Typical Schematic Cross-section of Winds During Advection Fog Season Along California Coast

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This highly schematic diagram shows the conceptual highlights of the circulation pattern during the summer associated with advection fog  along the California coast on a typical summer afternoon. The temperatures show surface temperatures.

Since the typical dew point temperature  is about 56F for the incoming air, moisture condenses when the air passes across the California current.  Surface air there is cooled to the dewpoint, but only in the air immediately in contact with the sea surface.  Some mixing does occur so that this marine layer  is about 500 ft to about 1000 ft thick.

The air is drawn inland through gaps or low passes through the Coast Range into the Central Valley, where the air is heated from beneath.  The fog evaporates (dissipates) as the difference between the temperature and the dewpoint gets large again.

Air in the Central Valley ascends.  However, since the depth of the atmosphere affected by this whole differential heating and cooling is only around 4000 feet or so, this ascending air does not get cooled to the dewpoint.  Hence, no clouds form over the Central Valley.

Notice that the marine layer is too shallow to be involved in the production of thunderstorms in the Sierra Nevada.

Conditions In Air Moving Inland Ocean
Beach
Downtown
San Francisco
Fairfield Sacramento
Temperature 56 62 80 105
Dewpoint Temp 56 56 56 56