Mountain Climate - Mountain Geography: Physical and Human Dimensions - page 74

and prevailing winds. Western Europe has a climate similar to the Pacific Northwest,
but the east-west orientation of the mountains allows the marine climate to extend far
inland, resulting in a milder climate throughout Europe.
Barrier Effects
While the Himalaya and Cascades are both outstanding climatic divides that create un-
like conditions on their windward and leeward sides, all mountains serve as barriers to
some extent, depending on their size, shape, orientation, and relative location. Specific-
ally, the barrier effects of mountains can be grouped under the following subheadings:
(1) damming, (2) deflection and funneling, (3) blocking and disturbance of the upper air,
(4) forced ascent, and (5) forced descent.
DAMMING
Damming of stable air occurs when mountains are high enough to prevent the passage
of an air mass across them. A steep pressure gradient may develop between the wind-
ward and leeward sides of the range. The effectiveness of the damming depends on the
depth of the air mass and the elevation of the lowest valleys or passes (Smith 1979). A
shallow, ground-hugging air mass may be effectively dammed, but a deep one is likely
to flow through higher gaps and transverse valleys to the other side. In the Los Angeles
Basin of southern California, for example, the San Gabriel, San Bernardino, and San
Jacinto Mountains act as dams for marine air from the Pacific Ocean. As the automobile-
based culture of southern California pollutes the air, the pollution can only be vented as
far east as the towns of San Bernardino and Riverside at the base of the mountains. In
the absence of a strong wind system, the pollution can build up to dangerous levels as
the air stagnates at the mountain barrier (Jerrett et al. 2005).
DEFLECTION AND FUNNELING
When a mountain range dams an air mass, the winds can be deflected around the
mountains if topographic gaps exist. Deflected winds can have higher velocities as their
streamlines are compressed, the so-called “Bernoulli effect” (Lin et al. 2005). In winter,
polar continental air coming from Canada across the central United States is channeled
to the south and east by the Rocky Mountains. Similarly, as the cold air progresses
southward, the Sierra Madre Oriental prevents it from crossing into the interior of Mex-
ico. The east coast of Mexico also provides an excellent example of deflection in sum-
mer: The northeast trade winds blowing across the Gulf of Mexico cannot cross the
mountains and are deflected southward through the Isthmus of Tehuantepec, where
they become northerly winds of unusual violence (Brennan et al. 2010). Maritime air
from the northeastern Pacific is deflected north and south around the Olympic Moun-
tains (Fig. 3.4). To their north, where wind is also deflected south from the Vancouver
Island Ranges, these winds converge into the topographic funnel of the Strait of Juan
de Fuca, resulting in much higher wind speeds (Mass 2008). A similar phenomenon oc-
curs around the Southern Alps of New Zealand, with winds funneled through the Cook
Strait between the islands (Sturman and Tapper 1996). These perturbations of the local