Why Deserts Are Where They Are

[A] Look at a globe and the great deserts line up in two bands, roughly thirty degrees north and thirty degrees south. The Sahara, the Arabian, the Sonoran, the Kalahari and the Australian interior all sit at about the same latitude, on different continents, which is too tidy to be a coincidence.

[B] The explanation is a circulation pattern in the atmosphere. Air heated at the equator rises, and rising air cools; cool air holds less moisture, so it drops its water as the tropical rain that keeps equatorial forests wet. [C] That air, now dry, travels north and south at high altitude and sinks back down at about thirty degrees. Sinking air warms, and warm air can hold more moisture than it is carrying, so instead of raining it pulls moisture out of whatever it touches. [D] Not every desert is made this way. Some sit in the rain shadow of a mountain range, where air has already lost its water climbing the windward side; others, like the Atacama, lie beside a cold ocean current that chills the air above it and suppresses evaporation. But the two great bands are the work of the circulation, and they are the reason a map of the world's deserts looks organised rather than random.