The decrease of tule fog in the Central Valley is linked to a decrease in air pollution, according to a study published by UC Berkeley researchers March 28.
Tule fog differs from the coastal fog experienced in Berkeley, according to study co-author and campus professor of environmental science, policy and management Dennis Baldocchi. It is a radiative, wintertime fog that arises between storms when temperatures are cold enough for the moistures in the air to condense and become tule fog.
Baldocchi said studying tule fog in the Central Valley is important because the fog has effects on the safety and economics of the region. He added that the fog is thick enough to cause more traffic accidents but helps fruit trees keep their buds cool.
“This is a fascinating study because it turns upside down the conventional thinking … that the reduction in tule fog is because of a warmer climate or less tule marshland,” said campus professor of environmental science, policy and management Robert Rhew in an email.
Lead study author and campus doctoral student Ellyn Gray said the main findings of the study showed that tule fog occurrences in the Central Valley increased by 80 percent between the 1930s and 1970s because of more air pollution. From the 1980s to the present day, however, tule fog decreased by 76 percent as air pollution also declined.
According to Gray, air pollution is responsible for tule fog fluctuations between the 1930s and now because air pollution provides condensation nuclei that facilitate the condensation of gaseous moisture.
“This study … makes a strong case that air pollution – especially particulate matter and nitrogen oxides from cars —can increase the frequency of tule fogs,” Rhew said in an email. “Thus, the cleaning up of California air over the last few decades with tighter emissions controls on automobiles could explain why tule fogs are less frequent.”
Gray said this study was built on a previous one authored primarily by Baldocchi, who began studying tule fog when a family member in the Central Valley noticed that tule fog occurrences had diminished in the area. She added that her study differs from Baldocchi’s in that she did not look at the agricultural effects of tule fog, used a wider variety of data to analyze tule fog and tracked the fog rates further back than the 1980s.
In order to conduct the study, Gray added that she had to take into consideration factors including historical and spatial correlation, wind speed, precipitation, sea level, correlations between fog and air pollution and statistical methods to separate long-term variability in tule fog from short-term variability.
“Ultimately, what we have here is a 50 percent decline in air pollution, which is good for the people living there and helps with safety concerns, though it may present challenges to farmers,” Gray said.