The Sahara, the Arabian Desert, the deserts of the American Southwest and northern Mexico, and Australia's interior deserts sit on entirely different continents, with no obvious geographic connection to each other. They do share one striking thing: nearly all of them cluster within a fairly narrow band of latitudes roughly twenty to thirty degrees north and south of the equator. That pattern isn't coincidence; it's a direct, predictable consequence of how the atmosphere circulates globally, a pattern called Hadley cell circulation.
Warm, wet air rises at the equator and falls, dry, further out
Air near the equator is heated intensely by strong, direct sunlight, causing it to rise. As it rises, it cools, and since cooler air holds less moisture, the rising air releases much of its water vapour as rainfall — which is exactly why equatorial regions tend to be lush and heavily rained on, supporting dense tropical rainforest. That same air, now considerably drier, continues moving through the upper atmosphere away from the equator, and by the time it descends back toward the surface — at roughly twenty to thirty degrees latitude, both north and south — it's lost most of its original moisture. This descending air also warms as it compresses on the way down, which further reduces its relative humidity and its tendency to produce rain, creating a broad band of persistently dry, sinking air at those specific latitudes.
The same circulation pattern repeats identically at every longitude
This circulation loop — air rising and losing moisture at the equator, then sinking dry roughly twenty to thirty degrees away from it — is called a Hadley cell, and it operates as a genuinely global atmospheric pattern, not a regional or continent-specific one, which is exactly why its effect shows up consistently at similar latitudes on entirely different, geographically unconnected continents. The zone of persistently dry, descending air the Hadley cell produces at those latitudes is a major reason so many of the world's largest deserts happen to sit in roughly the same narrow latitude band worldwide, despite having no other geographic connection to one another — the shared cause isn't regional geology or coincidence, it's the same predictable global circulation pattern operating identically at every longitude around the planet.
What we're still unsure about
The basic mechanism of Hadley cell circulation, and its strong general connection to the global distribution of subtropical deserts, is extremely well established atmospheric science, consistently observed and modelled. What's more genuinely variable, and depends on local factors the basic Hadley cell explanation alone doesn't capture, is exactly how a specific desert region's boundaries and severity are shaped by additional local influences — nearby mountain ranges, ocean currents, and continental interior effects can all significantly modify how a given stretch of subtropical latitude actually plays out on the ground, which is why real deserts don't form one single, perfectly uniform band around the globe even though the underlying Hadley cell driver behind their general location is the same everywhere.
This sits inside Climate Systems & Atmospheric Circulation, one of seven topics in Physical Geography, one of five domains in Geography, one of seventeen subjects the app can quiz you on.