Generally, the climate of high elevations. Mountain climates are distinguished by the departure of their characteristics from those of surrounding lowlands, and the one common basis for this distinction is that of atmospheric rarefaction. Aside from this, great variety is introduced by differences in latitude, elevation, and exposure to the sun. Thus, there exists no single, clearly defined, mountain climate. The most common climatic results of high elevation are those of decreased pressure, reduced oxygen availability, decreased temperature, and increased insolation; the last two combine to produce a typical “hot sun and cold shade” condition. Precipitation is heavier on the windward side of a mountain barrier than on the leeward (orographic precipitation), and on the windward side it increases upward to the zone of maximum precipitation, then decreases again. On many tropical mountains the forest zone extends into the level of average cloud height, which causes an excessively damp climate and produces the so-called fog forest. The orography gives rise to many local winds, chief among which are the foehn, mountain and valley winds, mountain-gap winds, and downslope winds of many sorts. Great interest in mountain climate has centered in the relatively well- populated, equatorial Andes. There, four zones of elevation are delimited: tierra caliente (hot land); tierra tamplada (temperate land); tierra fria (cool land); and tierra helada (land of frost).