Published Tuesday, December 4, 2001 in the San Jose Mercury News 
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WEATHER CORNER

Tule Fog Clouds the Valleys

Now that the region has seen its first significant rains of the season, 
we have to watch out for dense night and morning valley fog. 

It is commonly known as "tule fog" because of its prevalence in marshy 
areas populated by tule reeds or cattails.   Technically it's a 
radiation fog, which forms as the ground cools off at night and 
radiates heat into space.

Tule fog usually forms on the first or second clear night after it 
has rained, skies have cleared, and winds are light.  This happens 
when high pressure returns, creating an inversion with colder air near 
the earth's surface than aloft.  This in turn causes moisture on 
the ground to condense into a low-lying layer of fog that develops 
from the ground up.  Because the coldest air settles into the lowest 
elevations, such as along a river or stream, this is where fog is 
most likely.

Locally the areas with the highest incidence of tule fog are in the 
southern Santa Clara Valley and the Livermore Valley.  In these areas 
the fog will usually clear by mid-morning, but it may last all 
day in the Central Valley.

Dense fog can reduce the horizontal visibility to only a few feet, 
making driving extremely hazardous.  Each year there are numerous 
traffic accidents when fog limits visibility and vehicles can't stop 
in time.  Many are chain reaction pileups involving many cars that 
drive into the fog and encounter stopped vehicles.  In December 1997, 
five people died and 28 were injured when 25 cars and 12 big rig 
trucks collided inside a fog bank on Interstate 5 near Elk Grove 
south of Sacramento. 

The National Weather Service issues "dense fog advisories" when 
visibilities are expected to be an eighth of a mile or less.  Under 
those conditions, someone driving 65 miles per hour has only 7.5 
seconds to see a hazard and stop.

The California Highway Patrol's first recommendation when driving in 
fog is to slow down.  The CHP also suggests driving with headlights on, 
but use only the low beam because high beams would be reflected by 
the fog to reduce visibility further.  The CHP also recommend drivers 
delay trips through especially foggy areas until midday when visibilities 
usually improve.

And if you are stuck at home waiting for the fog to clear,  
check out an entire web site of haiku poetry about tule fog at 
http://wdsroot.ucdavis.edu/clients/pcbr/what/haiku.html.