The Aridity Gradient.

Deserts are not defined by heat, but by a deficit. They are regions where the potential evapotranspiration exceeds the actual annual precipitation.

The Sahara represents the absolute extreme of the aridity gradient. In these hyper-arid zones, rain may not fall for decades. The landscape is shaped almost entirely by aeolian processes—the mechanical force of wind moving sand into massive structures called ergs.

The Rain Shadow Effect

Most deserts are born from geography rather than latitude. As moisture-laden air hits a mountain range, it is forced upward, cools, and drops its water on the windward side. By the time the air reaches the leeward side—the rain shadow—it is bone dry. This is why the Atacama remains the driest place on Earth; it is trapped between the Andes and the Pacific coastal range.

Atmospheric Stability

High-pressure systems near the 30-degree latitudes create sinking air that prevents clouds from forming. This atmospheric "lid" keeps the desert in a state of constant solar bombardment. Without cloud cover to trap heat at night, the desert experiences the most violent temperature swings on the planet—plunging from 50°C at noon to below freezing at midnight.