Environmental Engineering Reference
In-Depth Information
Over the continents the convective area expands considerably, as seen over central Africa
in Plate 7.1.
What is the climate of the equatorial trough like? Figure 28.3 gives an example of
mean monthly temperature and rainfall for Manaus in Brazilian Amazonia. The mean
monthly maximum temperature varies by 2·8° C over the year and the mean monthly
minimum by only 0·6° C. Extremes are rare and insignificant by temperatelatitude
standards. The diurnal variation of temperature is more noticeable than the annual
variation. At Manaus mean annual rainfall is high, with 1811 mm, though even in this
zone there is a drier period when rain days are fewer. It is at this time of year that burning
of the forest takes place. This somewhat drier season is experienced in most of the
equatorial trough zone, though its intensity and duration vary. Only a few areas have no
drier season. For example, Padang in Sumatra (Indonesia) receives an average rainfall of
4427 mm and only one month has less than 250 mm. The driest season occurs when the
trough moves farthest polewards in response to continental heating in the summer
hemisphere. As one moves farther away from the equatorial trough zone so the dry
season lengthens and we reach the monsoon areas. Ironically annual rainfall totals in
monsoon areas can be even greater than in the humid tropics, where factors favour
rainfall, though there is always a strong seasonal pattern. At Cherrapunji in Assam the
Khasi hills assist orographic uplift of moist monsoonal air from the Bay of Bengal to give
a mean annual total of 11,074 mm but virtually no rain falls between November and
March (see Chapter 5).
The humid tropics cover a wide range of climatic types and hence show considerable
environmental diversity. Temperatures are fairly stable throughout the tropical world,
tending to be somewhat higher during any dry season and cooler and with a smaller
diurnal cycle during the wet season. The main variable is the quantity of rain and its
seasonal distribution. In this section on climate, for completeness, we have included some
areas which have a relatively short dry season because the origins of their climate are
similar to more humid areas. For the rest of the chapter we will concentrate on the more
humid areas where the dry season is shorter than the wet season and there is a annual
surplus of moisture.
