Foehn Winds
When air flows downhill from a high elevation, its
temperature is raised by adiabatic compression. Foehn
winds are katabatic winds caused by adiabatic heating
of air as it descends on the lee sides of mountains.
Foehn winds occur frequently in our western mountain
states and in Europe in the late fall and winter. In
Montana and Wyoming, the Chinook is a well-known
phenomenon; in southern California, the Santa Ana is
known particularly for its high-speed winds that easily
exceed 50 knots. For the purpose of illustrating a Foehn
wind, the Santa Ana is used.
The condition producing the Foehn wind is a
high-pressure area with a strong pressure gradient
situated near Salt Lake City, Utah. This gradient directs
the wind flow into a valley leading to the town of Santa
Ana near the coast of California. As the wind enters the
valley, its flow is sharply restricted by the funneling
effect of the mountainsides. This restriction causes the
wind speed to increase, bringing about a drop in
pressure in and near the valley. The Bernoulli effect
causes this pressure drop in and near a valley.
Generally speaking, when the Santa Ana blows
through the Santa Ana Canyon, a similar wind
simultaneously affects the entire southern California
area. Thus, when meteorological conditions are
favorable, this dry northeast wind blows through the
many passes and canyons, over all the mountainous
area, including the highest peaks, and quite often at
exposed places along the entire coast from Santa
Barbara to San Diego. Therefore, the term Santa Ana
refers to the general condition of a dry northeast wind
over southern California.
In the Rocky Mountain states, the onset of Foehn
winds have accounted for temperature rises of 50°F or
more in only a few minutes. In southern California, the
temperature, though less dramatically, also rises rapidly
and is accompanied by a rapid decrease in humidity (to
20 percent or less) and a strong shift and increase in
wind speeds. Although these winds may on occasion
reach destructive velocities, one beneficial aspect is
that these winds quickly disperse the severe air
pollutants that plague the Los Angeles Basin.
REVIEW QUESTIONS
Q3-15.
What is the cause of monsoon winds?
Q3-16.
What causes land and sea breezes?
Q3-17.
Describe Bernoulli's theorem.
Q3-18.
When does a valley breeze usually reach its
maximum?
Q3-19.
What causes eddies?
Q3-20.
What causes Foehn winds?
SUMMARY
In this chapter, we studied the primary, secondary
and tertiary circulation of the atmosphere. We learned
about large-scale circulations, worldwide locations of
major
pressure
systems,
horizontal
and
vertical
pressure systems. We studied how pressure systems,
temperature, and world winds relate to each other, and
finally we studied small-scale effects, due to local
features.
A good understanding of atmospheric
circulation is essential in order to understand the
characteristics of air masses and fronts.
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