CHAPTER IX
CLIMATOLOGY OF THE PERUVIAN ANDES
CLIMATIC BELTS
The noble proportions of the Peruvian Andes and their position in tropical latitudes have given them climatic conditions of great diversity. Moreover, their great breadth and continuously lofty summits have distributed the various climatic types over spaces sufficiently ample to affect large and important groups of people. When we add to this the fact that the topographic types developed on a large scale are distributed at varying elevations, and that upon them depend to a large degree the chief characteristics of the soil, another great factor in human distribution, we are prepared to see that the Peruvian Andes afford some striking illustrations of combined climatic and topographic control over man.
The topographic features in their relations to the people have been discussed in preceding chapters. We shall now examine the corresponding effects of climate. It goes without saying that the topographic and climatic controls cannot and need not be kept rigidly apart. Yet it seems desirable, for all their natural interdependence, to give them separate treatment, since the physical laws upon which their explanations depend are of course entirely distinct. Further, there is an independent group of human responses to detailed climatic features that have little or no connection with either topography or soil.
The chief climatic belts of Peru run roughly from north to south in the direction of the main features of the topography. Between 13° and 18° S., however, the Andes run from northwest to southeast, and in short stretches nearly west-east, with the result that the climatic belts likewise trend westward, a condition well illustrated on the seventy-third meridian. Here are developed important climatic features not found elsewhere in Peru. The trade winds are greatly modified in direction and effects; the northward-trending valleys, so deep as to be secluded from the trades, have floors that are nearly if not quite arid; a restricted coastal region enjoys a heavier rainfall; and the snowline is much more strongly canted from west to east than anywhere else in the long belt of mountains from Patagonia to Venezuela. These exceptional features depend, however, upon precisely the same physical laws as the normal climatic features of the Peruvian Andes. They can, therefore, be more easily understood after attention has been given to the larger aspects of the climatic problem of which they form a part.
The critical relations of trade winds, lofty mountains, and ocean currents that give distinction to Peruvian climate are shown in Figs. 71 to 73. From them and [74] it is clear that the two sides of the Peruvian mountains are in sharp contrast climatically. The eastern slopes have almost daily rains, even in the dry season, and are clothed with forest. The western leeward slopes are so dry that at 8,000 feet even the most drought-resisting grasses stop—only low shrubs live below this level, and over large areas there is no vegetation whatever. An exception is the Coast Range, not shown on these small maps, but exhibited in the succeeding diagram. These have moderate rains on their seaward (westerly) slopes during some years and grass and shrubby vegetation grow between the arid coastal terraces below them and the parched desert above. The greatest variety of climate is enjoyed by the mountain zone. Its deeper valleys and basins descend to tropical levels; its higher ranges and peaks are snow-covered. Between are the climates of half the world compressed, it may be, between 6,000 and 15,000 feet of elevation and with extremes only a day’s journey apart.