Tropics, the region circling the earth between the Tropic of Cancer (23°27' N.) and the Tropic of Capricorn (23°27' S.). These two parallels, or imaginary lines of latitude, mark the farthest points reached by the sun's vertical rays. At all points between the two parallels, the sun passes directly overhead twice during the year. Because the sun remains high in the sky all year, average annual temperatures in the tropics are greater than those elsewhere in the world. However, daily highs rarely equal the summer temperatures in the interior of Eurasia or North America—in the so-called temperate zones.
The usual four seasons of the world's temperate areas do not occur in the tropics. Instead, seasonal variations consist primarily of rainy and dry periods that vary with distance from the Equator. On or near the Equator, even this distinction is missing, since rainfall is evenly distributed throughout the year and there is no dry period. Equatorial areas frequently receive 100 inches (2,540 mm) or more of rain annually. In this warm, humid climate grows the tropical rain forest, a dense stand of tall, broad-leaved evergreen trees and undergrowth.
A few degrees of latitude away from the Equator, where alternate rainy and dry periods begin, the rain forest gives way gradually to more open wooded areas. Rainfall decreases steadily and grasses become the dominant vegetation as distance from the Equator lengthens. Near the margins of the tropics are the semiarid steppes and, finally, the desert. Most of the world's great deserts are crossed by either the Tropic of Cancer or the Tropic of Capricorn.
In the tropics, as in much of the rest of the world, wide variations in climate and vegetation are caused by winds, elevation, nearness to the coast, ocean currents, and the location of large mountain ranges. East Africa, for example, although crossed by the Equator, has a relatively dry, cool climate because of its high elevation. ( .)
