Persistent net radiational heating of the tropical earth surface leads to
widespread occurrence of static instability of the tropical atmosphere. Large
amounts of evaporation from the sun-warmed oceans also assures ample supplies of water
vapor in the air. There is, consequently, a large scale, persistent band of organized
convection throughout the tropics called the Inter Tropical Convergence Zone (ITCZ).
The ITCZ extends along a long band in the deep tropics, and can often be traced
aound the whole globe. This persistent band of organized convection draws air into
it al low levels from latitudes north and south of its location.
Gravity and mass conservation requires that the large amounts of rising air in this
convection zone bear off to horizontal flow at high altitudes. The need to transport
energy out of the tropics
dictates a bearing off towards the higher latitudes of the Northern
and Southern Hemispheres, rather than remaining in the tropics. As the air moves through
the subtropics and begins to interact with the flow regimes of the middle latitudes,
the mixed and cooled air subsides, thus completing a large scale, organized, tropical
convection system called the Hadley Circulation.
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THE TROPICAL HADLEY CIRCULATION
