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1
Moist air forced
upward by terrain,
causing condensation
3
Hot, dry air descends,
causing low relative humidity
2
Rain
Windward side
Leeward side
Figure 6.30 The development of Chinook (foehn) winds.
For these winds to develop, the pressure gradient must slope from
the windward to leeward side of a mountain range.
An excellent example of a valley wind is the mistral
wind , which occurs along the eastern Mediterranean coast of
France during the winter months. This wind develops when
cold air sweeps across France from the English Channel, hits
the Alps, and spills down the Rhône Valley into the coastal
region. On days when this process is especially active, winds
can gust to over 120 km/h (75 mph). Another great exam-
ple of downslope winds is the Santa Ana winds in Southern
California. These winds occur in winter when high-pressure
systems dominate over the Great Basin to the east. On the
southern side of the high, air flows west toward California,
rising up and over the Sierra Nevada and other mountain
ranges (Figure 6.31a). When the air descends on the western
side of these ranges, it picks up speed as it rushes into the Los
Angeles area.
Santa Ana winds are often beneficial because they clear
smog from the Los Angeles metro area. On the other hand,
they can fan wildfires during extremely dry conditions, some-
times resulting in catastrophic blazes that send people fleeing
from the area. A recent very bad year for such fires was 2007
when a series of wildfires spread across Southern California in
late October. Smoke plumes from these fires were visible from
space and clearly showed the influence of Santa Ana winds
(Figure 6.31b). At least 1500 homes were destroyed, and about
2000 km 2 (770 mi 2 ) of land was burned. Unfortunately, nine
people died as a result of these fires and over 80 were injured.
Bad fires also occurred in May of 2009 and 2013, which is un-
usual because fire season usually occurs at the end of summer
when the landscape is driest. The 2009 Jesusita Fire was note-
worthy because it threatened Santa Barbara, a wealthy coastal
enclave northwest of Los Angeles. Over 80 homes were de-
stroyed by the fire, which lit up the hills above the city at night
(Figure 6.31c).
Oceanic Circulation
Gyres and Thermohaline Circulation
So far in this chapter, we have seen how atmospheric circulato-
ry systems operate and how they distribute heat energy around
Earth. Another way that heat energy is moved is through ocean-
ic circulation (Figure 6.32), which is strongly related to atmo-
spheric circulation in many ways. Surface currents in the ocean
are driven by winds through the transfer of energy from the
air to the water by friction. As a result, the direction of surface
ocean currents is related to the same pressure factors that influ-
ence atmospheric circulation. For example, the tropical easterly
winds produce easterly oceanic currents in the low latitudes.
The Coriolis force also plays a major role in the movement
of oceanic currents. Notice in Figure 6.32, for example, that
ocean currents in the Northern Hemisphere generally move in
a clockwise direction, moving warm tropical water into higher
latitudes. A great example of such a current is the Gulf Stream,
which transports warm water north from the tropical latitudes
 
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