Delineation of the Tropics

The tropics comprise that area of the world between the Tropic of Cancer (latitude 23.5 °N) and the Tropic of Capricorn (latitude 23.5 °S). These parallels mark the points that the sun reaches at its greatest declination north or south. Outside these latitudes, the annual amount of solar energy reaching the earth's surface generally decreases as a function of increasing latitude (Kondratyev 1969). From an ecological perspective, it is more useful to delineate the tropics on the basis of energy balance, as indicated by temperature, rather than latitude. One approach has been to use the mean annual 20 °C

Fig. 2.5. Thermal delimitation of the tropics. The mean annual isotherms of 20, 25, and 30 °C are shown by lines of alternating dashes and dots. The 20 °C January (northern hemisphere) and July (southern hemisphere) isotherms, corresponding to the coldest months, are shown by solid lines with hachures. Areas where the mean daily temperature range exceeds the mean annual temperature range are indicated by dark shading inside the 20 °C isotherm and dots outside the 20 °C isotherm. (Adapted from Jordan 1985, with permission of John Wiley and Sons Ltd., publisher)

Fig. 2.5. Thermal delimitation of the tropics. The mean annual isotherms of 20, 25, and 30 °C are shown by lines of alternating dashes and dots. The 20 °C January (northern hemisphere) and July (southern hemisphere) isotherms, corresponding to the coldest months, are shown by solid lines with hachures. Areas where the mean daily temperature range exceeds the mean annual temperature range are indicated by dark shading inside the 20 °C isotherm and dots outside the 20 °C isotherm. (Adapted from Jordan 1985, with permission of John Wiley and Sons Ltd., publisher)

isotherm (Fig. 2.5). Other definitions are: areas with temperatures that exceed 20 °C during the coldest months of the year, and areas that have a mean daily temperature range greater than the mean annual range (Tricart 1972).

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