Deserts are classified by their geographical location and dominant weather
pattern as trade wind, midlatitude, rain shadow, coastal, monsoon, or polar
deserts. Former desert areas presently in nonarid environments are paleodeserts,
and extraterrestrial deserts exist on other planets.
Trade wind deserts
The trade winds in two belts on the equatorial sides of the
Horse Latitudes heat up as they move toward the Equator. These dry winds
dissipate cloud cover, allowing more sunlight to heat the land. Most of the major
deserts of the world lie in areas crossed by the trade winds. The world's largest
desert, the Sahara of North Africa, which has experienced temperatures as high as
57° C, is a trade wind desert.
The Sahara of Africa is the world's largest desert.
It contains complex linear dunes that are separated
by almost 6 kilometers. (Skylab photograph).
Midlatitude deserts occur between 30° and 50° N. and S.,
poleward of the subtropical highpressure zones. These deserts are in interior
drainage basins far from oceans and have a wide range of annual temperatures. The
Sonoran Desert of southwestern North America is a typical midlatitude desert.
A rare rain in the Tengger, a midlatitude desert of China,
exposes ripples and a small blowout on the left. Winds will
shortly cover or remove these features.
Rain shadow deserts
Rain shadow deserts are formed because tall mountain ranges
prevent moisture-rich clouds from reaching areas on the lee, or protected side,
of the range. As air rises over the mountain, water is precipitated and the air
loses its moisture content. A desert is formed in the leeside "shadow" of the
This Landsat image shows the Turpan Depression in the rain shadow
desert of the Tian Shan of China. A sand sea is in the lower center
on the right, but desert pavement, gray in color, dominates this
desert. The few oases in the desert and the vegetation in the
mountains at the top are in red. A blanket of snow separates the
vegetation in the Tian Shan from the rain shadow desert.
Coastal deserts generally are found on the western edges of
continents near the Tropics of Cancer and Capricorn. They are affected by cold
ocean currents that parallel the coast. Because local wind systems dominate the
trade winds, these deserts are less stable than other deserts. Winter fogs,
produced by upwelling cold currents, frequently blanket coastal deserts and block
solar radiation. Coastal deserts are relatively complex because they are at the
juncture of terrestrial, oceanic, and atmospheric systems. A coastal desert, the
Atacama of South America, is the Earth's driest desert. In the Atacama,
measurable rainfall--1 millimeter or more of rain--may occur as infrequently as
once every 5-20 years.
Crescent-shaped dunes are common in coastal deserts
such as the Namib, Africa, with prevailing onshore
winds. Low clouds cover parts of the Namib in this
space shuttle photo.
High dunes of the Namib desert near Sossus Viei (photograph by Georg Gerster).
Morning fog moistens the dunes of the Namib
coastal desert (photograph by Georg Gerster).
"Monsoon," derived from an Arabic word for "season," refers to a
wind system with pronounced seasonal reversal. Monsoons develop in response to
temperature variations between continents and oceans. The southeast trade winds
of the Indian Ocean, for example, provide heavy summer rains in India as they
move onshore. As the monsoon crosses India, it loses moisture on the eastern
slopes of the Aravalli Range. The Rajasthan Desert of India and the Thar Desert
of Pakistan are parts of a monsoon desert region west of the ranqe.
The Indus River floodplain, lower left, is the western border
of the Thar Desert. This Landsat image of the monsoon desert
shows small patches of sand sheets in the upper right, with
three types of dunes; some dunes are almost 3 kilometers long.
Polar deserts are areas with annual precipitation less than 250
millimeters and a mean temperature during the warmest month of less than 10° C.
Polar deserts on the Earth cover nearly 5 million square kilometers and are
mostly bedrock or gravel plains. Sand dunes are not prominent features in these
deserts, but snow dunes occur commonly in areas where precipitation is locally
more abundant. Temperature changes in polar deserts frequently cross the freezing
point of water. This "freeze-thaw" alternation forms patterned textures on the
ground, as much as 5 meters in diameter.
The Dry Valleys of Antarctica have been ice-free for
thousands of years (courtesy of USGS Image Processing
Facility. Flagstaff. Arizona).
Data on ancient sand seas (vast regions of sand dunes), changing
lake basins, archaeology, and vegetation analyses indicate that climatic
conditions have changed considerably over vast areas of the Earth in the recent
geologic past. During the last 12,500 years, for example, parts of the deserts
were more arid than they are today. About 10 percent of the land between 30? N.
and 30? S. is covered now by sand seas. Nearly 18,000 years ago, sand seas in two
vast belts occupied almost 50 percent of this land area. As is the case today,
tropical rain forests and savannahs were between the two belts.
Fossil desert sediments that are as much as 500 million years old have been found
in many parts of the world. Sand dune-like patterns have been recognized in
presently nonarid environments. Many such relict dunes now receive from 80 to 150
millimeters of rain each year. Some ancient dunes are in areas now occupied by
tropical rain forests.
The Nebraska Sand Hills is an inactive 57,000square kilometer dune field in
central Nebraska. The largest sand sea in the Western Hemisphere, it is now
stabilized by vegetation and receives about 500 millimeters of rain each year.
Dunes in the Sand Hills are up to 120 meters high.
This aerial photograph of the Nebraska Sand Hills
paleodesert shows a well-preserved crescent-shaped
dune (or barchan) about 60 to 75 meters high.
(Photograph by Thomas S. Ahlbrandt)
A dry community of vegetation grows among the
dunes of the Nebraska Sand Hills.
(Photograph by N.H. Darton)
Mars is the only other planet on which we have
identified wind-shaped (eolian) features. Although its surface atmospheric
pressure is only about one-hundredth that of Earth, global circulation patterns
on Mars have formed a circumpolar sand sea of more than five million square
kilometers, an area greater than the Empty Quarter of Saudi Arabia, the largest
sand sea on our planet. Martian sand seas consist predominantly of
crescent-shaped dunes on plains near the perennial ice cap of the north polar
area. Smaller dune fields occupy the floors of many large craters in the polar
This Viking spacecraft image of Mars shows
alternating layers of ice and windblown dust
near the north polar cap. Annual and other periodic
climatic changes due to orbit fluctuations may occur
on Mars (courtesy of USGS Image Processing Facility,
One of the first images taken at the Viking 2 landing site on Mars
shows the pink sky over Utopia and the desert pavement on the ground
(courtesy of NASA).
Some of the crescent-shaped dunes in this Viking image of Mars
are more than a kilometer wide. The dark material that streaks
from the horn-shaped features may be dust recently blown from
the dunes (courtesy of NASA).