A desert is defined not by its temperature, but by its lack of precipitation, typically receiving less than 10 inches (25 centimeters) of rain annually. While many deserts are associated with high temperatures, they can also be cold; the defining factor is aridity. Deserts cover approximately one-third of the Earth’s land surface. The formation of these arid regions results from large-scale atmospheric patterns and specific geographical features that prevent moisture from falling as rain. The primary mechanisms responsible for creating deserts are global air circulation, mountain barriers, and the influence of cold ocean currents.
Atmospheric Circulation and High-Pressure Zones
The existence of the world’s largest deserts, such as the Sahara and the Arabian Desert, is a direct consequence of global atmospheric patterns, specifically the massive circulation cells known as Hadley Cells. These cells begin near the equator, where intense solar heating causes warm, moist air to rise and create a zone of low pressure. As this air rises into the upper atmosphere, it cools and releases its moisture as heavy tropical rainfall, which is why rainforests thrive near the equator.
The now-dry air mass, depleted of its water vapor, moves poleward in the upper atmosphere. At approximately 30 degrees latitude north and south of the equator, this cool, dry air descends back toward the Earth’s surface, creating persistent belts of high pressure. This descending air compresses and warms significantly in a process called adiabatic heating.
Adiabatic heating means the air warms purely due to compression from the increasing atmospheric pressure as it sinks. This warming increases the air’s capacity to hold moisture, making it highly unlikely for water vapor to condense into clouds or fall as rain. The result is a vast, dry air mass that actively absorbs moisture from the land below, creating the arid conditions characteristic of the subtropical desert belts. These high-pressure zones, sometimes referred to as the Horse Latitudes, are home to the most extensive hot deserts globally.
The Rain Shadow Effect
The rain shadow effect is a major mechanism for desert formation, occurring in the lee of large mountain ranges. This process begins when prevailing winds carry moisture-laden air from an ocean or large body of water toward a continent. When this moist air encounters a tall mountain, it is forced upward, a process known as orographic lifting.
As the air mass rises, the drop in atmospheric pressure causes it to expand and cool. This cooling leads the water vapor within the air to condense, forming clouds that release their moisture as precipitation—rain or snow—on the windward side of the mountain. By the time the air reaches the mountain crest, it has lost the majority of its water content.
The now-dry air begins its descent on the leeward side of the mountain range. This descending air warms due to adiabatic compression. This warm, dry air has a low relative humidity and actively absorbs moisture from the ground, creating an arid region known as the rain shadow. Examples include the deserts of the Basin and Range Province in the western United States, which lie east of the Sierra Nevada and Cascade Mountains.
Coastal Deserts and Cold Ocean Currents
A distinct category of deserts forms immediately adjacent to an ocean, a seemingly contradictory location, such as the Atacama Desert in Chile and the Namib Desert in southwestern Africa. The formation of these coastal deserts is directly influenced by cold ocean currents, such as the Humboldt (or Peru) Current and the Benguela Current. These currents transport frigid water from polar regions toward the equator along the western edges of continents.
The cold water cools the layer of air directly above the ocean surface. This chilled air has a very limited capacity to hold moisture, often creating dense fog as the air cools to its dew point. When this cool, stable air moves over the adjacent land, its stability is maintained by a temperature inversion.
The inversion traps the cool air near the surface beneath a layer of warmer air higher up, preventing vertical air movement. Since air cannot rise, the necessary lifting and cooling processes required to form rain-producing clouds are suppressed, resulting in extreme aridity. These regions experience little rainfall, despite the high humidity and frequent fog along the coast. The Atacama, for example, is the driest nonpolar desert in the world.