Not What You Think

Say "desert" and most people picture rolling sand dunes. Yet only about a quarter of deserts are sandy. The rest are rocky plains, gravel flats, and bare bedrock. Stranger still, the largest desert on Earth is not the Sahara. It is Antarctica — a frozen continent that receives almost no precipitation. A desert is about how little falls from the sky, not how much sand lies on the ground.

A desert is defined by aridity, not heat. The standard line is less than 250 mm of precipitation a year — about the height of a basketball. By that rule, plenty of deserts are brutally cold. The Gobi drops to roughly -40 degrees in winter. Parts of Chile's Atacama have no record of rain ever falling. Dryness makes a desert; temperature is just the costume it wears.