Geography Reference
In-Depth Information
These pressure zones create the global circulation system (Fig. 3.2), which in turn dic-
tates the prevailing wind direction and types of storms that occur latitudinally. The east-
erly trade winds have warm, very moist, convective (tropical) storms. The subtropical
highs have slack winds and clear skies year round. The subpolar lows and polar front
are embedded in the westerlies, bringing cool, wet cyclonic storms and large seasonal
temperature fluctuations. The cold and dry Polar easterlies develop seasonally, dissipat-
ing in summer.
FIGURE 3.1 Length of daylight received at each latitude during summer (left) and winter (right) sol-
stice in the northern hemisphere. (After Rumney 1968: 90.)
Jet streams are high-velocity, relatively narrow air currents found at about 11 km
(36,000 ft) above the surface (Archer and Caldeira 2008). They form at the boundaries
of air masses with significant differences in temperature, such as at the polar front.
Thus they develop seasonally, and their strength and position are variable. While jets
form in several locations, the best developed are the polar jets, westerly winds in both
hemispheres at latitudes 30-70°. The path of a jet typically has a meandering shape
(Rossby waves), and these meanders propagate eastward, at lower speeds than that of
the actual wind within the flow. Jet streams are important in steering storms, partic-
ularly midlatitude cyclones. The altitude of jets is above most mountains; however, at
higher elevations, wind velocities generally increase.
Where mountains are located within the global circulation system greatly influences
their climate. Mountains near the equator—such as Kilimanjaro in East Africa, Kinabalu
in Borneo, or Cotopaxi in Ecuador—are under the influence of the equatorial low and
receive precipitation almost daily on their east-facing windward slopes. By contrast,
mountains located around 30° latitude may experience considerable aridity, as do the
northern Himalayas, Tibetan highlands, the Puna de Atacama in the Andes, the Atlas
Mountains of North Africa, and the mountains of the southwestern United States and
northern Mexico (Troll 1968). Farther poleward, the Alps, Rockies, Cascades, southern
Andes, and the Southern Alps of New Zealand receive heavy precipitation on westward
slopes facing the prevailing westerlies. Leeward-facing slopes and lands downwind are
notably arid. Polar mountains are cold and dry year round.
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