Atmospheric Circulation
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Global Circulation The animation to the left shows global air circulation. It spans one year and uses hourly data. Cloud cover is shown in white, and orange shows precipitation. Look out for seasonal weather phenomenon that you can see in different regions at different times of the year. This animation was produced by the Community Climate System Model. |
The General Atmospheric Circulation System
Atmospheric circulation is the large scale movement of air around the globe. The three-cell model of atmospheric circulation can be used to explain this movement of air and air pressure. The differential heating of the surface of the Earth creates a pattern of pressure cells. These cells are called the Hadley, Ferrel and Polar cells of atmospheric circulation. The air in these cells generally moves in a circular direction and it is these cells that cause the transfers of energy from the equator to other parts of the Earth. These three cells are positioned in both the northern and southern hemispheres and due to the circular motion of the cells, areas of high pressure are created when air falls and low pressure when air rises.
The major factor driving the atmospheric circulation system is solar heating. This is generally higher at the equator and lower at the poles.
The major factor driving the atmospheric circulation system is solar heating. This is generally higher at the equator and lower at the poles.
Hadley CellThe Hadley cells are responsible for the seasonal changes in climates of regions that experience a wet and dry climate.
Between the two Hadley cells at the equator is an area of low pressure called the ITCZ (Inter-Tropical Convergence Zone). There is a lot of surface evaporation as the sun is always high in the sky and therefore heats the ground quickly. As the hot air rises in convection currents, an area of low pressure is developed. The rising air cools and water vapour condenses, giving heavy rainfall. |
Ferrel CellThe Ferrel cells occur at higher latitudes than the Hadley cells (between 30 and 60 degrees north and and 30 and 60 degrees south.
They are responsible for the climate types occurring in the mid-latitudes. Air on the surface is pulled towards the poles, forming the warm south-westerly winds in the northern hemisphere and the north-westerly winds in the southern hemisphere. L Warmer air from the tropics is lighter than the dense, cold air from the poles so it rises where the two masses meet. This uplift causes unstable conditions and mid-latitude depressions experienced in cool temperate western maritime (CTWM) climates. |
Polar CellAt the north and south poles, descending air from the Polar cell results in high pressure.
Wind always blows away from areas of high pressure to areas of low pressure. At both the north and south poles, the air is pulled towards the mid-latitude low-pressure belt. Due to the Earth spinning on its axis, cold air is transported away from high latitudes and warm air is brought in by the Polar cell. |