Hadley Cells and Rossby Waves

Warming at the equator sets up two huge convection cells which circulate air to the North and South. These are called Hadley Cells, after the 18th century English lawyer and amateur meteorologist who discovered them. Watch this video to learn more about how the Hadley cells are formed.

Source: global circulation (1-54) | samwsm1 | YouTube

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Since installation is strongest when the Sun is directly overhead, the equator is heated more strongly than other places on earth. Heated air rises and cold air sinks resulting in low surface pressure at the equator and high surface pressure at the polls. The engine of low latitude atmospheric circulation is the Hadley cell. Convection occurs at the thermal equator. Poleward-moving air is forced to descend. This produces two sub-tropical belts of high-pressure, centered about thirty degrees latitude. Surface winds spiral out from the sub-tropical highs, moving toward the equator as well as the mid-latitudes. Above 30 degrees latitude wind patterns are more complex, in a belt of conflict between polar and sub-tropical air known as the polar front. The latitude which the Sun is directly overhead changes with the seasons. Since Hadley cell circulation is driven by this heating, we can expect elements of the Hadley cell to migrate as well. At high altitude, air moves without the drag of surface friction. This geostrophic wind moves along rather than across the pressure gradient. The westerly flow of upper air frequently forms undulations called Rossby waves. Warm air pushes poleward while troughs of cold air from the South are pinched off leaving pools of cool air in the mid-latitudes.