June 5, 2015
Mysterious fog rolling in and shrouding whole cities is a well-worn horror trope, creeping us out in everything from Stephen King’s The Mist to the “Silent Hill” video games. What’s scarier, though, is that the opposite is happening in real life: Cities are making fog disappear.
In cities on the Southern California coast, there are fewer foggy mornings than there used to be. Around Los Angeles, for example, the frequency of fog (which is a cloud that sits at or near ground surface) has declined by almost two-thirds in the last six decades. Around San Diego, it’s dropped by 25 percent. Clouds also appear less frequently higher up in the atmosphere. The change has implications for everything from drought to energy use.
“The importance of fog to Southern California ecosystems is what made me want to know whether coastal California has undergone any change … and whether changes should be expected in the future,” climatologist Park Williamssays.
Williams’ interest in clouds started one summer in college, when he was working as an undergraduate researcher in southern Costa Rica, where he watched clouds roll in off the ocean every day and “inundate the mountains.” Later, in grad school at UC Santa Barbara, he studied pine forests on the Channel Islands of the Southern California coast and saw how much fog can matter.
“Those forests probably owe their survival after the end of the last glacial period to the existence of summer coast fog,” he says.
In 2009, he began work on figuring out what makes SoCal’s summer cloud system tick. He and colleagues from the University of California, Oregon State University and NASA’s Goddard Institute for Space Studies gathered detailed meteorological data from 24 airports and airfields around L.A., San Diego, Santa Barbara and the Channel Islands. They combed through records of summer cloud frequency and height going back to 1948, looking for trends and connections to other variables like sea and air temperatures and wind patterns.
The airport records showed conflicting trends.
“Some places saw major decreases in summer cloud frequency while others saw little change,” Park says. Cloud frequency declined across the L.A. and San Diego areas, especially at the lowest heights, indicating that the clouds were being pushed upward. The Channel Islands, meanwhile, saw a slight decrease in overall cloud frequency, but a huge increase in fog. This suggested, says Park, that while large-scale climate processes were at work, something was also happening on a smaller scale, and some local process was changing cloudiness for better or worse at some sites, but not others.