Deserts are commonly defined as regions that receive extremely low amounts of precipitation, typically less than 250 millimeters of rain per year. This lack of moisture creates environments where plant and animal life must possess specialized adaptations to survive. Deserts cover roughly one-third of Earth’s land surface. Their placement is governed by predictable, large-scale atmospheric and geographic forces that prevent moisture-laden air from reaching or releasing its water content in specific locations around the world.
Defining Arid and Semi-Arid Zones
The official classification of a desert focuses on aridity, which is the imbalance between the moisture an area receives and the moisture it loses. This balance is often quantified using an aridity index, comparing precipitation to potential evapotranspiration. In a true arid zone, precipitation is substantially less than the potential water loss.
The widely accepted threshold for a true arid zone is an annual precipitation rate that rarely exceeds 250 millimeters. Regions that are slightly less dry, known as semi-arid zones or steppes, generally receive between 250 and 500 millimeters of annual precipitation. These semi-arid areas form transitional belts around the true deserts, supporting sparse grassland or scrub vegetation.
Deserts are categorized by their lack of water, not their temperature. Hot deserts, such as the Sahara, have a mean annual temperature above 18° Celsius. Conversely, cold deserts, like the Gobi, have a mean annual temperature below 18°C and often experience freezing winters. Both types are characterized by the severe deficit of moisture.
Global Distribution Patterns
The distribution of the world’s major deserts follows three distinct geographic patterns, each linked to a specific climate-forming process. The most significant pattern involves the subtropical desert belts, which host the largest hot deserts on Earth. These arid zones are consistently found in bands centered near 30 degrees north and 30 degrees south of the equator.
Examples include the Sahara and Arabian Deserts in the Northern Hemisphere and the Kalahari and Australian Deserts in the Southern Hemisphere. This placement is a direct consequence of global atmospheric circulation, which creates persistent zones of high pressure and sinking dry air at these latitudes.
A second pattern involves coastal deserts, found along the western edges of continents. The Atacama Desert in Chile and the Namib Desert in Namibia are prime examples. These deserts remain intensely dry due to the influence of cold ocean currents that run parallel to the coastlines.
The third pattern is represented by interior continental deserts, located deep within the landmasses of large continents. The Gobi Desert is a classic illustration. These regions are far removed from any significant oceanic moisture source, meaning that air masses reaching them have already shed most of their water content.
The Mechanisms of Desert Formation
Hadley Cell Circulation
The primary reason for the existence of the largest deserts is the operation of global atmospheric circulation cells, specifically the Hadley Cell. This circulation is a vast, continuous “conveyor belt” of air driven by intense solar heating near the equator. Warm, moist air rises powerfully, creating a persistent low-pressure zone. As this air ascends, it cools, and the water vapor condenses, leading to the heavy, frequent rainfall characteristic of tropical rainforests.
After shedding its moisture, this now-dry air flows poleward in the upper atmosphere. Around 30 degrees latitude, the air becomes dense and begins to sink back down toward the Earth’s surface, creating persistent high-pressure systems. As the air sinks, it warms through compression, a process called adiabatic heating. This warming rapidly increases the air’s capacity to absorb water, suppressing cloud formation and preventing precipitation. This continuous cycle maintains the great subtropical deserts.
Rain Shadow Effect
A second major mechanism is the rain shadow effect, which occurs when mountain ranges physically intercept moisture-laden air. As prevailing winds force moist air upward over a mountain range, the air expands and cools adiabatically. This cooling causes the water vapor to condense and precipitate as rain or snow on the windward side of the mountain.
Once the air mass crosses the summit, it has lost much of its moisture content. The air then descends the leeward side of the range, where it undergoes compression and warms substantially. This warm, dry, descending air absorbs any remaining surface moisture and creates an arid “shadow” zone. The Great Basin Desert in the western United States, situated behind the Sierra Nevada, is largely a product of this effect.
Cold Ocean Currents
The third significant mechanism is the influence of cold ocean currents, which produce deserts along the western coasts of continents. Currents like the Humboldt Current off South America and the Benguela Current off Africa carry frigid water toward the equator. When warm, moist air from the ocean passes over this cold water, the air is chilled from below, which stabilizes the atmosphere.
This cooling prevents the air from rising high enough to condense its moisture and form rain-producing clouds. While the cold water often results in heavy coastal fog, the atmosphere above remains too stable for significant vertical movement and precipitation to occur. The resulting environment is intensely dry, yet often cool and foggy, exemplified by the hyper-arid Atacama Desert.
Continentality
Finally, the factor of continentality contributes to the aridity of regions located deep within large landmasses. Even without the influence of a rain shadow, air masses lose moisture progressively as they travel inland over a continent. By the time the air reaches the deep interior, such as in parts of Central Asia, it has been effectively dried out by distance and previous precipitation events. This geographical isolation from oceanic moisture sources reinforces the aridity created by atmospheric patterns and mountain barriers.