Thursday, January 20, 2011

Tule Fog

Source: CIMSS Satellite Blog "... showed a persistent Tule fog event across much of the Central Valley of California on 05 January 2011 ... snow-covered Sierra Nevada Range can be seen to the east of the Central Valley [San Francisco Bay Area Region seen to the northwest]."

Ain't No Sunshine ...
Tule fog is a phenomenon that can cause a near white-out condition at ground level with an air temperature just above freezing. Unpleasant, dangerous, and lasting for as long as days. Occasionally, when intermittently intense ground level tule fog is combined with even longer periods of higher level fog cover, full sunlight and warmer air temperatures may not reach down onto the valley floor in hundreds of square miles for several consecutive weeks during winter. This huge fog blanket can be photographed from space and may appear conforming closely to the actual shapes of the contiguous Sacramento and San Joaquin Valleys that are a central geological feature in California. (Total area greater than shown in satellite photo above.)

Condensed Water Vapor
(Excerpt) "Fog is water vapor that has condensed onto microscopic particles in the air and formed a cloud at ground level. When much of Earth’s heat is radiated out to space, usually on clear, windless nights, it cools the moist, dense and heavier layer of air near the valley’s floor. When air temperature decreases, humidity levels increase and the valley begins to take on a grayish tint. When the air has reached its dew point temperature, the relative humidity is at 100 percent. Water droplets become visible to the naked eye. At that point, tule fog can rapidly develop.

During periods of tranquil weather ... the San Joaquin Valley tule fog can last for days on end as it settles between the Sierra Nevada [Mountains] to the east and the coastal range to the west under a strong temperature inversion layer.

The air above the inversion layer is drier, warmer and hence lighter. Like a cover over a bed, this further serves to trap the cold, dense and heavier air within the valley.

... [During winter], the sun is low in the southern sky and much of its light is reflected back out to space off the top of the fog layer.

Very small amounts of sunlight penetrate the fog and warm the valley floor. Afternoon high temperatures may only reach the high 30s with little temperature change through the night."

Question for Blog Readers
Why is this phenomenon called "tule fog"?

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