Cold ocean fog gets sucked through the Golden Gate, the narrow opening to the bay. The bridge that connects San Francisco at the south with Marin County at the north, though the second longest suspension bridge in the country, is only about 1.7 miles long, including the portions that extend over land. The narrowness of the Golden Gate prevented the immense bay (not nearly so large today as it used to be) from being discovered by early European explorers. When the fog would lift, Angel Island would create the illusion of a solid landmass.
The concentrated fog often takes weird winding routes through the city, contributing to its pronounced microclimates. One neighborhood can be warm and sunny while a neighboring one is suffused with a penetrating chill. This video was shot from a helicopter over the bridge.