Many of the world’s largest and driest deserts are found around 30 degrees north and south of the equator. This global pattern, evident in regions like the Sahara in North Africa or the Atacama in South America, is explained by the planet’s intricate system of heat distribution and air circulation.
Global Air Circulation: The Hadley Cells
The sun’s energy at the equator heats the Earth’s surface and the air above it. This warm, moist air becomes less dense and rises into the atmosphere. As it ascends, the air cools, and the water vapor it carries condenses, forming clouds and causing frequent, heavy rainfall in equatorial regions.
After releasing its moisture, this drier air continues to rise to the upper troposphere. From there, it flows poleward. As this air travels, it continues to cool and gradually loses altitude, eventually sinks back towards the Earth’s surface at approximately 30 degrees latitude in both hemispheres. This continuous circulation pattern, involving rising air at the equator and descending air at 30 degrees latitude, is known as a Hadley Cell.
Descending Air and High-Pressure Zones
As the cool, dry air from the upper atmosphere descends at these subtropical latitudes, it undergoes compression and warms significantly. As air warms, its capacity to hold moisture increases. This descending air then actively absorbs moisture from the land below, rather than releasing it as precipitation.
This continuous sinking of dry, warming air creates persistent high-pressure systems at 30 degrees north and south latitude. High-pressure systems are characterized by stable atmospheric conditions, clear skies, and suppressed rainfall. These stable, high-pressure zones directly contribute to the arid conditions observed in these regions, making them ideal environments for desert formation.
Reinforcing Factors for Arid Regions
While Hadley Cell circulation is the primary reason for deserts at 30 degrees latitude, other factors can intensify aridity. Cold ocean currents flow along the western coasts of continents at these latitudes, such such as the Benguela Current or the Peru Current. These currents cool the air above them, reducing its ability to pick up moisture and inhibiting rainfall, sometimes creating fog but little actual precipitation.
Regions deep within continental interiors, far from oceanic moisture sources, also experience increased dryness. Air masses often lose much of their moisture content over coastal regions before reaching these inland areas. Mountain ranges can also contribute to desertification through the “rain shadow” effect. As moist air is forced to rise over mountains, it cools and releases its moisture on the windward side; the descending air on the leeward side is then dry and warms, creating arid conditions.