The Tropics, namely the zone between the Tropic of Cancer and the Tropic of Capricorn was named the Torrid Zone in the past on the basis of solar climate. However, neither the Tropic of Cancer nor the Tropic of Capricorn are boundaries of a temperature zone defined by a specific isotherm. From the standpoint of solar climate, the Tropic of Cancer as well as the Tropic of Capricorn should be the most basic climatic boundaries, as Louis (1958) and Lauer and Frankenberg (1988) maintained and presented climatic maps of the world.
It was geographer Supan (1879) and climatologist Köppen (1884) who defined the Tropics as a temperature zone characterized by a mean temperature of 20°C or above for the coldest month. Therefore, the boundary of this temperature zone, “the Tropics”, should correspond with the 20°C isotherm of the monthly mean temperature of the coldest month. It may be assumed that the above mentioned two scholars adopted this critical value, considering the relationship between distribution of tropical plants and of temperature in the coldest month, although they did not state this distinctly. Since then, this value has often been cited to delimit the Tropics.
In the year 1918, Köppen wrote a famous paper on the classification of climates on the basis of temperature, precipitation and annual trend, and in substance, however, on the basis of the relationship between the distribution of plants and climates of the world. This epochmaking paper played an important role in pioneering the field of climatology. In that paper he defined the Tropics as a temperature zone with mean monthly temperature of the coldest month of 18°C or above.
Since then, regarding the classification of climates, numerous results have been published, on the basis of the so-called effective method. They may be classified into two groups : climatic systems initiated by Köppen (1918), Köppen and Geiger (1928), de Martonne (1909), Gorczynski (1935, 1942), Trewartha (1954, 1968), and Miller (1959), for example, belong to the former, and those which were initiated by Philippson (1921), v. Wissmann (1939, 1948, 1962), Blair (1949), Creutzburg (1950, 1964), Troll (1955), Troll and Paffen (1964), for instance, to the latter.
Climate systems initiated by these two groups differ. Köppen et al
. divided climates into humid, arid and cold climates, and the humid climates into, for instance, tropical, (subtropical), temperate and polar. In this way, the tropical climates, defined by a critical value of mean monthly temperature of the coldest month (for instance, 18°C or above, or an approximate critical value) set bounds to the humid tropics alone. Therefore, according to this climatic system, arid tropical region should inevitably be outside the Tropics.
According to the latter climatic system, climates were divided into, for instance, tropical, subtropical temperate, boreal and subpolar-polar zones, on the basis of the specific isotherms, and then, except subpolar-polar climates, they are divided into climate types on the basis of, for instance, the annual trend of humidity (or aridity).
Therefore, according to this climatic system, the tropical zone was defined on the basis, of, for instance, either the suitable isotherms, the absolute frost limit, or the line on which diurnal and annual range of temperature are equal. Accordingly, the tropical climatic region covers the humid as well as the dry tropical climates.
It was v. Wissmann (1948) who delimited the warm tropics of the world on the basis of absolute frost limit, and moreover, in his climatic system (v. Wissmann, 1962) defined warm-, temperate-, cool-, and cold tropics, analysing the vertical structure of climatic regionality of the tropical highlands and mountains.