A desert is defined by a lack of moisture, receiving less than 10 inches (250 millimeters) of precipitation annually. This low precipitation occurs in both hot environments, like the Sahara, and cold ones, such as Antarctica or the Gobi Desert. Deserts form where evaporation greatly exceeds the water gained from rain or snow. Arid landscapes result from global atmospheric patterns, physical barriers on the Earth’s surface, and the influence of nearby oceans.
Global Air Circulation Patterns
The main driver for the world’s largest deserts is the planet’s global air circulation system, driven by solar energy. At the equator, sunlight heats the surface, causing air to warm and rise in a process known as convection. This rising, moisture-rich air cools as it ascends, leading to condensation and heavy rainfall that defines the wet tropical climate of the Intertropical Convergence Zone.
Once this air reaches the upper atmosphere, it flows poleward, away from the equator, having released its moisture. The air eventually descends back toward the surface around 30 degrees latitude north and south, completing a convection loop called the Hadley Cell. This descent creates a permanent belt of high atmospheric pressure known as the Subtropical High-Pressure Zone.
As the air sinks, increasing atmospheric pressure compresses it, causing it to warm adiabatically. This warming increases the air’s capacity to hold water, making it dry and stable. The resulting warm, dry air mass suppresses cloud formation and precipitation, leading to the formation of subtropical deserts. Examples include the Sahara and the Great Australian Desert.
The Role of Topography and Rain Shadows
Mountain ranges act as physical barriers that force moisture out of moving air masses, creating localized deserts in their lee. This process begins with orographic lifting, where prevailing winds push moist air up the windward slope. As the air is forced upward, pressure decreases, causing the air to expand and cool adiabatically.
The cooling air reaches its dew point, leading to condensation, cloud formation, and precipitation on the windward side. By the time the air crests the peak, it has lost most of its moisture. This dry air then begins its descent down the leeward side, where it experiences compression.
The descending air warms rapidly due to adiabatic heating, making precipitation unlikely. This effect creates an arid region known as a rain shadow. The Great Basin Desert lies in the rain shadow of the Sierra Nevada mountains. The Patagonian Desert is kept dry by the Andes mountain range, which strips moisture from Pacific air.
Influence of Ocean Currents and Continentality
The oceans contribute to desert formation in two ways: cold currents and the distance of a landmass from a water source. Cold ocean currents, such as the Humboldt Current or the Benguela Current, flow along the western coasts of continents. These currents cool the air immediately above the ocean surface.
When warmer, moist air flows over this cold water, the air mass base cools, creating a temperature inversion that stabilizes the atmosphere. This stable layer prevents the vertical air movement necessary for developing rain-producing clouds. While the air may produce dense coastal fog, it rarely generates rainfall, leading to environments like the Atacama Desert and the Namib Desert.
Continentality
Continentality refers to the dryness of land located deep in a continent’s interior. Air masses must travel over large bodies of water to carry moisture. As these air masses move inland, they progressively release moisture as precipitation. By the time the air reaches the central regions, it is depleted of water vapor. This explains deserts like the Gobi in Central Asia, which is thousands of miles from the ocean and shielded by mountain ranges.