The rise and fall of tule fog -- the thick, white, low-lying fog that can make winter driving in the Central Valley so harrowing – has a lot to tell us about the effectiveness of anti-pollution laws, a surprising new Berkeley study reports. Examining meteorological and air pollution data dating back to 1930, the researchers found an unmistakable correlation between fog and particulate air pollution. Fog days increased by 85 percent between 1930 and 1970, then dropped by 76 percent between 1980 and 2016. During the upward trend, there was significant growth in farming and industry in the valley, while the drop-off correlated with the beginning of air pollution regulations. The findings help explain why that type of fog forms in the first place, with the droplets forming around pollution particles. "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," says graduate environmental science, policy and management student Ellyn Gray, the study's first author. Environmental science, policy, and management professor Allen Goldstein, the study's senior author, says: "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. ... 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." For more on this, see our press release at Berkeley News.