A lovely new satellite image from NASA illustrates what San Francisco's infamous marine layer looks like from above on an average summer day. Once and for all, this is why the Golden Gate Bridge and the Sunset are always socked in on days when it should be warm, and why our summer fog is different from our winter fog:
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. ...
During the summer, the strengthening of the high pushes the California Current’s surface waters away from the shore. As this happens, deeper, colder water rises up to replace it, a process called upwelling. Upwelling causes water temperatures in the Pacific to be frigid, but biologically productive. When sea breezes blow over this cold water, water vapor is forced to condense out of the air, forming advection fog. A different type of fog—tule fog—forms in San Francisco during the winter due to a separate set of meteorological conditions.
And, as you can see, at least in the middle of the afternoon, downtown and the Potrero, along with the Mission, Bernal Heights, Dogpatch, and the Bayview, are all quite sunny, meanwhile that fog encroaches, and will stop burning off around 5 p.m. and start to make your park-sitting frigid, and all travel to the avenues ill-advised.