The Foggy Season
in the San Joaquin Valley
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ASHLEE BUSSIERE
"The fog comes on little cat feet.
It sits looking over harbor and city on silent haunches and then moves on". FOG - Carl Sandburg - 1878
If one were to describe the most notable weather characteristics of the San Joaquin Valley, undoubtedly
they would include the occurrence of dense fog episodes during the winter. Many who move to the interior
of Central California seem to express surprise at the persistence and intensity of the fog that can
develop during the winter months. Fog Season is one of the most dangerous parts of the year in
Central California. Associated with the rainy season in the San Joaquin Valley the dense fog
is almost a daily occurrence from mid-October to early March.
Before one can understand why Central California has such a problem with fog, one must first understand
the climate that supports the phenomenon and the geography of the land. The San Joaquin Valley
stretches from the Sacramento- San Joaquin Delta to the North Tehachapi Mountains. The Valley
has the various coastal ranges to the west and the Sierra Nevada to the east. In conjunction
with the Sacramento Valley the Great Valley of California is essentially a closed air basin. The
climate in Central California is one of long hot dry summers, occasional blowing dust events
in the fall and spring and cool winters with dense fog. The introduction of moisture is not
removed from the air basin unless pushed or lifted out by a weather event. In the fall the
cool season begins and more frontal systems begin to move in and bring the much needed rain
to Central California. After the rain, low level moisture is added thereby setting the
stage for fog to form.
Fog - as defined in the encyclopedia, is a cloud of condensed water vapor droplets or ice crystals, suspended
in the atmosphere just over the surface of the earth. In simple terms fog is a cloud in contact with
the ground. However, this cloud is different than any other and comes in many shapes and forms. The
main fog that forms in the San Joaquin Valley is called radiational fog. First a good rainfall
covers much of Central California then high pressure builds behind the passing storm and settles
the atmosphere. Once the air is quiet and the night is long, clear skies and light wind allow
heat gained during the day to radiate out. The temperature falls to the dew point and the
air near the surface becomes saturated. Fog needs some microscopic particles (condensation
nuclei) to cling to and the valley usually has plenty of that due to pollution and dust. So
the cool air lies near the valley floor and fog forms throughout the night. The temperature
inversion layer can also play a roll in fog formation. The inversion layer, warmer
temperatures above cooler temperatures, also sets up over a period of time and that
can eventually lead to a layer of fog from about 600 feet to 3000 feet above the
surface. This sometimes allows the fog to form in the Foothills of the Mountains
while the Valley floor is cold and dreary with peeks of sunshine. The lack of
strong sunshine during daytime hours does not provide sufficient incoming energy
to always evaporate the overnight fog, thus fog can and does last several days
at a time. The atmosphere needs additional drying or mixing of its air basin
for the fog to lift out.
One form of radiation fog that is the leading cause of weather-related casualties in Central California is
Tule fog. This thick ground fog condenses when there is a high relative humidity, which usually
comes after a heavy rain, light winds and rapid over night cooling. Tule fog forms when the cold
mountain air flows into the valley during the night, pooling in the low areas and filling the
valley to the brim. Because of the density of the cold air, winds are not able to dislodge the
fog and the high pressure of the warmer air above the mountains presses down on the cold air
and traps the fog in the valley. The result is a dense immobile fog that reduces visibility
to a mere foot. The Tule fog usually develops below 1,000 feet and above the layer of cold
air is typically a layer of warmer dry clear air. Once the fog has formed, strong turbulent
air is necessary to break through the temperature inversion layer. Daytime heating can
sometimes help evaporate the fog. There is not much that one can see in Tule fog. Visibility
is usually reduced to a few hundred feet but can sometimes be less than 10 feet. Visibility
in Tule fog varies rapidly and is the cause of many chain reaction pile-ups on the
highways and roads in Central California.
In the morning hours of dense fog on November 14th 1998 reports began pouring into the California Highway
Patrol at 8:50-AM of a multiple vehicle pile-up on highway 99. More than 74 vehicles, including 19
tractor trailers were involved in the one mile stretch just two miles southeast of Kingsburg, CA. there
were two fatalities and 51 injuries. Over a 132 people were involved in the accident. On the same
day another eight accidents occurred in the Fresno area as well as an eight car pile-up which
occurred just two miles away from the accident near Kingsburg. The accident was recorded as
the worst event until the winter of 2007. On November 1st, 2007 a 108 car pile up occurred
on Highway 99 in Fresno killing three people and injuring over 100. The pile-up was later
blamed on a drunk driver going to fast in the morning fog causing a change reaction accident.
Forecasting Fog is difficult to predict but is easier during the months of December and January. It becomes
harder to predict just before and near the end of winter. The difficulty lies with predicting widespread
or patchy dense fog. If the weather pattern is extremely stagnant, then widespread dense fog is a
good bet. Studies are being done on better handling of fog prediction using the 700 millibar computer
models. According to National Weather Service Meteorologist and Author Mark Burger "Forecasters
often rely on inspection of low level winds, the presence of a large-scale anticyclone aloft,
initially clear skies, damp surface conditions, and climatology to gauge the potential for dense
fog in the San Joaquin Valley. However, many of these methods are quite subjective in nature
and/or are overly reliant on model guidance, as opposed to observational data. Thus, my
study seeks to objectively examine one parameter, upper level heights derived from radiosonde
data, and correlate its effectiveness in diagnosing and predicting dense fog across the
central San Joaquin Valley". The advancement of Satellite images from the National Weather
Service and ground level cameras provided by the CHP, non-traditional technology is helping
in showing the areas of fog.
Fog patterns in Central California have not changed in the past few years. The thickness, duration
and amount of days all depends on how much rain the Valley receives and how strong the high is over
the Valley after the rain. The San Joaquin Valley averages about 36 days of dense fog every winter
when the visibility is a quarter of a mile or less. But when the Valley's weather pattern is
active like in an El Nino pattern, the air can remain to mixed for fog to form. If it does form,
a succeeding storm would prevent the fog from reforming and if the atmosphere simply remains to
mixed, the air dries and fog has a tougher time forming.
Therefore, with the combination of geography, weather conditions and scientific data, we can better
prepare for the dense fog in the San Joaquin Valley.
References:
1. Mark Burger - The Application of Upper Level Heights in Diagnosing and Forecasting San
Joaquin Valley Dense Fog Episodes
www.wrh.noaa.gov/wrh/03TAs/0309/index.html
2. Tule Fog
www.en.wikipedia.org/wiki/Tule_fog
3. Highway Fog related accidents
www.abc30.com
4. Fog
www.encarta.msn.com/encyclopedia
5. Fog Season
www.wrh.noaa.gov/hnx/newslet/winter/fogseason.htm
6. California Highway Patrol
www.chp.gov
7. San Joaquin Valley
www.answers.com/sanjoaquinvalley
8. Fog and Speed Kills
www.wrh.noaa.gov/hnx/newslet/winter98/story1.htm
9. Carl Sandburg "Fog" Poem
www.poemhunter.com
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