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Encyclopedia of Biomes: Volume 1 – Finals/ 11/4/2009 12:35 Page 123
Desert
ll deserts have two things in common: they are dry, and they support
little plant and animal life. If a region receives an average of fewer
than 10 inches (25 centimeters) of rain each year, it is classified as a desert.
Contrary to what most people believe, not all deserts are hot. Some
deserts near the North and South Poles are so cold that all moisture is
frozen—these are called polar deserts. Tropical desert areas are near the
equator. Temperate desert areas are between the tropics and the North
and South Poles.
True deserts cover about one-fifth of the world’s land area. With the
addition of polar deserts, the total rises to 30 percent. Another 25 percent
of Earth’s land surface possesses desertlike characteristics. In all, deserts
constitute 33 million square miles (86 million square kilometers). Most
deserts lie near the tropic of Cancer and the tropic of Capricorn, two lines
of latitude about 25 degrees from the equator. The area between these two
lines is called the Torrid Zone (torrid means very hot).
A
How Deserts Are Formed
Deserts are generally caused by the presence of dry air. The average
humidity (moisture in the air) is between 10 and 30 percent. In some
cases, mountain ranges prevent moisture-laden clouds from reaching the
area. Mountains can cause heavy, moisture-filled clouds to rise into the
colder atmosphere. There, the moisture condenses and falls in the form of
rain, leaving the air free of moisture as it crosses the range. In other cases,
certain wind patterns along the equator bring air in from dry regions.
Cold-water ocean currents can cause moist air to drop its moisture over
the ocean. The resulting dry air quickly evaporates (dries up) ground
moisture along the coastal regions as it moves inland.
Deserts have always existed, even when glaciers covered large portions
of Earth during the great Ice Ages. Although geological evidence is scarce,
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WORDS TO KNOW
Arroyo: The dry bed of a stream that flows only
after rain; also called a wash or a wadi.
Estivation: An inactive period experienced by
some animals during very hot months.
Butte: A small hill.
Mesa: A flat topped hill.
Deforestation: The cutting down of all the trees in a
forest.
Oasis (plural is oases): A fertile area in the desert
having a water supply that enables trees and
other plants to grow there.
Desertification: The changing of fertile lands into
deserts through destruction of vegetation (plant
life) or depletion of soil nutrients. Topsoil and
groundwater are eventually lost as well.
Dormant: A suspension of growing (plants) or
activity (animals) when conditions are harsh.
Wadi : The dry bed of a stream that flows after a
rain; also called a wash, or an arroyo.
Xeriscaping: Landscaping method that uses
drought tolerant plants and efficient watering
techniques.
scientists tend to agree that some desert areas have always been present,
but they were probably smaller than those of today. Fossils, the ancient
remains of living organisms that have turned to stone, can reveal the
climatic history of a region. For example, scientists believe that the
Arabian Desert, which covers most of the Arabian Peninsula to the east
of North Africa, once included wetlands because fossils of a small species
of hippopotamus have been found there. In the Sahara Desert of North
Africa, rock paintings made 5,000 years ago show pictures of elephants,
giraffes, and herds of antelope that are no longer present.
Desertification (DES-aurt-ih-fih-KAY-shun; desert formation) occurs
continuously, primarily on the edges of existing deserts. It is caused by a
combination of droughts (rainless periods) and human activity such as
deforestation (cutting down forests) or overgrazing of herd animals. When
all the grass is used and rain is scarce, plants do not grow back. Without
plants to hold the soil in place, wind blows away the smaller and finer
particles of soil, exposing the less compacted layer of sand. This leaves a
barren, unprotected surface. Eventually, even groundwater disappears.
Kinds of Deserts
Scientists measure a region’s aridity (dryness) by comparing the amount
of precipitation (rain, sleet, or snow) to the rate of evaporation.
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Evaporation always exceeds precipitation. Deserts can be classified as
hyperarid (less than 1 inch [2.5 centimeters] of rain per year); arid (up
to 10 inches [25 centimeters] of rain per year); and semiarid (as much as
20 inches [50 centimeters] of rain per year, but are so hot that moisture
evaporates rapidly). Most true deserts receive fewer than 4 inches (10
centimeters) of rain annually.
Except for those at the North and South Poles, which are special cases,
deserts are classified as hot or cold. Daytime average temperatures in hot
deserts are warm during all seasons of the year, usually above 65˚F (18˚C).
Nighttime temperatures are chilly and sometimes go below freezing.
Typical hot deserts include the Sahara and the Namib Desert of Namibia.
Cold deserts have hot summers and cold winters. At least one month
during the year the mean temperature is below 45˚F (7˚C). Cold deserts
include Turkestan in Kazakhstan and Uzbekistan, Gobi (GOH-bee)
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Desert
The oblong-shaped Ayers Rock
is located in the Australian
Desert. I MA GE C OPY RI GH T
RO NAL D SU MNE RS , 20 07.
U SE D UND ER L IC EN SE F ROM
SH UTT ER STO CK .C OM.
in China and Mongolia, and the Great Salt Lake Desert in Utah. These
deserts usually get some precipitation in the form of snow.
Deserts can be further characterized by their appearance and plant
life. They may be flat, mountainous, broken by gorges and ravines, or
covered by a sea of sand. Plants may range from nearly invisible fungi to
towering cacti and trees.
Climate
Although desert climates vary from very hot to very cold, they are always
arid (dry).
In hot deserts, days are usually sunny and skies are cloudless. During the summer, daytime air temperatures between 105˚ and
110˚F (43.8˚ and 46.8˚C) are not unusual. A record air temperature of
136.4˚F (62.6˚C) was measured in the Sahara Desert, in a place called El
Azizia, on September 13, 1922. The absence of vegetation exposes rocks
and soil to the sun, which may cause ground temperatures in the hottest
deserts to reach 170˚F (77˚C). Nights are much cooler. The lack of cloud
cover allows heat to escape and the temperature may drop 25 degrees or
more after the sun sets. At night, temperatures of 50˚F (10˚C) or less are
common, and they may even drop below freezing.
Temperature
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The Sands of Time
When living things die, moisture in the air aids the
bacteria that cause decay. Before long, tissues
dissolve and eventually disappear. Desert air is so
dry that decay does not take place or occurs
extremely slowly. Instead, tissues dry out and
shrink, turning an animal or human being into a
mummy.
In ancient Egypt around 3000 BC, the dead were
buried in shallow graves in the sand. The very dry
conditions mummified the bodies, preserving
them. Later, for those who could afford it, Egyptian burials became more complex. Internal
organs were removed, and the bodies underwent
special treatments designed to preserve them.
They were then placed into tombs dug into rocky
cliffs or, in the case of certain pharaohs (kings),
placed within huge pyramids of stone. In most
cases, bodies of the ancient Egyptians are so well
preserved that much can still be learned about
what they ate, how they lived, and what caused
their deaths.
Graves discovered in the Takla Makan (TAHK-lah
mah-KAN) Desert of China have also given scientists important information. (The name Takla
Makan means ‘‘the place from which there is no
return.’’) Well-preserved mummies as much as
3,800 years old have been found in the graves.
The mummies have European features and some
are dressed in fine woolens woven in tartan
(plaid) patterns commonly used by the ancient
Celts and Saxons of Northern Europe. Scientists
believe these mummies were the first Europeans
to enter China, which was officially closed to
outsiders for thousands of years. Evidence exists
that these people rode horses using saddles as
early as 800 BC, and they may have introduced
the wheel to China. Their descendants, who have
intermarried with the Chinese, still live in the
Takla Makan.
Winters in cold deserts at latitudes midway between the polar and
equatorial regions can be bitter. In the Gobi Desert, for example, temperatures below freezing are common. Blizzards and violent winds often
accompany the icy temperatures.
Rainfall varies from desert to desert and from year to year. The
driest deserts may receive no rainfall for several years, or as much as 17 inches
(43 centimeters) in a single year. Rainfall may be spread out over many months
or fall within a few hours. In the Atacama Desert of Chile, considered the
world’s driest desert, more than half an inch (1.3 centimeters) of rain fell in one
shower after four years of drought. Such conditions often cause flash floods,
which sweep vast quantities of mud, sand, and boulders through dry washes,
gullies, and dry river beds (called wadis or arroyos). The water soon evaporates
or disappears into the ground. The Atacama Desert is the site of the world’s
longest known drought; no rain fell for 400 years (from 1571 until 1971).
Precipitation
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In coastal deserts, fog and mist may be common. Fog occurs when
cold-water ocean currents cool the air and moisture condenses. The
Atacama Desert lies in a depression behind mountains, so most of its
precipitation is received in fog form.
Some deserts, such as the polar deserts, experience snow rather than
rain or fog.
Geography of Deserts
The geography of deserts involves landforms, elevation, soil, mineral
resources, and water resources.
Desert terrain may consist of mountains, a basin surrounded
by mountains, or a high plain. Many desert areas were once lake beds that
show the effect of erosion and soil deposits carried there by rivers. Wind
Landforms
Sandstone formations are seen in Monument Valley, Utah. IM AGE COP YR IGH T SI GE N, 2 007 . US ED U NDE R LI CE NS E FR OM
SH UT TER ST OCK .C OM .
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