The elevated summer temperatures in California are causing decreased levels of the low-atmosphere clouds that were once common throughout the southern coastal regions of the state. A new study has found that because these clouds are dissipating from the increased heat, the region is now facing an increased risk of wildfire.

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Smoke from forest fire rising behind LA skyline

“Clouds that used to burn off by noon or 1 o’clock are now gone by 10 or 11, if they form at all,” bioclimatologist and study lead author Park Williams told Due to a warming climate and an expanding urban heat island, cloud cover is trapped in a positive feedback loop where less clouds mean higher temperatures, and higher temperatures mean less clouds.

Published in the journal Geophysical Research Letters, the new study concludes that there has been a 25 to 50 percent decrease in low-lying summer clouds since the 1970s. “Cloud cover is plummeting in southern coastal California,” said Williams, “and as clouds decrease, that increases the chance of bigger and more intense fires.” The low-lying stratus clouds in the area typically form in the early morning in a thin, wet layer of coastal air that exists between land and drier air masses. The increased heat from climate change and the urban heat island effect has caused the clouds to dissolve earlier in the day, leaving little cover during the hottest parts of the afternoon.

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To study the changes in cloud cover, Williams and his team analyzed hour-by-hour cloud data gathered by California airports over the past several decades. The data was then compared with vegetation moisture data from the U.S. Wildland Fire Assessment System. This comparison enabled the team to conclude that the decreased cloud cover has led to an increased wildfire risk. “Even though the danger has increased, people in these areas are very good at putting out fires, so the area burned hasn’t gone up,” Williams explained. “But the dice are now loaded, and in areas where clouds have decreased, the fires should be getting more intense and harder to contain. At some point, we’ll see if people can continue to keep up.”

Geophysical Research Letters


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