Geoscience Reference
In-Depth Information
27
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 arbi-
trary, 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 dry
climates.
It is an area of the world which has come into promi-
nence 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.
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
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 drier, 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.
Equatorial trough
The traditional idea of the equatorial climate involves the
daytime build-up of convectional clouds into massive
cumulonimbus displays. Rainfall is frequent and abun-
dant, temperatures and humidity are high, acting together
to give us the tropical rain forests. At night the air is humid
and still. Condensation takes place on to the vegetation,
and the sound of moisture dripping to the forest floor
competes with that of the wildlife.
The structure of the atmosphere, though, is not as
simple as this model may suggest. The multitude of names
which have been used for the area give some idea of its
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
