There is another California weather characteristic that results from the location of the Pacific high.  The steady flow of air from the northwest during the summer helps to drive the California Current of the Pacific Ocean as it sweeps southward almost parallel to the California coastline.  However, since the mean drift is slightly offshore, there is a band of upwelling immediately off the coast as water from deeper layers is drawn into the surface circulation.  The water from below the surface is colder than the semi-permanent band of cold water just offshore, which ranges from 25 to 50 miles in width.

The temperature of water reaching the surface from deeper levels varies from about 49° F. in winter to 55° F. in late summer along the northern California coast, and from 57° to 65° F. on the southern California coast.  At a distance of 200 to 300 miles offshore, surface water temperatures range from 51° to 65° F. in the north and from 60° to 67° F. in the south.  Thus, the water near the coastline is as much as 10° colder during the summer than is the water farther west.

Comparatively warm, moist Pacific air masses drifting over this band of cold water form a bank of fog which is often swept inland by the prevailing northwest winds out of the high pressure center.  In general, heat is added to the air as it moves inland during these summer months, and the fog quickly lifts to form a deck of low clouds that extend inland only a short distance before evaporating completely.  Characteristically, this deck of clouds extends inland further during the night and then recedes to the vicinity of the coast during the day.  This layer of maritime air is usually from 1,500 to 2,000 feet deep, while above this layer the air is relatively warm, dry, and cloudless.