Appendix A - Definition of the Humid Tropics

Summary

The humid tropics can be defined in different ways, depending on the objectives. The three best known approaches may be briefly summarized.

Köppen classification. Wladimir Köppen, German biologist, was the pioneer of climatic classification with his original work being published in 1900 and revised in 1936.

Köppen presented formulas, which included monthly temperature and rainfall data, to delineate arid (desert) and semiarid (steppe) climates. Outside the arid and semiarid zones, all climates are forest climates, except in polar regions and high mountains where low temperatures prohibit the growth of trees.

Within the forest climates, three major types are recognized according to temperature. Köppen's tropical climate (A climate) includes areas where the mean temperature of the coldest month exceeds 18°C, the optimum temperature for human comfort. The 18°C isotherm also agrees well with the distribution of palm trees, a typical tropical plant.

The A climate has three sub-types: Af (tropical rain forest), Am (monsoon forest), and Aw or As (savanna). In the Af climate, monthly rainfall exceeds 60 mm throughout the year. In the Am climate, the dry season is either very short or the rainfall during the wet season is copious. Both the Aw and As climates have a distinct dry and wet season, with the dry season occurring during the winter and summer, respectively. However, the As climate has a limited distribution.

Some climatologists consider all A climates to be humid tropical since they occur outside the arid and semiarid zone. Köppen's Aw climate, however, is too extensive and consequently includes a wide range of ecological and agroclimatic zones.