The major types of desert are commonly classified according to the climatic and geographical processes responsible for their aridity. Important categories include subtropical deserts, coastal deserts, rain shadow deserts, continental interior deserts and polar deserts. Some classifications also distinguish cold winter deserts and other specialised arid environments. These categories are not always mutually exclusive because a single desert can be influenced by more than one mechanism.
The Sahara, for example, is primarily a subtropical desert, while the Atacama combines subtropical, coastal and rain shadow influences. Antarctica is a polar desert, and the Great Basin contains strong rain shadow and continental influences. The essential common feature is water scarcity, while temperature and geographical setting determine the specific form of desert environment.
Subtropical deserts occur mainly in the subtropical high pressure belts, commonly around 20 to 30 degrees latitude in both hemispheres. Air that has risen in the tropics eventually descends in the subtropics as part of the Hadley circulation. Descending air warms adiabatically, reducing relative humidity and suppressing cloud formation. This produces broad belts of dry climate around the globe. The Sahara, Arabian Desert, Kalahari region, Australian interior and Sonoran and Chihuahuan desert regions are influenced by subtropical circulation, although local geography modifies their climates. These deserts can be extremely hot during the day and can experience large daily temperature ranges.
Coastal deserts occur near coastlines where cold ocean currents and atmospheric stability suppress rainfall. The Atacama Desert along the Pacific coast of South America is a classic example. The Humboldt or Peru Current carries cold water northward along the coast, reducing evaporation and helping maintain a stable layer of cool air. Moist air can be present offshore without producing substantial rainfall over land. The Namib Desert is similarly influenced by the cold Benguela Current along southwestern Africa. Coastal deserts demonstrate that proximity to an ocean does not necessarily mean a region is wet. Ocean temperature and atmospheric circulation determine whether the sea supplies moisture effectively to the adjacent land.
Rain shadow deserts form on the leeward side of mountains. Moist air approaches a mountain range and is forced upward. As it rises, it expands and cools, causing condensation and precipitation on the windward side. After crossing the crest, the now drier air descends and warms, increasing its capacity to hold water and reducing the likelihood of further precipitation. This creates an arid zone on the leeward side. The Great Basin of North America is a major example, with mountain ranges intercepting moisture moving inland from the Pacific. Rain shadow effects can also contribute to deserts in South America, Central Asia and other mountainous regions.
Continental interior deserts occur deep within large landmasses where atmospheric moisture has difficulty reaching the interior. Central Asia contains extensive arid and semi arid environments influenced by great distances from oceans and complex mountain systems. Interior deserts often experience strong continentality, with hot summers and very cold winters. Precipitation may be low and irregular, and snowfall can contribute a significant fraction of annual moisture in colder regions. The Gobi is an important example of a cold continental desert. It is not primarily a sand sea but contains rocky and gravelly terrain, demonstrating again that desert classification concerns climate rather than surface material.
Polar deserts are deserts because they receive very little precipitation, despite being covered extensively by snow and ice. Antarctica is the largest polar desert and is also the driest continent by average precipitation. The Arctic contains polar desert environments on some islands and high latitude land areas, although the Arctic as a whole is not a continent. Extremely cold air holds little water vapour, so precipitation is limited. Much of what precipitation occurs falls as snow. Because temperatures are low, snow and ice can persist for long periods, creating an apparent paradox in which a region contains abundant frozen water while remaining climatically extremely dry.
Desert categories can overlap, and boundaries between them are often gradual rather than sharp. A desert may be subtropical in latitude, coastal in location and rain shadow affected because several climatic mechanisms operate simultaneously. The Atacama is a particularly clear example of overlapping controls, involving the subtropical high pressure belt, the cold Humboldt Current and the Andes rain shadow. The Namib similarly combines subtropical circulation with strong coastal oceanic influence. Classification systems also differ in their definitions and thresholds, so the exact assignment of a desert can vary between climatic, ecological and geographical sources.
Understanding desert types is useful because the mechanism producing aridity determines the environment, seasonal pattern and potential response to environmental change. A coastal desert may remain relatively cool and foggy despite extremely low rainfall, while a subtropical interior desert may experience intense heat and large daily temperature ranges. A polar desert is dominated by snow and ice rather than dunes, and a rain shadow desert may occur immediately beside a comparatively wet mountain slope. The major desert types therefore demonstrate that desert is an ecological and climatic category rather than a single landscape form.
