San Francisco Fog - Land-sea Temperature Gradient

Land-sea Temperature Gradient

The prevailing wind along the California coast is from the northwest owing to the normal location of the Pacific High, a large area of high atmospheric pressure. As the coastline is oriented from northwest to southeast, the marine layer and any clouds present within it would be confined to the coast and adjacent offshore waters, and often are, but for the large difference in temperature between the coastal waters and the inland valleys, especially, the Central Valley. In the summer, inland temperatures can soar as high as 100 F. This large difference creates a strong pressure gradient which turns the prevailing northwest flow to a westerly and even southwesterly direction, driving the marine layer and its clouds onshore and through any gaps in the Coast Ranges. The largest gap is the Golden Gate at the entrance to San Francisco Bay which also communicates via the Bay with the Carquinez Strait and the Central Valley.

As the City of San Francisco lies adjacent to the Golden Gate, it is often subject to the fog and low clouds blowing in on the marine layer. Even when the clouds are not present, the coolness of the marine layer exacerbated by the strong winds can chill the City even in mid-summer. Because of this, San Francisco is sometimes referred to as "naturally air conditioned".

Under normal summertime conditions, a diurnal (day-night) 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. 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.