"Intrusions by the marine layer—and all of the accompanying fog and clouds—are routine in San Francisco during the summer. The intrusions are caused by westerly breezes that push cold air inland to replace the warm air rising off of California’s Central Valley. As it did on the day this image was taken, the marine layer often completely envelops the Golden Gate Bridge in a thick cloak of fog and clouds. (The bridge visible in the lower part of the image is the Bay Bridge; to the north is the Richmond-San Rafael Bridge.)"

Via +SFist.
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