Why are the world's great deserts in two belts around the globe?

The world's great deserts sit where they do because of a giant loop of air. Near the equator, the Sun heats the surface hard, and warm, humid air rises and dumps the heaviest rain on Earth. That air spreads toward the poles high above the ground, cools, and sinks again around 30 degrees latitude, north and south. This loop is called the Hadley cell. The sinking air suppresses rain, so many of the Earth's deserts lie in two belts just where it comes down.

The Sahara is the textbook case. Geographers call it a trade wind desert: it sits in the belt around 30 to 35 degrees north, linked to a vast descent of dry air. The same story, mirrored in the south, explains why so many of the world's driest lands sit in belts on either side of the tropics. Those belts are also called the horse latitudes, known for sunny skies, calm winds and very little precipitation.

Nobody is quite sure where that name came from. One documented explanation is a sailors' ritual: crews paraded a straw-stuffed horse around the deck, then threw it overboard. A popular alternative, usually counted as folk etymology, is a tale of Spanish ships carrying horses to the Americas.

The loop is named after George Hadley, who in 1735 proposed that circulation cells driven by differences in heating explain the trade winds. Edmund Halley had sketched a similar idea in 1685, but Hadley's version did not catch on for more than a century, partly because it was unintuitive and there were no observations yet to confirm it.