of 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."

The study results helped explain an increase, then decrease, in the number of "fog days" in the region. Fog days increased by 85 percent from 1930 to 1970 and then dropped by 76 percent 1980 to 2016. This upward trend of fog days matches the era when the Central Valley was increasingly industrialized and farmed, and the downward trend matches the era after air pollution started being regulated in the 1970s.

The results also help explain why fog is more frequent in the southern part of the Central Valley – even though it should be less frequent because of how high temperatures normally suppress the formation of fog.

"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 told Berkeley News.

She explained that nitrogen oxides combine with ammonia to create ammonium nitrate particles, which helps cause water vapor to create small fog droplets. But there has been a steady decrease in nitrogen emissions since the 1980s due to air pollution regulations.

Next, the team of scientists are going to study the link between air pollution, tule fog and traffic safety in the Central Valley. The tule fog is infamous for creating dangerous driving conditions when it descends on California roads. In some instances, the fog has even frosted over roadways in the Bay Area.

"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."