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Cacti dominate the Sonoran Desert vegetation
near Tucson, Arizona. (Photograph by Peter Kresan) |
Approximately one-third of the Earth's land
surface is desert, arid land with meager rainfall that supports only
sparse vegetation and a limited population of people and animals.
Deserts--stark, sometimes mysterious worlds--have been portrayed as
fascinating environments of adventure and exploration from narratives such
as that of Lawrence of Arabia to movies such as "Dune." These
arid regions are called deserts because they are dry. They may be hot,
they may be cold. They may be regions of sand or vast areas of rocks and
gravel peppered with occasional plants. But deserts are always dry.
Deserts are natural laboratories in which
to study the interactions of wind and sometimes water on the arid surfaces
of planets. They contain valuable mineral deposits that were formed in the
arid environment or that were exposed by erosion. Because deserts are dry,
they are ideal places for human artifacts and fossils to be preserved.
Deserts are also fragile environments. The misuse of these lands is a
serious and growing problem in parts of our world.
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Ripples on a dune in the Gran Desierto,
Mexico. (Photograph by Peter Kresan)
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There are almost as many
definitions of deserts and classification systems as there are deserts in
the world. Most classifications rely on some combination of the number of
days of rainfall, the total amount of annual rainfall, temperature,
humidity, or other factors. In 1953, Peveril Meigs divided desert regions
on Earth into three categories according to the amount of precipitation
they received. In this now widely accepted system, extremely arid lands
have at least 12 consecutive months without rainfall, arid lands have less
than 250 millimeters of annual rainfall, and semiarid lands have a mean
annual precipitation of between 250 and 500 millimeters. Arid and
extremely arid land are deserts, and semiarid grasslands generally are
referred to as steppes.
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Distribution of non-polar arid land (Click
on image to see full map) |
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