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Message: <blockquote data-quote="Jeff Wilder" data-source="post: 4747890" data-attributes="member: 5122">In the Bay Area, fog usually occurs on a warm day followed by a rapid cool-off at night and in the early morning.  That happens quite a lot here.  And yes, fog can produce rain and other forms of precipitation.From <a href="http://www.babelation.com/?q=node/971" target="_blank">HERE</a>:"Advection fogs occur when warm moist air moves over a cold surface. Advection is the horizontal transport of heat by winds and slow drifting movements of the air. The stronger the wind, the deeper the fog layer. In the San Francisco Bay Area, the movement of the air, the sea-breeze, from the Pacific Ocean inland to radiation-cooled land, produces its spectacular fog. Along the coast, during the clear summer days, the land warms more than the adjacent water. As the air warms over the land, it expands. Land air flows up and out over the ocean. The air pressure is decreased over the land and increases over the ocean. Air circulation begins, and the movement of the air is now from the ocean to the land, a sea breeze. Although little of original air returns to the land, most if not all of the lower air along the surface of the ocean flows inland. It usually starts around 10:00 a.m. far offshore, and ends up extending seaward and inland about 10 to 30 miles each way. At evening it subsides. Then the land breeze starts up because the land cools more than the water. The air over the land becomes denser as it cools. On top of this cooled land air mass, the air from the ocean begins to flow inward. The air pressure increases over the land. The land breeze removes the cooled air. Land breezes usually only extend about 5-6 miles over the sea, and inland. That is because the temperature differences between land and sea are less during the night. The fogs seen further offshore are also advection as they are the result of adjacent bodies of contrasting temperature water.Evaporative fogs occur when cold air moves over warm water. The fog looks like steam rising as it begins at the surface of the water as moisture is evaporated from the air, and builds upward. An evaporative fog also develops when rain falls from a warm layer of air to a colder layer of air below it and the air is stable or there is only a light wind. The boundary between the two layers of air is known as a front.Ground fogs and high-inversion fogs are radiation fogs as their air radiates/loses heat. Ground fogs occur primarily in the clear-weather mornings as they form from the cooling of the surface of the earth, and the lower air at night producing an inversion of temperature. The result is convection prevention and reduction of turbulence. Convection is the vertical movement of the air brought about by the transfer by internal mass movements of the air containing the heat. It can only occur in liquids and gases. Ground fogs rarely go above 100 feet above the surface of the earth. High-inversion fogs are prevalent and dense in winter in the San Joaquin Valley and in valleys near the southern and central California coast. They are really low stratus clouds. Cool quiet moist air lies on the earth. Over it at about 300 feet to 2,000 feet above the surface of the earth is warmer drier air. The result is fog.Upslope fogs are formed due to adiabatic cooling of air moving upslope against a mountainside or a gradually up-slope plain. When air ascends, the pressure on it decreases. Therefore, the air expands losing energy/heat in its expansion. The change in temperature is an internal change because of pressure. Adiabatic is the term for it meaning “without transfer of heat.” When dry air rises above the ground, its cooling is at a fairly uniform rate of 5.5 F per 1,000 feet. When dry air sinks to the ground, its heating is at the same rate."Interesting, that site has more complete info than the Wikipedia entry.(BTW, the fog is one of the reasons I moved to San Francisco, and one of the reasons I now live overlooking the Pacific, only a half-mile inland.  The fog here gets crazy, and even though I ride a motorcycle, I love it.)</blockquote>

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