*Tropical Regions*
Tropical Zone, one of the world's major climato-vegetational zones lying predominantly between the Tropic of Cancer (23°27¢ north) and the Tropic of Capricorn (23°27¢ south) and characterized by a minimum mean annual temperature, at sea level, of 18° C (64.4° F) in the coldest month. It covers around 40 per cent of the Earth's surface, taking in large parts of South East Asia (including India and China), Australia, Africa, and Central and South America. The Tropics receive large amounts of solar radiation (sunshine) throughout the year, so seasonal fluctuations in temperature are minimal. The amount and timing of rainfall, however, often varies considerably and it is rainfall that is commonly used to subdivide the Tropics. Rainfall directly and indirectly affects vegetation, soils, landform processes, agriculture, and even economic activity. Based on rainfall and vegetation, three major subdivisions of the tropical zone can be distinguished. The humid Tropics have at least 1,000 mm (39 in) of rainfall per annum and no distinct dry season. They are typically covered with forest vegetation, notably rainforest. The wet and dry (wet-dry) Tropics receive between 250 and 2,000 mm (10 and 79 in) of rain annually and have one or two distinct dry seasons. They are typically covered with grassland (savannah) vegetation. The dry, or arid, Tropics receive less than 250 mm of rainfall per annum and have only a short wet season. They are typically covered with sparse, shrubby xerophilous vegetation; that is, plants adapted to survive very dry conditions. This article deals primarily with the humid Tropics. The wet-dry Tropics and dry Tropics are described in the Arid Zone and Savannah articles.