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“The Global Circulation can be described as the worldwide system of winds where the necessary transport of heat from tropical to polar latitudes is accomplished,” (metoffice.gov.uk).These atmospheric heating and circulation patterns are controlled by three cells of air flow between the Earth’s poles and the equator. These three cells are the Polar Cell, Ferrel Cell, and the Hadley Cell. The polar cell originates at lower latitudes, its warm air rises and move to higher altitudes, where the cooled air will descend due to the high pressure. The Ferrel cells winds are complicated; it is a boomerang flow of air at high altitudes towards the tropics that join sinking air at lower latitudes near the Hadley cell. When humid and warm air from both hemispheres brought by trade winds joins in the Intertropical Convergence Zone the low-pressure areas move up to 30 degrees latitude until it descends in high-pressure areas which then causes convection in the atmosphere resulting in high precipitation in the tropics. This cycling of winds is called Hadley cell.
High precipitation occurs in zones where there is the continual and rapid ascension of air masses. In the tropics, there is a convergence of trade winds in the ITCZ which ascend due to the low pressure. Also, the evaporation rates are high. Thus, moist air rises as it cools. On the other hand, warm air is able to condense and hold more water leading to high precipitation (Schumacher, 2003).
It is at the temperate latitudes where the convergence of moist subtropical air and cold polar air causes forced condensation. Also, the region receives the most intense sun radiation per unit area, thus, increased evaporation. Besides, the low-pressure system causes a rise in more air for precipitation. Clouds build up due to the rising air that picks up moisture from the dry land at lower latitudes (Schumacher, 2003).
In tropical zone, low precipitation can occur as a result of warm, prevailing winds being impeded by mountain ranges since the mountains force the moisture in those winds to come down on the mountainside that is blocking the winds as rain leaving the other side with low precipitation. When air rises moisture is picked up from dry regions at lower latitudes it causes clouds to build up bringing a high amount of precipitation since precipitation is largely involved with temperature that means the lower the temperature the less moisture it can hold because it is very warm near the tropics the air moisture will have to interact with colder weather in order to lose it. Therefore causing the precipitation to stay low. This levitation is known as rain shadow.
The tropical dry forests and savanna biomes are located in the Inter-Tropical Convergence Zone (ITCZ) where high precipitation is observed, due to the amount of sunlight near the equator. Tropical savannas are covered with grassland and a few scattered trees while tropical dry forests have more trees that are green during rainy seasons and withered in dry seasons. The two biomes are usually warm throughout the year as the wind movement cause shifting of Inter Tropical Convergence Zone (ITCZ). The sunlight causes air to rise which causes the pressure to drop, and makes the warm low-pressure air to rise and condenses into clouds high precipitation occurs. The ITCZ shifts between Northern and Southern hemispheres causing seasonal variations in pressure and hence precipitation. In the dry season, however, the ITCZ moves to the warm and dry trade winds flow to the other side of the equator.
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