When someone says the word "desert," it's almost certain that you immediately picture the stereotype depicted in movies and other forms of popular culture: Sand as far as the eye can see in all directions, no plants with the possible exception of cactus or two, a total absence of water and an abundance of searing sunlight. Deserts appear, in a word, inhospitable. Yet few people in North America have any first-hand experience with deserts.
While in general the above impressions are reasonably accurate, a desert is not merely a patch of arid land; rather, a desert constitutes a biome, or community of living things connected to a particular type of geography. In addition, deserts are anything but scarce. Deserts, in fact, account for one-fifth of Earth's land area, and come in four distinct varieties.
What Is a Desert?
Deserts are characterized by extreme environmental conditions. They get at most 50 centimeters (cm), or 20 inches, a year of precipitation; more commonly, they're lucky to get half of that. Most of them are found at low latitudes, that is, closer to the equator than to the poles. The massive Sahara, probably the most famous desert on Earth and its third largest, lies just north of the equator in Africa. While they are far less densely populated than other biomes owing to how dry they are and being poorly hospitable overall, most deserts do feature a range of vegetation as well as both vertebrate and invertebrate animal life.
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Large mammals are uncommon in deserts because most of them cannot store sufficient water and tolerate the heat (camels are a notable exception). While smaller animals might be able to find patches of shade sufficient to cover their bodies, deserts typically offer little protection from the sun for larger animals. The dominant animals of the warm deserts are non-mammalian vertebrates, mainly reptiles. Whatever mammals have managed to thrive in these biomes tend to be small, such as the kangaroo mice that inhabit some deserts in North America.
A few sentences ago, you read that the Sahara is the third-largest desert in the world. Did this perhaps surprise you? Have you heard elsewhere that the Sahara is far and away the world's biggest desert? The explanation for this is surprising and powerful.
How Many Types of Deserts Are There in the World?
While ecologists agree that there are four fundamental types of deserts, the nomenclature of these four desert biomes varies slightly from source to source. The four basic desert types are the hot-and-dry (or subtropical) desert, the semiarid (or cold-winter) desert, the coastal desert and the cold (or polar) desert. These are described individually in detail later, but a brief overview is helpful to start.
Hot-and-dry deserts are, well, hot and dry. Various kinds of deserts experience very hot weather, but this type gets it all year. Cold winter deserts have long, arid summers and a small amount of rainfall in the winter. Coastal deserts have cool winters, but warm summers. Polar deserts are cold year-round.
To continue the intrigue from the preceding section, the two largest deserts in the world are polar deserts. One is the Antarctic Polar Desert, and the other is the Arctic Polar Desert. How can vast areas covered mainly or entirely in snow and ice, which is clearly a form of moisture, qualify as deserts?
What Are the Four Different Types of Desert?
Hot-and-dry deserts probably best fit the average person's idea of what a desert should look and feel like. The Sahara is one such desert. Others appear in Australia, South Asia, and Central and South America. The U.S. features the Chihuahuan, Sonoran, Mojave and Great Basin deserts.
The seasons are warm to quite hot year-round, and owing to the low humidity of these environments, the temperature swing from the hottest time of day to the coldest time of day may be extreme – over 45 C (about 80 F) in some regions. This is mainly because the surface receives twice as much solar radiation during the day as would the surface in comparable but more humid environment, and loses twice as much heat at night.
Rainfall is usually very sparse in hot-and-dry deserts, and evaporation rates routinely outpace rainfall rates. Falling rain has even been noted to evaporate before reaching the ground. What little rain these deserts get occurs in short, brief and sometimes intense bursts, though monsoons and remnants of tropical systems that drift into some deserts can provide abundant moisture at times. The Atacama Desert in Chile on the western coast of South America, known as the world's driest place, receives an average of 1.5 cm per year of rain – barely a half-inch.
Plants in hot-and-dry deserts are mostly low shrubs and short, woody trees. Animals include small nocturnal carnivores, with comparatively high populations of burrowers and kangaroo rats. Insects, arachnids, reptiles and birds are also common. The animals hide from the sun and then come out to forage at dusk or at night, when the desert is coolest.
Cold winter deserts, also called semiarid deserts, are characterized by moderately long, dry summers and winters that include brief intervals of rain. This pattern is like that of hot-and-dry deserts, but the overall temperatures are somewhat cooler. U.S. examples include the sagebrush zones of Utah, Montana and the Great Basin. They also include northern, but subarctic, portions of North America, Newfoundland, Greenland, Russia, Europe and northern Asia.
Summer temperatures in these deserts usually average between 21-27 C (70-80 F). It normally does not rise above 38 C (100 F), and evening temperatures are cool, at around 10 C (50 F). Annual rainfall can be as low as only 2 to 4 cm (about 0.8 to 1.5 inches).
Soil can range from sandy and fine-textured to loose rock fragments, gravel or sand. There is no subsurface water in these environments. As for vegetation, cacti (the plural of "cactus") are found here. The spines of cacti and other plants in cold winter deserts provide protection in a difficult natural setting. The multitude of spines offer enough shade for the surface of these plants to reduce water losses through transpiration. Many plants have glossy leaves, which lets them reflect more light energy. Semiarid desert plants include Creosote bush, bur sage, white thorn, cat claw, mesquite, brittle bushes, lyciums and jujube.
As for animals, insects and jack rabbits are seen during the day, staying in shadow as much as possible. Many animals seek protection in burrows underground, where they are insulated from the hot, dry air. These include kangaroo rats, rabbits, skunks, some insects, birds and reptiles.
Coastal deserts are found in regions that are generally cool to moderately warm. Portions of the aforementioned Atacama Desert in Chile represent the coastal desert biome. Here, cool winters alternate with relatively long and warm summers. Temperatures are moderate compared to the two desert biomes already discussed. Average summer temperatures range from 13-24 C (55-75 F); winter temperatures are 5 C (41 F) or colder. The maximum annual temperature is close to 35 C (95 F), and the minimum is about -4 C (25 F).
Rainfall, while sparse, exceeds that of hot-and-dry and cold-winter deserts, averaging around 8 to 13 cm (3 to 5 inches) per year. The soil in these deserts is high in salt and other nutrients. Some of the plants have extensive root systems, unlike the flora in the aforementioned desert types. These plants are almost botanical analogs of camels in that they can store very large amounts of water for future use when it is available. These plants include the salt bush, buckwheat bush, black bush, rice grass, little leaf horsebrush, black sage and chrysothamnus.
Coastal-desert animals boast special adaptations for dealing with heat and lack of water. For example, some toad species seal themselves in burrows with sticky, gel-like secretions and remain inactive for eight or nine months until a heavy rain washes them out. Amphibians that include larval stages of development feature accelerated life cycles, bettering their chances of reaching maturity before the rainwater evaporates. Some insects lay eggs that are able to stay dormant in adverse conditions, maturing only when their environment is more suitable for hatching; fairy shrimps do the same. Coastal-desert mammals include coyotes and badgers; birds include the famed great horned owl, golden eagle and the bald eagle. Lizards and snakes are the chief reptilian representatives.
Polar deserts or cold deserts are curiosities, like almost everything about the Earth's poles. Compared to other desert biomes, they receive a veritable flood of precipitation, especially in the winter months. Mean annual precipitation is about 15 to 26 cm (6 to 10 in). Winter in the Arctic Polar Desert – which spans 5.4 million square miles in parts of Alaska, Canada, Greenland, Iceland, Norway, Sweden, Finland and Russia – occurs between mid-December and mid-March, while that in the 5.5-million-square-mile Antarctic Desert that spans the continent after which is was named falls between mid-June and mid-September.
Polar desert plants are widely scattered over the vast lands in which they grow. Plant heights can reach 122 cm (about 4 feet) in some areas. The main plants are deciduous, meaning that they have leaves they shed seasonally with most of these having spiny leaves. Fungi and dwarf shrubs are also common.
What Are the Major Types of Desert Biomes?
Some sources list more than four desert types to better account for the variability in geographic and ecological factors from place to place. For example, the U.S. Geological Survey lists eight types of deserts: trade wind, mid-latitude, rain shadow, coastal, monsoon, polar deserts, paleodeserts and extraterrestrial deserts. The last two are not found on Earth; paleodeserts are areas that show evidence of having been deserts in the recent geological past, whereas extraterrestrial deserts are found on other planets, such as Mars.
Trade wind deserts are analogous to hot-and-dry (subtropical) deserts. Mid-latitude deserts overlap with the cold-winter deserts in the four-desert-type scheme. Rain shadow deserts, which are also cold-winter-style deserts, form on the sides of tall mountain ranges blocked from receiving much moisture. Monsoon deserts are seen in India and Pakistan. Coastal and polar deserts retain the same basic definitions as before.
What Are the Five Largest Deserts in the World?
The two largest deserts in the world are the Antarctic Polar Desert, which is 5.5 square million miles in area, and its northern counterpart, the Arctic Polar Desert, which includes 5.4 million square miles. The United States, by way of comparison, is about 3.5 million square miles in size. The Antarctic Polar Desert is more easily visualized because it is confined to a single, large, somewhat circular land mass.
The Sahara Desert in northern Africa covers about 3.5 million square miles and is recognized by some sources as being the world's largest desert because the polar deserts are not traditional deserts. The fourth-largest at 1 million square miles is the Arabian Desert that takes up the Arabian Peninsula in the Middle East, while the fifth-largest is the Gobi Desert of China and Mongolia, which covers 500,000 square miles.