A desert is not defined by sand, and it is not defined by heat. It is defined by the absence of water. Any region receiving less than roughly 10 inches (250 millimeters) of precipitation a year meets the standard. Antarctica’s frozen interior qualifies. So do the dunes of North Africa.

That single measurement hides a lot of machinery. Four mechanisms do most of the work. Sinking air suppresses cloud formation, and mountains strip moisture from storms before they cross the crest. Cold ocean water shuts down evaporation. Some interiors simply sit too far from the sea.

Warm, moist air rises near the equator. It cools as it climbs, releases its water as tropical rain, then drifts poleward at high altitude. Near 30 degrees north and south, that air has cooled enough to sink back toward the surface. Descending air compresses and warms. Warming air holds more water vapor rather than less, so clouds evaporate instead of thickening. This overturning pattern is called Hadley circulation. It parks a belt of high pressure across both subtropics and keeps rain away for months at a stretch.

The result is visible on any globe. The Sahara, the Arabian Desert, the Kalahari, the Thar, and most of arid Australia all sit inside those two bands. The Sahara alone spreads across about 3.6 million square miles (9.2 million square kilometers). Terrain builds deserts, too. Air pushed up the windward flank of a mountain range cools and condenses. It drops that moisture as rain or snow before clearing the crest. Whatever descends the far side arrives warm and wrung out.

Death Valley sits behind several ranges standing between it and the Pacific. The floor at Badwater Basin lies 282 feet (86 meters) below sea level. Central Asia’s Gobi exists in part because the Himalaya intercepts the summer monsoon. Some of the driest ground on the planet sits within sight of the ocean. Cold water offshore is the reason. The Humboldt Current carries chilled sub-Antarctic water up the coast of Chile and Peru. The Benguela Current does the same along the coast of Namibia.

Cold water evaporates slowly, and it chills the air directly above it, creating a temperature inversion that traps whatever moisture forms into a shallow fog layer near the surface. Fog rolls inland and coats the ground. Rain almost never follows.

The Atacama shows the effect at its most extreme. Some weather stations in the desert’s core have logged annual rainfall near 0.02 inches (0.5 millimeters). Even so, the ground is not sterile; researchers continue to find microscopic life beneath the surface.

A fourth mechanism needs no mountains and no ocean current. It just needs distance. Air masses lose moisture steadily as they travel over land. Continental interiors dry out because storm systems reach them exhausted. The Taklamakan and the drylands of Kazakhstan and western China sit thousands of miles from any significant body of water. These are cold deserts, with brutal winters and short, hot summers. Aridity has nothing inherent to do with temperature.

Antarctica pushes that point further. The continent covers roughly 5.5 million square miles (14.2 million square kilometers). Most of its interior receives very little snowfall, which makes it the largest desert on Earth by a wide margin.

Frigid air holds almost no water vapor, so there is nothing available to precipitate. In the McMurdo Dry Valleys, katabatic winds pour down off the polar plateau and strip away what little ice remains, leaving bare rock and gravel.