The essential feature of a dry climate is that annual losses of water through evaporation at the earth's surface exceed annual water gains from precipitation. Due to the resulting water deficiency, no permanent streams originate in dry climate zones. Because evaporation, which depends chiefly on temperature, varies greatly from one part of the earth to another, no specific value for precipitation can be used as the boundary for all dry climates. For example, 25 in (610 mm) of annual precipitation may produce a humid climate and forest cover in cool northwestern Europe, but the same amount in the hot tropics produces semiarid conditions.
Two divisions of dry climates are commonly recognized: the arid desert, and the semiarid steppe. Generally, the steppe is a transitional belt surrounding the desert and separating it from humid climates beyond. The boundary between arid and semiarid climates is arbitrary but commonly defined as one-half the amount of precipitation separating steppe from humid climates.
Of all the climatic groups, dry climates are the most extensive; they occupy a quarter or more of the earth's land surface.