California’s “Central Valley”:1 runs from Redding in the north to just past Bakersfield in the south, and is the home of frequent fall and winter “Tule fog”:2. Since the Central Valley is almost completely surrounded by mountains (the Coastal Range to the west, Sierra to the east, Tehachapi Mountains to the south and Cascade Range to the north), air tends to become very stagnant in the Valley. This combined with frequent low-level temperature inversions (the air near the ground is colder than the air higher up) and moisture from fall/winter storm systems creates one of the world’s largest fog-bound areas. In fact, for weeks at a time the majority of the Central Valley can be entrapped in fog.

Even worse, when the rest of California is enjoying sunny, warm weather (like today), it is frequently cold, damp and foggy/overcast in the Valley. For example, the high temperature along California’s central coast will peak in the “mid 70s”:2 today, while cities in the Valley will be lucky to get over “50 degrees”:3.

The GEOS West satellite has a “fog spectrum view”:4 which vividly shows the presence of Tule fog in the Valley.

Up and down the Central Valley the National Weather Service maintains a “series of profilers”:5 that measure wind and temperature at various altitudes. The temperature inversions can easily be see on these. Here’s one for today from the “Lost Hills”:6 profiler.

The classic set up for Tule Fog is the presence of high-pressure in the “Great Basin”:9 region. These high-pressure systems effectively put the lid on the Central Valley with warm air aloft, but also heat up California’s coastal areas through “adiabatic heating”:8. Today’s surface-level pressure map shows high pressure in the Great Basin.

What’s the view like in the fog? Flickr user “emdot”:10 shows us.

Zoinger says, no PTHs (peak tanning hours).

[10(Emdot in the fog)]http://flickr.com/photos/emdot/73257387/

[9(Wikipedia on the Great Basin)]http://en.wikipedia.org/wiki/Great_Basin

[8(Wikipedia on adiabatic heating)]http://en.wikipedia.org/wiki/Adiabatic_lapse_rate

[7(Wikipedia on Tule Fog)]http://en.wikipedia.org/wiki/Tule_fog

[6(Google Map for Lost Hills, CA)]http://maps.google.com/maps?f=q&hl=en&q=lost+hills,+ca&ll=35.617396,-119.689608&spn=0.044516,0.078878

[5(California profilers)]http://www.weather.nps.navy.mil/profiler/coastprof.html

[4(GEOS West California fog spectrum)]http://sat.wrh.noaa.gov/satellite/2km/hnx/FOG2HNX.GIF

[3(Visalia current weather)]http://weather.noaa.gov/weather/current/KVIS.html

[2(Santa Maria current weather)]http://weather.noaa.gov/weather/current/KSMX.html

[1(Wikipedia on the Central Valley)]http://en.wikipedia.org/wiki/California_Central_Valley