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Southern California Losing Cloud Cover

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Sunny California may be getting too sunny. Increasing summer temperatures brought on by a combination of intensifying urbanization and warming climate are driving off once common low-lying morning clouds in many southern coastal areas of the state, leading to increased risk of wildfires, says a new study.  “Cloud cover is plummeting in southern coastal California,” said Park Williams, a bioclimatologist at Columbia University’s Lamont-Doherty Earth Observatory and lead author of the research. “And as clouds decrease, that increases the chance of bigger and more intense fires.” Williams said the decrease is driven mainly by urban sprawl, which increases near-surface temperatures, but that overall warming climate is contributing, too. Increasing heat drives away clouds, which admits more sunlight, which heats the ground further, leading to dryer vegetation, and higher fire risk, said Williams. The study appears this week in the journal Geophysical Research Letters.  The research follows a 2015 study in which Williams first documented a decrease in cloud cover around the sprawling Los Angeles and San Diego areas. Urban pavement and infrastructure absorb more solar energy than does the countryside, and that heat gets radiated back out into the air–a major part of the so-called heat-island effect, which makes cities generally hotter than the rural areas. At the same time, overall temperatures have been rising in California due to global warming, and this has boosted the effect. In the new study, Williams and his colleagues have found a 25 to 50 percent decrease in low-lying summer clouds since the 1970s in the greater Los Angeles area.

From Columbia University  Photo John Vlahakis

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