Global atmospheric circulation (GAC)
The movement of air across the planet occurs in a specific pattern. The whole system is driven by the equator, which is the hottest part of the Earth. Air rises at the equator, leading to low pressure and rainfall.
When the air reaches the edge of the atmosphere, it cannot go any further and so it travels to the north and south. The air becomes cold and falls to create high pressure and dry conditions at around 30° north and south of the equator. Large cells of air are created in this way. Air rises again at around 60° north and south and descends again at around 90° north and south. Global atmospheric circulation creates winds across the planet and leads to areas of high rainfall, like tropical rainforests, and areas of dry air, like deserts.
Global circulation can be described as the worldwide system of winds by which the necessary transport of heat from tropical to polar latitudes is accomplished. In each hemisphere, there are three cells (Hadley cell, Ferrel cell, and Polar cell) in which air circulates through the entire depth of the troposphere. The troposphere is the name given to the vertical extent of the atmosphere from the surface, right up to between 10 and 15 km high.
Hadley cell
Hadley Cells are the low-latitude overturning circulations that have air rising at the equator and air sinking at roughly 30° latitude. They are responsible for the trade winds in the Tropics and control low-latitude weather patterns.
The largest cells extend from the equator to between 30 and 40 degrees north and south and are named Hadley cells, after English meteorologist George Hadley. Within the Hadley cells, the trade winds blow towards the equator, then ascend near the equator as a broken line of thunderstorms, which forms the Inter-Tropical-Convergence Zone (ITCZ). From the tops of these storms, the air flows towards higher latitudes, where it sinks to produce high-pressure regions over the subtropical oceans and the world's hot deserts, such as the Sahara desert in North Africa
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