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Southern California clouds are vanishing, but why?

Dr. Mario Picazo
Meteorologist, PhD

Tuesday, June 5, 2018, 16:49 - Southern California is known for "May Gray" and "June Gloom" weather, endless days of coastal low cloud cover that usually "burns off" in the afternoon hours as the solar rays are able to transmit some energy on to the surface. 

Despite the persistent cloud cover this May and early June of 2018, a recent study has found that clouds above areas like Los Angeles are vanishing. 

The findings published in the journal of Geophysical Research Letters, show that summer clouds over Southern California have become less abundant over the past few decades, mainly as a result of higher temperatures and more heat produced by the city itself. 


Photo: Mario Picazo

The cloud deck that used to hang around until sometime in the afternoon is now normally gone by 10 or 11 a.m. These low clouds are also known as stratus clouds, and they normally form in a deep marine layer with a temperature inversion aloft, a common situation along the Southern California coast between May and August. 

Image: NOAA

Cloud cover is not actually the type of data that is easy to measure, however, the research team, led by Park Williams, a bioclimatologist at Columbia University, was able to gather a large database of cloud information from Southern California airports going back to the 1970s.

Image: UCLA

The cloud data showed that during the summer months, the low cloud deck, which usually hangs around between 1,000 and 3,000 feet above the surface, had decreased between 25 and 50 per cent over the last 50 years.

In a second stage of the study, the idea was to find a relationship between the vanishing clouds and moisture content in Southern California's vegetation. This time, the authors found that with less cloud cover, more solar radiation is available, causing the ground to become drier, a situation which can create an environment favorable for fires to ignite.

According to Brandon Collins, a research scientist at UC Berkeley´s Center for Fire Research and Outreach, the fact that the ground is drier does not necessarily mean it is the cause of major wildfires today, "it's one piece of the fire story in Southern California, but maybe not even a big part of it."

The city is growing, and as it does, asphalt is replacing vegetated or even bare soil-covered land. This will certainly bump-up the intensity of your urban heat island and extend it further out from the city center. Increased temperatures could be eating away coastal stratus clouds, but for now, there is no formal study that proves this is the main cause.

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