What is a desert? A desert is a region where only special
forms of life can exist. All deserts have a shortage of moisture, which means
the life that exists in them must be able to get along with almost no water.
The amount of rainfall largely controls the amount and kinds
of plan life in a region. Forests grow where there is much rain. Grassland
grows where the rainfall is less. Where there is still less rain, only
scattered, specialized desert plants can live.
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| Sahara in Africa |
The hot deserts near the Equator, such as the Sahara in
Africa, lie in subtropical land where the air settles downward, becoming warmer
and drier as it does so. Lands in these areas are dry, even though they are
next to the ocean. The same is true of deserts in northwest Africa and Western
Australia.
Deserts farther away from the Equator are generally caused
by the great distance from the sea and its moist winds, and by the mountains
between the desert and the sea. These mountain barriers may catch rainfall on
their seaward side but the interior, leeward region remains dry.
This is known as "the rain shadow" effect. The
deserts of Central Asia lie in the rain shadow of the great Himalayan ranges
and the plateau of Tibet. The deserts of the Great Basin, in Western United
States, lie in the rain shadow of high mountain ranges on their west, such as
the Sierra Nevada.
Deserts differ greatly in appearance. Where sand is
abundant, the winds may build sand hills or dunes. These are sand deserts. Rock
deserts consist mostly of bare rock, which forms fantastic cliffs and hills, or
rough, jagged plains. Other deserts, like much of Southwestern United States,
include barren Rocky Mountains and dry plains of soil and gravel. The wind
sweeps away finer soil and the stones which are left form a gravelly surface
called "a desert pavement."
Most deserts have some sort of plant and animal life. Desert
plants often have little or no leaf surface to reduce evaporation of water from
the plant. Or they have thorns or spines to discourage animals from eating
them. Animals which can live in deserts can go for long periods without water
and get liquid from plants or night dew.
