Three convection cells per hemisphere
The global circulation emerges from a simple thermodynamic principle: the equator receives more solar energy than the poles, driving convection. Warm air rises at the equator, flows poleward at the top of the troposphere, and sinks near 30 degrees latitude. This creates the Hadley cell, dominant in the tropics. The Ferrel cell occupies the mid-latitudes, driven partly by Hadley cell dynamics and partly by poleward transport of heat. The Polar cell completes the set, with cold dense air descending at the poles and flowing equatorward near the surface.
Each cell carries its own wind signature. Hadley cells produce the trade winds that blow eastward near the equator. Ferrel cells generate the westerlies of temperate latitudes. Polar cells yield the cold easterly winds of high latitudes. These bands are not perfectly stationary; they shift seasonally, with the entire pattern migrating toward the summer pole.
Latitude and deserts
Deserts cluster near 30 degrees latitude almost everywhere on Earth because that is where the Hadley cell sinks. Sinking air warms adiabatically, relative humidity drops, and clouds dissipate. The Sahara, Arabian, Australian, and Kalahari deserts all occupy this latitude band. The wet tropics sit at the equator where air rises and rains persistently. Mid-latitudes are wetter on average because the Ferrel cell does not produce a strong sinking zone.
This circulation pattern persists because it is a natural consequence of unequal solar heating and Earth's rotation. Changing the energy input from the sun would alter these cells, shifting desert locations and rainfall patterns poleward or equatorward. The cells are not permanent fixtures but they are remarkably stable features of Earth's climate system.