Wanderlust World Wanderlust World The correct answer is **expanding (subsiding) air**.
Deserts are defined by aridity. The primary mechanism is **subsiding air** in subtropical high-pressure belts (Hadley Cell circulation). As air descends, it compresses and warms adiabatically. This **expansion/warming drastically lowers relative humidity**, inhibiting cloud formation and precipitation. The result: persistent high pressure, clear skies, intense solar radiation, and extreme dryness—the classic desert climate.
While cold air holds less moisture, the key driver for major hot deserts (Sahara, Arabian, Australian) is the *dynamical drying* caused by large-scale subsidence, not just cold temperatures.