One vast loop of air builds the rainforests and the deserts
Near the equator warm air rises, flows toward the poles about 12 to 15 kilometres up, then sinks around 30 degrees latitude and returns as the trade winds. This loop, the Hadley circulation, covers almost half the planet. It waters the tropics, dries out many of the world's deserts, and is widening as climate warms.
The engine is uneven sunlight. Over a year the equator absorbs more solar energy than it radiates, while higher latitudes lose more than they gain. Without some way to move heat poleward, the tropics would keep warming and the rest of the planet keep cooling. Rising air over the hot equator and sinking air in the cooler subtropics set up pressure differences that drive the flow, filling the full depth of the troposphere across the whole band between the two tropics.
Each cell has distinct branches. Near the surface, the trade winds carry moist ocean air toward the equator, bent partly westward by the Coriolis effect. They meet in the Intertropical Convergence Zone, home to Earth's heaviest rainfall. The upward leg happens mostly inside towering thunderstorms that cover only about one percent of the tropics; somewhere between 1,500 and 5,000 of these hot towers a day are needed to lift the heat. High up, the air turns poleward, cools, and descends over the subtropics, creating high-pressure ridges that suppress rain. Many of the world's deserts sit beneath those sinking branches.
Averaged over a year there are two cells, one per hemisphere, with the southern one slightly stronger. In solstice seasons a single cell dominates instead, with air ascending on the summer side of the equator and descending on the winter side. As the rainy convergence zone shifts with the seasons, it brings monsoons. The circulation also feeds the subtropical jet stream and carries momentum and moisture, and weaker Ferrel cells operate over the mid-latitudes. Similar patterns may exist on Venus and Mars.
The idea is almost three centuries old. In 1735 George Hadley proposed hemisphere-wide loops driven by heating differences to explain the trade winds. Others later refined his qualitative argument, and the circulation was confirmed in the mid-20th century once radiosonde balloons routinely sampled the upper air. Since at least the 1980s observations and models show the Hadley cells expanding poleward, and projections suggest they will widen and weaken through this century.
Source: Hadley cell
