An understanding of the processes of condensation and precipitation allows us now to take another look at the global energy balance, and to include the mechanism of latent heat transport.
We can now also summarize the pattern of transport of water vapour across the parallels of latitude; this is a transport of matter and is part of the global water balance. Let us review the energy exchanges and water transports for each latitude zone.
The equatorial zone is characterized by a rise of moist, warm air in innumerable convectional cells reaching to the upper limits of the troposphere. As condensation and precipitation occur here, enormous amounts of latent heat energy are liberated. This zone has been called the 'fire box' of the globe, in recognition of this intense production of sensible heat by condensation. Moving equator ward to replace the rising air are the tropical easterlies, or trades, within the Hadley cell circulation. Water evaporates from the ocean surface over the subtropical highs and is carried equator ward in vapour form.
Next, we move into middle latitudes, where upper-air waves constantly form and dissolve in the westerlies. Here, cyclones and anticyclones exchange cold, polar air from warm, tropical air across the parallels of latitude. Water vapour is also transported poleward and reaches a peak rate of flow at about 40° N and 40° S lat. Condensation in cyclonic storms removes water vapour from the troposphere in increasing quantities into high latitudes, so that the movement of water vapour declines and reaches zero at the poles.
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