File:Sanfrancisco Bay Fog ali 2012229 lrg.jpg

English: The Advanced Land Imager (ALI) on NASA’s Earth Observing-1 (EO-1) satellite acquired this view of fog encroaching on the city on August 16, 2012. The fog is part of the marine layer, a mass of cool, dense air from the sea that was sandwiched beneath a layer of warmer air as part of a temperature inversion. Fog is often present in the lower part of the marine layer, whereas wispy stratus clouds form in the upper part. 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.)