Geography Reference
In-Depth Information
of the Himalaya permits tropical climates to extend farther north in India and South-
east Asia than anywhere else in the world (Xu et al. 2010). One of the wettest places
on Earth is Cherrapunji, near the base of the Himalaya in Assam, with an annual rain-
fall of 10,871 mm (428 in.; Cerveny et al. 2007); the record for a single day is 1,041
mm (41 in.), as much as Chicago or London receives in an entire year! On the north
side of the Himalaya, however, there are extensive deserts, and temperatures are ab-
normally low for the latitude. This contrast in environment between north and south
derives almost entirely from the presence of the mountains, whose east-west orienta-
tion and great height prevent the invasion of warm air into central Asia just as surely as
they prevent major invasions of cold air into India. It is no wonder that the Hindus pay
homage to Shiva, the great god of the Himalaya.
External Climatic Controls
Mountain climates occur within the framework of the surrounding regional climate and
are controlled by the same factors, including latitude, altitude, continentality, and re-
gional circumstances such as ocean currents, prevailing wind direction, and the loca-
tion of semipermanent high-and low-pressure cells. Mountains themselves, by acting as
a barrier, affect regional climate and modify passing storms. Our primary concern is in
the significance of all these more or less independent controls for mountain weather
and climate.
Latitude
Distance from the equator governs the angle at which the sun's rays strike the Earth,
day length, and thus the amount of solar radiation arriving at the surface. In the tropics,
the sun is always high overhead at midday, and days and nights are of nearly equal
length throughout the year. There is no winter or summer; one day differs from anoth-
er only in the amount of cloud cover. With increasing latitude, however, the height of
the sun changes throughout the year, and days and nights become longer or shorter de-
pending on the season (Fig. 3.1).
Although the highest latitudes receive the lowest annual amounts of heat energy,
middle latitudes frequently experience higher temperatures in summer than the tropics.
This is because of the moderate sun heights and longer days of the middle latitudes.
Mountains in these latitudes may experience even greater solar intensity than lowlands,
both because the atmosphere is thinner and because the sun's rays strike slopes orien-
ted toward the sun at a higher angle than level surfaces. A surface inclined 20° toward
the sun in middle latitudes receives about twice as much radiation during the winter as
a level surface. Thus, slope angle and orientation with respect to the sun are very im-
portant and may partially compensate for latitude.
The basic pattern of global atmospheric pressure systems—the equatorial low
(0°-20° latitude), subtropical high (20°-40° latitude), polar front and subpolar lows
(40°-70° latitude), and polar high (70°-90° latitude)—reflects the role of latitude in de-
termining climatic patterns. The equatorial and subpolar lows are zones of relatively
heavy precipitation, while the subtropical and polar highs are areas of low precipitation.
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