The Tropical Environment

The term “tropics” refers to the continuously warm and frost-free zone of the world that lies approximately between the Tropic of Cancer (or latitude 23.5° north of the equator) and the Tropic of Capricorn (or latitude 23.5° south of the equator). The tropical region comprises approximately 36% of the world’s land surface. Geographically, the tropics encompasses the entire region of Southeast Asia, Central America, the islands in the South Pacific and the Caribbean Basin, a major part of Africa, South America, a large portion of the Indian subcontinent, and a small part of northern Australia. Within a tropical region, natural vegetation and agriculture vary with elevation and rainfall regime. Within the tropical belt, mean annual temperature at sea level is about 26 °C, and it decreases approximately 0.6 °C with every 100 m increase in elevation. On the basis of elevation, the tropics may be further divided into • lowland tropics (areas below 600 m), • midaltitude tropics (areas between 600 and 900 m), and • high-altitude tropics or tropical highlands (areas above 900 m). Tropical highlands account for 23% of the tropics whereas the low- and midaltitude regions together comprise about 87% of the total area. Tropical highlands usually have cool air temperatures with a mean annual temperature of 20 °C or lower. Rainfall on tropical highlands can be extremely variable within a short distance. Because of the year-round comfortable temperature, areas of tropical highlands with favorable rainfall and fertile soils are usually densely populated and hence intensively cultivated. Climates in the lowland and midaltitude tropics generally share three common features, namely, a year-round warm temperature, rainfall of high intensity and short duration, and a high rate of evaporation. Climates are characterized principally by mean monthly air temperature, and the amount and distribution of rainfall.