The world’s largest and most persistent deserts, such as the Sahara in North Africa and the vast Australian Outback, are not scattered randomly across the globe. This striking geographical pattern sees major arid regions clustered around 30 degrees north and 30 degrees south of the equator. The consistent placement of these dry landscapes points to a powerful, large-scale meteorological engine that dictates the distribution of moisture across the planet. Uncovering the reason for this arrangement requires understanding the mechanics of global atmospheric movement.
Defining the Subtropical Desert Belt
The subtropical zone is generally defined by the latitudinal bands roughly between 15 and 35 degrees north and south of the equator. Within these regions lie the planet’s most extensive hot deserts, a direct result of global air circulation. These areas are characterized by extremely low annual precipitation, often less than 30 centimeters, and high daytime temperatures. Major examples include the massive Sahara and Arabian Deserts in the Northern Hemisphere and the Kalahari and Great Australian Deserts in the Southern Hemisphere. The arid conditions here result in landscapes where evaporation significantly exceeds precipitation.
The Engine of Atmospheric Circulation
The source of this global pattern begins at the equator, a region known as the Intertropical Convergence Zone (ITCZ), where intense solar radiation heats the Earth’s surface, causing the air above it to warm and become buoyant. This warm, moist air rises vigorously in a continuous column, creating a persistent zone of low pressure at the surface.
As the air ascends, it cools and expands due to the lower pressure of the upper atmosphere. Since cooler air holds less water vapor, the massive amount of moisture carried aloft condenses into immense cloud formations. This process results in the heavy, frequent rainfall that sustains the world’s tropical rainforests located along the equator.
Having shed the vast majority of its water content, the now-dry air mass continues to rise until it hits the boundary of the stratosphere. At this high altitude, the air mass must diverge and flow horizontally toward the poles, both north and south, away from the equatorial low-pressure zone. This entire circulation system, involving the air rising at the equator and flowing poleward aloft, is known as the Hadley Cell.
High Pressure and Descending Dry Air
The poleward-moving air from the equator eventually accumulates and begins to sink back toward the Earth’s surface at latitudes around 30 degrees. This downward movement of air, known as subsidence, creates a continuous belt of high atmospheric pressure (the subtropical high or anticyclone), which directly causes the desert belt. This sinking action suppresses the vertical movement of air, acting as a lid that prevents the formation of clouds and rain-producing weather systems.
The primary factor in creating arid conditions is the effect of adiabatic warming on the descending air. As the air sinks, it is compressed by the increasing weight of the atmosphere above it, causing its temperature to rise. This warming is a result of the compression, occurring at a rate of approximately 10°C per kilometer of descent.
This rapid warming dramatically increases the air’s capacity to hold water vapor, even though the air mass is already moisture-depleted from its equatorial journey. When this warm, dry air reaches the surface, it acts like a sponge, drawing any residual moisture out of the landscape below. The combination of high pressure inhibiting cloud formation and adiabatic warming desiccating the land is the primary reason why the world’s major deserts align perfectly with the sinking branches of the Hadley Cells.
Contributing Local Factors
While the Hadley Cell circulation is the fundamental cause of subtropical deserts, local geographical features can intensify the arid conditions. One such intensifier is the rain shadow effect, where mountain ranges block the passage of moisture-laden air.
Rain Shadow Effect
As air is forced to rise over a mountain barrier, it cools, and moisture condenses, releasing precipitation on the windward side. By the time the air descends on the leeward side, it is dry, and the subsequent adiabatic warming creates a dry, hot region, intensifying the desert conditions. The Andes Mountains, for example, contribute to the extreme aridity of the Atacama Desert in South America through this process.
Cold Ocean Currents
Another influential factor is the presence of cold ocean currents running along the western coasts of continents in the subtropics. Currents like the Humboldt and Benguela chill the air directly above the ocean surface. This cold air stabilizes the atmosphere, preventing the formation of convective currents that would normally lead to clouds and rainfall over the adjacent land. This stabilization limits potential moisture from the ocean from moving inland, resulting in coastal deserts like the Namib and Atacama, which are among the driest places on Earth.