The Tropics

The nature of the tropics is complex, the definition of the term therefore hardly clear. In terms of radiation climate, the tropics are bounded by the northern and southern tropical cycles. Tropics and subtropics occupy about two-thirds of the earth's surface. Climatologists and geographers have made various attempts to define the tropics within the framework of genetic and effective climate classifications. A hygrothermal classification of the tropics is made by Lauer (1975). He draws up a synthesis of thermal characteristics (warm tropics – cold tropics), hygric characteristics (humid tropics – dry tropics) and synoptic-climatic phenomena, which are strongly influenced by the atmospheric circulation. The latter is in turn modified by relief, distribution of land and sea, etc. Lauer's classification of the tropics is used as the basis for this book. Quan et al. (2013) have also delineated the tropics based on climatic patterns of precipitation, evaporation and atmospheric circulation to simulate climate in models. A review of numerous observations has shown that the tropical zone has become larger in recent decades in association with global warming. The expansion of the tropical belt is significant because it is associated with a large-scale shift in atmospheric circulation. Regional changes in tropical boundaries are not captured by modelling.