Global Air Circulation Patterns
Deserts are defined by extreme aridity and very low annual precipitation. While many are hot, their defining characteristic is a lack of moisture, allowing unique ecosystems to survive.
Global air circulation patterns contribute to the formation of many expansive deserts. These patterns create persistent high-pressure zones where dry air descends. Hadley cells, atmospheric convection currents, exemplify this, circulating air between the equator and approximately 30 degrees latitude in both hemispheres.
Near the equator, intense solar radiation heats the Earth, causing warm, moist air to rise. As it ascends, this air cools and condenses, leading to cloud formation and heavy rainfall in equatorial regions. After releasing its moisture, the air moves poleward in the upper atmosphere, then descends around 30 degrees north and south of the equator.
The descending air compresses and warms, inhibiting cloud formation and precipitation. This creates stable, high-pressure systems with clear skies and minimal rainfall. Many large deserts, like the Sahara and Arabian Deserts, are located within these subtropical high-pressure belts, a direct consequence of this global atmospheric circulation.
Mountain Barriers and Rain Shadows
Mountain ranges create localized desert conditions through the rain shadow effect. They act as barriers to moisture-laden air masses moving across a continent. As moist air encounters a tall mountain range, it is forced to ascend the windward side.
As the air rises, it cools, and water vapor condenses to form clouds and precipitation. This results in abundant rainfall or snowfall on the windward slopes, supporting lush vegetation. By the time the air reaches the mountain crest and descends on the leeward side, it has lost most of its moisture.
The now dry air warms as it descends the leeward slope, reducing its relative humidity and ability to produce precipitation. This creates a “rain shadow” effect, where the land on the leeward side receives very little rainfall, leading to arid conditions. Examples include the Sierra Nevada’s contribution to Death Valley’s aridity and the Andes Mountains’ role in Patagonia’s dryness.
Ocean Currents and Landmass Size
Cold ocean currents shape coastal desert environments. Flowing along western coasts, they cool the overlying air, creating stable atmospheric conditions. This cool, stable air has a reduced capacity to hold moisture, inhibiting rain-producing clouds. As a result, these coastal areas receive very little precipitation, despite being adjacent to water.
The Atacama Desert in Chile, one of Earth’s driest places, exemplifies deserts formed by cold currents like the Humboldt. The Namib Desert in southwestern Africa is similarly influenced by the cold Benguela Current. These regions often experience frequent fog, but it rarely translates into significant rainfall.
The size of a landmass, known as continentality, also contributes to interior desert formation. Areas deep within large continents are far removed from oceanic moisture. As air masses travel inland, they gradually lose moisture through precipitation, becoming significantly drier by the time they reach the interior.
This distance from oceanic moisture results in lower annual precipitation and more extreme seasonal temperature variations. The Gobi Desert in Central Asia, for example, is a large interior desert receiving minimal moisture due to its vast distance from any ocean. This combination contributes to arid conditions in continental interiors.
Extreme Cold and Dryness
Deserts are not exclusively defined by heat; some arid regions are found in extremely cold environments. These “polar deserts” exist in the Arctic and Antarctic, with temperatures well below freezing most of the year. Despite vast ice sheets, these areas receive surprisingly little precipitation, often less than 250 millimeters annually, classifying them as deserts.
The primary reason for this aridity is that extremely cold air holds very little moisture. As air temperatures drop, air’s capacity to retain water vapor decreases significantly. Even if moisture is present, it is unlikely to condense into clouds and fall as snow. Existing moisture is often locked in massive ice formations, unavailable for atmospheric circulation.
Limited snowfall often remains on the ground for extended periods without melting, contributing to dry conditions. The dry valleys of Antarctica, for instance, are polar deserts. These areas demonstrate that dryness can be a function of temperature’s effect on air’s moisture-holding capacity, not just a lack of available water.