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"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.
abhi raj's profile photoDoug Landauer's profile photoDoug Kramer's profile photo
Many days, that wisp of fog goes straight or bears right just a bit, to aim straight at Berkeley.  SF Bay sailors call this "the slot".  You can be sailing in a beautiful sunny day south of the Bay Bridge, head north past it, and be thrust into a deep, dark, foggy but windy cold day for ten minutes, and emerge into the sunny day north of it.
It's interesting how a westerly breeze flows from west to east, but a westerly ocean or river current flows east to west. I believe that's because on a boat, you feel that westerly breeze from the west, while that westerly water current carries your boat to the west.
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