DESERTS Never-ending sand dunes. Scorching temperatures. Vultures circling over the remains of an unlucky traveler. These are images the word “desert” brings to mind for many people. December and January trigger phone calls to desert parks from potential visitors hoping to escape the chill of winter. They are amazed to hear of nighttime temperatures below freezing. Snow in the desert is an impossibility to them. Isn’t the desert always hot and dry? DESERTS ����� ����� ������� ������ ������ ������ ���������� ������ �������� ������ ������������ �� ��� ������� ������� distributed throughout the year. Weather receives only 40% of possible solar What Is A patterns often create short, violent radiation.) The hot, dry air causes any Desert Anyway? downpours. Flash floods, characteristic available water to evaporate quickly. of some desert areas, are produced. When temperatures are extremely hot, heck five sources and you will Much of the water runs off before it can rain can evaporate before it reaches probably find five, slightly different C soak into the soil. Earth. definitions. Some sources define a A lot of moisture is also lost to The conditions producing high desert as an area receiving no more than evaporation. Many deserts lie in areas daytime temperatures reverse the ten inches of precipitation annually. of high pressure systems where there process after sundown. Approximately However, many areas receiving this is little cloud cover. At least 90% of 90% of the day’s accumulated heat amount of precipitation are not deserts. the sun’s rays reach Earth, producing radiates back toward the sky. In moister This simple definition is not complete. seasonal hot temperatures. (For climates only about 50% of this heat Both the timing and type of comparison, the surface of more humid is lost. These conditions produce precipitation determine the environment lands, covered with more vegetation, the wide range of daily temperatures established. In a desert, rain isn’t evenly UNIT II–2 UNIT II–3 DESERTS DESERTS characteristic of deserts. This range is It cools, dropping heavy rains on often fifty degrees or more. tropical areas. The resulting cooler, The rapid heating and cooling of air drier air then descends, creating zones GLOSSARY create another characteristic of most of high atmospheric pressure as it endemic — belonging exclusively or deserts — strong winds. These winds, moves away from the equator. The confined to a particular place. circulating air which is often hot and descending air hinders cloud formation environment — all those factors, dry, increase the already high rate of and precipitation. It also warms up, both living and non-living, which evaporation. Evaporation in American absorbing any available moisture. The make up the surroundings of an deserts ranges from 70˝ – 160˝ per year. Sahara, the world’s largest hot desert, is organism. A desert then is not so easily defined. a subtropical desert about the size of the evaporation — the process by All these characteristics — seasonal, United States. which water changes into vapor. high temperatures; low, sporadic Coastal deserts are also in areas of rainfall; a high rate of evaporation; wide high pressure. Damp, chilly fog forms flash flood — a sudden, rising flood temperature ranges; strong winds — are when air, chilled by water contact as caused by heavy rainfall. part of the definition. it blows toward shore, meets warm air gneiss — a metamorphic rock where over land. Although humidity is high, intense pressures and temperatures atmospheric disturbances that can cause have caused minerals to segregate Where Are The rainfall are not present. Two coastal giving the rock a banded appearance. Deserts? deserts, the Atacama of Chile and the groundwater — water stored Namib in southern Africa, are among the raw a line around the world, beneath the surface of the ground, driest places in the world. starting mid-center between Joshua coming from precipitation and D Interior deserts, like the Gobi, exist Tree and Death Valley, and you will touch surface water that has percolated because they are too far from moisture- or come close to many of the world’s down. laden, ocean winds. By the time these great deserts — Mojave, Great Basin, metamorphic — changed by great winds reach the center of a large Sahara, Arabian, Iranian, Gobi. Most pressure, stress, and/or chemical landmass, the air is very dry. deserts occur between the latitudes of changes, usually at depth in the crust, Rain shadow deserts are created 15° – 40° on either side of the equator. from pre-existing rocks. when mountain ranges lie parallel to They are found around the world on moist, coastal areas. Prevailing winds native — a plant or animal that every continent, covering approximately moving inland cool as air is forced to rise evolved or was transported to an 20% of the Earth’s land area. over the mountains. Carried moisture falls area through natural means. Sand dunes cover only about 10% on slopes facing the winds. When the Precambrian — all geologic time of this area. Some deserts are very winds move over the crest and down the prior to the Paleozoic Era (prior to mountainous. Most are hot, with warm far side, they are very dry. Descending 570 million years ago). daytime temperatures much of the year, air also makes it hard for additional but others are cold, getting over half precipitation — water received on clouds and precipitation to form. their moisture from snow. the Earth directly from clouds as rain, Without another source of moisture, hail, sleet, or snow. rain shadow deserts are formed on the Why Are Deserts far side of these mountain ranges (see Discovery Activity Page #1). the south (mainly between 34° – 38°N Where They Are? latitudes). Elevations are generally eserts can be divided into four types The Mojave Desert between three and six thousand feet, D— subtropical, coastal, interior, rain although Death Valley National Park shadow — depending on the conditions he Mojave Desert is a rain includes both 11,049-foot Telescope Peak creating them. T shadow desert. It is defined by and the lowest point in the United States, Subtropical deserts lie along the a combination of latitude, elevation, at Badwater, 282 feet below sea level. Tropic of Cancer (23°N latitude) and geology, and indicator plants. It is Temperatures are a function of both the Tropic of Capricorn (23°S latitude). situated between the Great Basin latitude and altitude. Although the Near the equator hot, moist air rises. Desert to the north and the Sonoran to Mojave Desert has the lowest absolute UNIT II–2 UNIT II–3 DESERTS DESERTS elevation and the highest maximum more water available for irrigation, Coast. These mountain ranges create temperature (134°F in Death Valley), it farming in desert areas has increased. the rain shadow effect described in the is north of the Sonoran Desert and its This, along with human population introduction to this unit. average elevations are higher. As a result, growth and its water demands, has 1. Enlarge the rain shadow diagram its average temperatures are lower than depleted groundwater supplies faster on the activity page on a piece of poster those of the Sonoran. than they can replenish themselves. In board. Cut it into pieces to create a The Mojave Desert is in what some some cases, desert plant and animal puzzle. If you have a large group, you geographers call the Basin and Range communities can no longer be supported. may want to make more than one puzzle. Province, a landscape of alternating As the popularity of driving off-road Laminating or covering the pieces with mountain ranges and their adjacent vehicles across the desert increases, clear contact paper will extend their life. basins. Common rock substrates so does the damage done to plants and 2. Give a copy of the activity page include Precambrian gneisses and fragile desert soils. Deserts are mined for to each student. Use the diagram to granites. Slopes are often composed of copper, silver, gold, and other minerals, explain rain shadow deserts. metamorphic rocks, such as gneisses. leaving tremendous scars on the land. 3. Leaving their activity pages Mojave Desert vegetation is Many people seem to have a desert behind, have students stand in a circle, dominated by low, widely spaced shrubs. wasteland mentality. They view deserts surrounding the area where the puzzle Vegetation in the desert’s northern as lifeless, valueless areas. Resource will be assembled. Give puzzle pieces to half closely resembles that of the Great destruction, that might concern them if members in your group. Ask participants Basin Desert, as that in the southern done elsewhere, may not bother them to step in one at a time. Each may either half does the Sonoran. However, nearly when desert areas are the victims. Desert attempt to place the puzzle piece in the one quarter of all Mojave Desert plants education is necessary to increase right location or move a piece already are endemics. Synonymous with the appreciation of these biologically and placed. After the puzzle is completed, Mojave Desert is the Joshua tree. aesthetically rich lands. have a volunteer explain how the rain Other endemics are Parry saltbush and shadow effect creates dry areas. Mojave sage. A more widely distributed EXTENDING THE EXPERIENCE: plant, the creosote bush, dominates Contact areas on both sides of the much of the land surface, often in close Activity 1 mountain ranges and in the mountains association with species of bur-sage. Desert Puzzler to collect rainfall totals for the past three years (such as Los Angeles, Creating Desert OBJECTIVES: Explain the creation of a Big Bear, and Joshua Tree and Death rain shadow desert. Name one desert Valley national parks in California; and Wastelands created this way. Las Vegas and Lake Mead National eserts are growing by an estimated MATERIALS: Discovery Activity Page Recreation Area in Nevada).