why is san francisco foggy in the summertime
San Francisco fog Under normal summertime conditions, a daily pattern of fog and low clouds occurs. Morning sun heats the ground which in turn heats the marine layer over the land areas. This creates convective turbulence within the marine layer as well as evaporation of any clouds within it. The marine layer clears back toward the coast, usually by noon. By mid-afternoon, inland areas have heated sufficiently to decrease the air pressure and increase the onshore flow. By late afternoon, the wind increases and begins to cool the onshore marine layer, allowing the fog and low clouds offshore to progress inland without evaporating. Cloudiness streams in over the Bay and through the various gaps. How far the clouds can penetrate inland depends on the depth of the marine layer and the strength of the cooling winds. As night falls and inland areas cool down, the winds usually decrease, but the fog and clouds remain wherever they have blown in until the following morning when the cycle repeats.[7]