| Where are Deserts? |
Draw a line around the world, starting mid-center between
Joshua Tree and Death Valley, and you will touch or come close to many of
the world’s great deserts: Mojave, Great Basin, Sahara, Arabian, Iranian,
Gobi. Most deserts occur between the latitudes of 15 and 40 degrees on either
side of the equator. They are found around the world on every continent,
covering approximately 20 percent of Earth’s land area. Sand dunes
cover only about 10 percent of this area. Some deserts are very mountainous.
Most are hot, with warm daytime temperatures much of the year, but others
are cold, getting over half their moisture from snow.
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| Deserts can be divided into four types: subtropical, coastal, interior,
and rain shadow, depending on the conditions creating them. Subtropical
deserts lie along the Tropic of Cancer (23°N latitude) and the Tropic
of Capricorn (23°S latitude). Near the equator hot, moist air rises.
It cools, dropping heavy rains on tropical areas. The resulting cooler,
drier air then descends, creating zones of high atmospheric pressure as
it moves away from the equator. The descending air hinders cloud formation
and precipitation. It also warms up, absorbing any available moisture. The
Sahara, the world’s largest hot desert, is a subtropical desert about
the size of the United States. |
| Coastal deserts are also in areas of high pressure. Damp, chilly fog forms
when air, chilled by water contact as it blows toward shore, meets warm
air over land. Although humidity is high, atmospheric disturbances that
can cause rainfall are not present. Two coastal deserts, the Atacama of
Chile and the Namib in southern Africa, are among the driest places in the
world. |
| Interior deserts, like the Gobi, exist because they are too far from moisture-laden,
ocean winds. By the time these winds reach the center of a large landmass,
the air is very dry. |
| Rain shadow deserts are created when mountain ranges lie parallel to moist,
coastal areas. Prevailing winds moving inland cool as air is forced to rise
over the mountains. Carried moisture falls on slopes facing the winds. When
the winds move over the crest and down the far side, they are very dry.
Descending air also makes it hard for additional clouds and precipitation
to form. Without another source of moisture, rain shadow deserts are formed
on the far side of these mountain ranges. The Mojave Desert, which includes
most of Joshua Tree National Park, is a rain shadow desert. |
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http://www.nps.gov/jotr/nature/features/deserts/where.html
last modified: 03/26/02
web editor: Sandra kaye |
