All deserts have two things in common: they are dry, and they support

Transcription

1 All 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). 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, 123

2 WORDS TO KNOW Arroyo: The dry bed of a stream that flows only after rain; also called a wash or a wadi. Butte: A small hill. Deforestation: The cutting down of all the trees in a forest. 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. Estivation: An inactive period experienced by some animals during very hot months. Mesa: A flat topped hill. Oasis (plural is oases): A fertile area in the desert having a water supply that enables trees and other plants to grow there. 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. 124 U X L Encyclopedia of Biomes: Volume 1

3 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) U X L Encyclopedia of Biomes: Volume 1 125

4 Encyclopedia of Biomes: Volume 1 Finals/ 11/4/ :35 Page 126 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, 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 F (62.6 C) was measured in the Sahara Desert, in a place called El Azizia, on September 13, 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 126 U X L Encyclopedia of Biomes: Volume 1

5 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. Precipitation 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). U X L Encyclopedia of Biomes: Volume 1 127

6 Encyclopedia of Biomes: Volume 1 Finals/ 11/4/ :35 Page 128 Desert 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, US ED U NDE R LI CE NS E FR OM SH UT TER ST OCK.C OM. 128 U X L Encyclopedia of Biomes: Volume 1