Tropical climates - The Global Climate System: Patterns, Processes, and Teleconnections - page 59

Chapter 3
Tropical climates
3.1 Introduction
For many years, from early exploration to perhaps the middle of the twentieth
century, the weather and climate of the tropical world were considered among
the most easily explained of all world climate systems. The daily rainfall of the
equatorial zone, the constancy of the trade winds, and the unrelenting aridity of
the tropical deserts gave an impression of benign and unchanging conditions.
Such is far from the case. Within the tropics are some of the most interesting and
difficult to explain phenomena of the world's climates.
The Earth's tropical regions are, in terms of geographic location, the area
between the Tropic of Cancer (23.58 N), and the Tropic of Capricorn (23.58 S).
Such a definition is not suitable, however, for identification of the climatic
regimes and over the years a number definitions have been suggested. In his
classification of world climate in 1896, Supan proposed that locations with
annual average temperature greater than 20 8C might be considered as the
tropics. In his widely used 1918 classification, K¨ppen classed tropical climates
as having the average temperature of each month greater than 18 8C. Using this
criterion, the wet tropical climates occupy some 36% of the Earth's surface.
If the tropical deserts are added to this class, then the tropics comprise almost
50% of the surface area of the world.
Additional thermal boundaries have been proposed by other authors, but in
reality the tropical climates of the world may be considered as those latitudes
that lie between, and partly include, the subtropical high-pressure regions that
are centered at about 308-358 N and S. The equatorial zone is a subset of the
tropical regions, occurring perhaps 108 north and south of the equator.
3.2 The climatic controls
As with all Earth climates, the most basic control is the energy balance. Within
the tropics, although varying slightly with the seasons, intensity of solar radia-
tion is high all year and there is very little variation in the length of the day from
one part of the year to the next. The photoperiod, the relative lengths of day and
59