The distribution of Earth’s major deserts, such as the Sahara, Atacama, and Great Australian Desert, reveals a striking pattern: they are consistently found close to 30 degrees latitude, both north and south of the equator. This geographical curiosity is not an accident of local terrain, but the direct outcome of the planet’s largest-scale atmospheric circulation system. This global engine moves heat and moisture across the globe, creating predictable bands of intense rainfall and extreme dryness.
Solar Energy and Equatorial Uplift
The driving force behind global circulation begins with the sun, which delivers its most concentrated energy to the equatorial regions. This intense solar radiation heats the surface, causing warm, moist air to rise vigorously in a belt known as the Intertropical Convergence Zone (ITCZ). This creates a persistent zone of low atmospheric pressure. As the air ascends, it cools and the water vapor condenses, resulting in the heavy precipitation characteristic of tropical rainforests. This upward motion effectively wrings most of the moisture out of the air near the equator, leaving the air mass relatively dry as it continues to rise.
The Mechanism of the Hadley Cell
Once the air reaches the upper atmosphere, it diverges and moves horizontally toward the poles. This large-scale, closed-loop circulation pattern is known as the Hadley Cell, which transports energy from the equator toward the subtropics. The air mass begins to cool through radiation and expansion as it travels poleward. This high-altitude flow carries the dried air mass away from the equatorial uplift zone until it reaches approximately 30 degrees latitude. At this point, the air has cooled sufficiently and become dense enough to begin its descent back toward the Earth’s surface. The Hadley Cell connects the wet tropics with the dry subtropics, ensuring the descending air is already moisture-depleted. The Coriolis effect plays a role in determining the precise latitude where this air mass sinks.
Compression and Drying at Subtropical Latitudes
The descent of this dry air mass back to the surface around 30 degrees latitude is the most direct cause of desert formation. As the air sinks, it is subjected to increasing atmospheric pressure, causing the air temperature to rise dramatically through adiabatic warming. This heating occurs because the air parcel is being squeezed, which significantly increases its capacity to hold water vapor. The relative humidity plummets, turning the descending air into a powerful drying agent upon reaching the surface.
This constant downward movement creates persistent, stable high-pressure systems at the surface, known as the subtropical high-pressure belts. High pressure suppresses vertical air movement, preventing the formation of clouds and storm systems. The resulting conditions—clear skies, minimal precipitation, and intense solar radiation—are the hallmarks of desert climates.
The descending air then flows along the surface back toward the equator to complete the Hadley Cell circulation, forming the trade winds. This mechanism explains why the world’s largest arid areas, such as the Sahara and the Kalahari, are situated beneath these regions of subsiding, warming air.