Variations
A land-sea temperature-pressure gradient is not always necessary to drive the marine layer and low clouds onshore into the Bay Area. Winds ahead of an approaching cold front or low pressure system can also draw the marine layer onshore.
Another pattern variation occurs in connection with heat spells that reach the coast from inland. Such heat waves typically occur when an area of high atmospheric pressure orients itself in such a way that the northerly to northeasterly gradient becomes dominant, driving the marine layer out to sea south and west of the California coast. These spells typically end with what is called a "southerly surge", when the northerly gradient relaxes, allowing the marine layer to "slosh back" up the coastline.
Yet another variation occurs when the upper air becomes turbulent. Turbulence above the marine layer inversion can, depending on its severity, break up the marine layer. The most common causes of such turbulence are strong upper level low pressure areas, or the Southwest Monsoon which occasionally extends northwestward from the desert areas of the U.S.
Read more about this topic: San Francisco Fog
Famous quotes containing the word variations:
“I may be able to spot arrowheads on the desert but a refrigerator is a jungle in which I am easily lost. My wife, however, will unerringly point out that the cheese or the leftover roast is hiding right in front of my eyes. Hundreds of such experiences convince me that men and women often inhabit quite different visual worlds. These are differences which cannot be attributed to variations in visual acuity. Man and women simply have learned to use their eyes in very different ways.”
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