How Deserts Form

Most hot deserts sit in two bands near 30 degrees latitude, north and south of the equator. The cause is the air overhead. At the equator, warm air rises and wrings out its moisture as tropical rain. That now-dry air drifts poleward and sinks back down near 30 degrees, forming a zone of high pressure that smothers rainfall. This sinking-air belt is why the Sahara and the Arabian deserts line up like beads on a string.

Other deserts form in a rain shadow. When moist air slams into a mountain range, it is forced to climb. Climbing air cools, and cool air cannot hold its water, so rain falls on the upwind slope. By the time the air crests the peak and slides down the far side, it is wrung dry. California's Mojave Desert exists for exactly this reason — the Sierra Nevada steals the moisture out of Pacific winds before they ever arrive.