The word desert is derived from the Latin word desertum, which means “abandoned.” Although there are various ways to define desert, the widely used Koppen classification system defines desert climates as those averaging at least twice as much potential evapotranspiration as precipitation during the year. This definition fits large stretches of the planet as well as other, adjacent land considered to be “semiarid” that present substantial problems for their use. The world’s largest desert is the Sahara, occupying about 3.5 million square miles of northern Africa. The arid region of which it is a part extends through Southwest Asia and Central Asia. All continents except for Antarctica contain dry, hot deserts. Australia is usually considered to be the world’s driest continent in that more than 40% of its surface is desert, with the significant adjacent semiarid land. The long, narrow strip of South America’s Atacama is believed to be the driest area region on the planet, containing locations where many years pass between rainfall events.
During the settlement of North America’s trans-Mississippi west in the 1800s, the term “Great American Desert” was commonly applied to semiarid areas thought not to be capable of supporting agriculture and significant habitation. Occasionally, modern writers will refer to “polar deserts” because polar climates are some of the driest on the planet. For instance, places in Antarctica’s interior receive less than one inch of precipitation a year. Moreover, the precipitated water is in frozen form, not directly usable by life forms.
Three types of deserts are described according to their causes. Subtropical deserts inhabit the latitudes between 20° and 30° north and south of the equator. They are due to the year-round presence of subtropical highs in the Hadley Circulation (the strongest of the Earth’s three cells of circulation, formed as warm air rises above the equator and flows northward.) The intense descent of air in the subtropical highs precludes significant cloudiness, and these are latitudes in which the solar angles are relatively high year-round. Thus, the subtropical deserts are the hottest places on Earth. The Sahara is this type of desert.
The second type of desert comprises the middle latitude deserts. They are caused by the summertime influence of subtropical highs and also lack precipitation because of topographic blockage and great distance inland from an oceanic source of precipitable water. The middle latitude locations mean that summer temperatures are not as high so that the imbalance between precipitation and evapotranspiration is not nearly as great and the aridity is not as great as in the subtropical deserts. A noteworthy aspect of such regions is that winter brings cold temperatures and some snow with the passage of middle latitude cyclones. The Gobi is an outstanding example of a middle latitude desert.
The third type of desert is a coastal desert. Whereas one might suppose the presence of a coast would make for a water source and a wetter environment, certain coasts are impressively dry. Coastal deserts might be subtropical, but their coastal locations amplify their aridity. This desert type is found along coasts with cold currents bolstered by upwelling. Air passing over these near-shore waters is cooled and stabilized, sapping any chance to rise to initiate the precipitation processes. Accordingly, these coastal deserts are quite a bit cooler than their inland counterparts. The Atacama and eastern African Namib deserts are examples of this extraordinary dryness.
There are several common misconceptions of the desert environment. One is that sand covers them. Dry places have much slower rates of weathering of rocks into the tiny pieces of clay so prevalent in moist areas, so deserts have greater amounts of sand on their surfaces. Sand dunes do not represent the “average” desert surface. It is believed that sand covers only about 15% of the Earth’s desert surfaces. More common are rocky surfaces with high percentages of sand in the soil material.
Of course, sand dunes are the iconic symbols of deserts. Several types of sand dunes are classified based on the local amount of sand supply and the seasonal nature of direction and strength of the wind. The world’s tallest dunes approach 1,600 feet (Algeria). The largest stretch of sand surface is the Rub’ al Kali (Empty Quarter), covering about 250,000 square miles of southern Saudi Arabia, Yemen, Oman, and the United Arab Emirates.
The fact that wind-crafted sand dunes are associated with deserts does not mean that they are windier places than their moister counterparts. The Wind is simply more active in deserts because of the lack of vegetation to hold soils and small pieces of weathered rock in place. Although sand blown by the wind polishes some desert surfaces, water is a far greater shape of desert landscapes. In the driest deserts, the abundance of dry stream channels is ample evidence that water erodes and deposits much more total material than the wind.
Desert precipitation is noticeably scarce. How dry is a desert? Quantitative definitions such as that of Koppen are based on rainfall amounts that change with temperature. For instance, Albuquerque, New Mexico garners a little precipitation of about 11 inches per year. This renders Albuquerque quite dry with a natural cover consisting of scattered bushes and grasses, which grew in response to the high evapotranspiration of the summer. The northeastern portion of the province of Alberta, Canada has similar yearly precipitation, but it is a land of forest and swamp because of considerably lower evapotranspiration. A hallmark of deserts is the unreliability of rainfall. There is enormous year-to-year variability in the amount and timing of precipitation. In both the subtropical and middle latitude deserts, rain is intense when it does come because a significant percentage of it originates from convectional storms.
The traditional image of a desert is a hot, dry area devoid of vegetation, but this is a stereotype not uniformly true. Net primary productivities are lower than any land regions except for those covered by ice. Both plants and animals are scarcer and less diverse than in other realms, though they do exist in deserts. Plant life is adapted to extreme dryness through the development of deep or wide root systems or forms that lessen transpiration such as leaves modified into thorns. In short, desert plants are very efficient at conserving water. Most desert animals are nocturnal, and so are not visible to the casual observer.
An oasis is a desert locale in which there is verdant life clustered around springs, lakes, ponds, or streams emanating from groundwater sources. The water table is close to the surface, and artesian water emerges under its pressure. The vegetation around oases provides food for limited numbers of grazing animals, so for thousands of years, nomadic peoples have traveled from oasis to oasis to allow their animals to eat and drink.
Deserts make up about one-third of Earth’s lands, and so have shaped the habitation patterns of the planet. Perhaps 5% of humans live in deserts. There are the big cities located in deserts (for example, Phoenix, Arizona and Cairo, Egypt), but these places can obtain water not dependent on local rainfall. In the case of Phoenix, the city uses groundwater and an aqueduct bringing water from the Colorado River almost 200 miles away. Cairo is situated on the Nile River, which flows without significant tributaries carrying water from wet tropics near the equator thousands of miles away.