Why are clouds disappearing in coastal Southern California?

Clouds above Los Angeles are vanishing.

A new study recently published in the journal Geophysical Research Letters suggests that summer clouds over Southern California have dwindled as both increasing temperatures and heat-radiated from urban sprawl have driven clouds away.

“Cloud cover is plummeting in southern coastal California,” Park Williams, a bioclimatologist at Columbia University and lead author of the research, said in a statement.

“Clouds that used to burn off by noon or 1 o’clock are now gone by 10 [a.m.] or 11 [a.m.], if they form at all,” Williams said.

The clouds in question are called stratus clouds, which hover around 1,000 to 3,000 feet above the ground, and are often referred to as the marine layer by coastal dwellers. During summer, the researchers found that this type of cloud cover has decreased by between 25 to 50 percent over the last five decades.

Low-level clouds over Los Angeles.

There aren’t many places that keep records of old cloud cover, but researchers were able to gather a rich collection of clouds documented in both big and small Southern California airports dating back to the 1970s. 

But at the end of the day, what do these disappearing clouds mean for the region?

By comparing their trove of cloud cover data to a decades-long history of moisture in Southern California’s vegetation, the study’s authors found a link between less cloud cover and drier ground, which creates an environment ripe for fire danger. Without cloud cover, the ground can dry out more fully under the baking sun.

That said, however, the disappearing clouds don’t seem to be driving meaningful changes in the region’s fire activity today.

“It’s one piece of the fire story in Southern California — but maybe not even a big part of it,” Brandon Collins, a research scientist at UC Berkeley’s Center for Fire Research and Outreach, said in an interview. 

“It’s one piece of the fire story in Southern California”

For instance, the December 2018 fires in Southern California — the largest in the fire-prone state’s history — were predominately caused by hot, dry winds, called Santa Anas, blowing fire over vegetation that had grow thick after a drenching winter and then dried out, effectively making it the perfect kindling. 

“Don’t jump to the conclusion that changing cloud cover had anything to do with the fires last December,” said Collins, who had no role in the study. “That was the Santa Anas.”

“Really, what’s been shown over and over is the defining driver of fires in Southern California is the Santa Ana winds,” he added.

Williams, the study’s lead author, did note that less cloud cover itself didn’t mean more burned land. Year to year, different conditions like rainfall and the location of fire affect how much land ultimately gets burned.

“But the dice are now loaded, and in areas where clouds have decreased, the fires should be getting more intense and harder to contain,” said Williams.

Is climate change contributing to the clouds’ disappearance?

Heat, radiating off of Southern California’s sprawling, asphalt-blanketed land and further exacerbated by rising temperatures, appears to be eating away at these coastal, stratus clouds. 

But warming temperatures haven’t necessarily been shown to decimate these coastal clouds. Rather, the researchers only demonstrate that cloud cover is being lost in the region, and it’s linked to an increased drying out of the land. 

“If you put lots of concrete and buildings on the surface, you do change the properties of the local microclimate,” Daniel Swain, a climate scientist at the University of California, Los Angeles’s Institute of the Environment and Sustainability who had no role in the study, said in an interview. 

“But no one has done a formal study why [clouds are decreasing]. We just know that we do observe this trend — it is happening.”

The aftermath of the Thomas fire, the largest in California' s history, in Ventura, California.

The aftermath of the Thomas fire, the largest in California’ s history, in Ventura, California.

Image: Visions of America/UIG via Getty Images

It’s possible, though, this trend could be driven by increased warming in urban areas, boosted by climate change. 

“Los Angeles and San Diego are definitely both warmer than they used to be, particularly at night and in the warm season,” said Swain. “There is a hint, that might be what’s going on here.”

But it’s certainly not as if increased warming or whatever else has totally driven Southern California’s famous marine layer away. 

“This year has been particularly foggy in May,” said Swain, who noted he was standing outside in a late-May morning fog. 

“It’s the classic May and June gloom,” said Swain. “It’s so characteristic of the climate here.”

“Yet that may be changing in a meaningful way,” he added. “And it’s not totally clear why.”

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