Environmental Engineering Reference
In-Depth Information
Chapter
28
Humid tropical environments
The humid tropics are those parts of the world within the tropical belt where, on balance,
precipitation is greater than evapotranspiration. Its definition is somewhat arbitrary, as
there is a continuous poleward gradient from areas with rain throughout the year, through
the areas of seasonal rainfall, or monsoon areas, to the deserts. On the eastern side of the
continents the gradient is less strong, with a moist climate dominating despite a gradual
decrease in mean annual temperature as found from Malaysia through Vietnam to Hong
Kong and China and also in Brazil. Not all equatorial areas are humid, as parts of Kenya,
north-east Brazil and northern Peru are all close to the equator yet for different reasons
experience very dry climates.
It is an area of the world which has come into prominence through concern about the
destruction of the tropical rain forest and its implications for the global environment.
Commercial exploitation and population pressure have led to stresses in such areas, with
a loss of resources, such as soil. We will devote a considerable part of this chapter to
these problems, as the use and exploitation of the forests of the humid tropics is a major
international dilemma.
CLIMATE
The tropics have been described as the firebox of our atmospheric engine. Much of the
sun's energy is absorbed here - energy which is transferred eventually into cooler,
energy-poor latitudes. There are three main circulation features of the tropics that
influence the climate. First there is the equatorial trough zone (or intertropical
convergence zone, ITCZ) which meets the popular idea of the humid tropics. It represents
the low-pressure zone along the thermal equator and moves polewards into the summer
hemisphere, interacting with the trade wind flows from the subtropical high-pressure cells
(Figure 28.1). In this area we encounter the monsoon zones. These are areas affected by a
seasonal wind reversal, usually in association with the movements of the equatorial
trough. There is normally a wet season and a dry season, with the wet season occurring
during the influx of maritime air associated with the trough and the dry season
developing when the trough moves away and draws dryer, continental air from more
polar regions. The classic area for this is India but other parts of the world have similar
seasonal wind reversals associated with wet and dry seasons, such as West Africa and
Australia. Each may differ in detail, depending upon geographical factors.
