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


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

Weather gifts for the holidays

In response to several requests about holiday gifts for weather enthusiasts, I have put together a webpage at that has links to several online catalogs with everything from weather stations to weathervanes.

Q  What is a ""Chinook wind?'' How or why does this occur? Where and when is this common in Canada?  Nicole Dallo - Windsor, Ont., Canada

A  Chinooks are downslope winds on the eastern slopes of the Rockies, including the Canadian Rockies.   These winds blow from west to east and warm up as they descend into the Plains of the
United States and Alberta Province in Canada. They can reach near hurricane force as they blast out of canyons at the base of the mountains.

The term Chinook is a Blackfoot Indian word meaning ""snow-eater,'' which refers to its warmth and ability to melt snow on the Plains very rapidly.  A Chinook wind can melt a foot of snow in just a few hours. On Jan. 22, 1943, a Chinook raised the temperature in Spearfish, S.D., from
minus 4 degrees F at 7:30 a.m. to 47 degrees F just two minutes later. And by 8 a.m., the Chinook had abated and the temperature dipped back to minus 4 degrees F.

Q  I read in your column that an aurora was visible from Silicon Valley last month. What is the easiest way to know when we might be able to see an aurora from the Bay Area?  Is there a timetable that I can look up and plan for it? How often is it visible from the Bay Area? I saw the aurora borealis when I was in Alaska and it took my breath away.  Lilly Koi - Sunnyvale

A  Historically, an aurora is visible in the Bay Area about once every five to ten years.  Because of light pollution from the metropolitan area, it takes a fairly strong aurora event to be visible.

To predict a strong aurora requires monitoring the weather on the sun, in particular the eruption of solar flares. These are related to sunspot activity, which now is just passing its peak in an 11-year
cycle.  To keep track of current solar activity, monitor .

Q  Why does daylight savings time end about seven weeks before the winter equinox, yet begins about 14 weeks afterward? Why not allow the same number of weeks before and after the equinox to maintain a daylight equilibrium?  John Kenny - Los Altos

A  Your idea certainly makes sense, but keep in mind that daylight saving time was established by Congress. Until the Uniform Time Act of 1966 set the start of daylight savings time as the first Sunday of April and the end on the last Sunday of October, each state was pretty much on its own.  The exception was in 1973 and 1974 when daylight saving time was extended to 10
months during that energy crisis.

Jan Null, founder of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send questions to him c/o WeatherCorner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. You also can telephone questions at (510) 657-2246, fax them to (510) 315-3015 or e-mail them to Please indicate in your e-mail what city you live in.