Wednesday, 7 March 2012

Climate & Atmospheric circulation

Hadley, Ferrel and Polar Cell

These terms are related to the general circulation of the atmosphere. In this circulation there are three cells of vertical circulation. Try to picture a cross-section of the earth. At the equator the air rises with heating and in the upper atmosphere begins to flow toward the north. At about 30 degrees north latitude it sinks and flows back toward the equator forming the easterlies in the band from 30 N to the equator. This cell is the Hadley circulation.

Between 30 N and about 60 N there is a similar circulation but in a reverse mode. It flows at the surface from 30 N to 60 N where it rises along the Polar front and returns aloft by flowing southward where it sinks at 30N. This cell is called the Ferrel cell.

Finally, north of 60 N there is the Polar cell which in the upper atmosphere flows toward the Pole where it sinks and flows southward near the surface.

Where the Ferrell and Hadley cells sink at 30 N we have an area of generally high pressure. The Ferrell cell flow at the surface is the cause of the westerlies. And with the Polar cell the surface winds are called the polar easterlies. This same pattern is repeated in the southern hemisphere.