Fig. 71—The three chief topographic regions of Peru.
Fig. 72—The wind belts of Peru and ocean currents of adjacent waters.
Fig. 73—The climatic belts of Peru.
Fig. 74—Belts of vegetation in Peru.
In the explanation of these contrasts we have to deal with relatively simple facts and principles; but the reader who is interested chiefly in the human aspects of the region should turn to p. [138] where the effects of the climate on man are set forth. The ascending trades on the eastern slopes pass successively into atmospheric levels of diminishing pressure; hence they expand, deriving the required energy for expansion from the heat of the air itself. The air thereby cooled has a lower capacity for the retention of water vapor, a function of its temperature; the colder the air the less water vapor it can take up. As long as the actual amount of water vapor in the air is less than that which the air can hold, no rain falls. But the cooling process tends constantly to bring the warm, moist, ascending air currents to the limit of their capacity for water vapor by diminishing the temperature. Eventually the air is saturated and if the capacity diminishes still further through diminishing temperature some of the water vapor must be condensed from a gaseous to a liquid form and be dropped as rain.
The air currents that rise thousands of feet per day on the eastern slopes of the Andes pass again and again through this practically continuous process and the eastern aspect of the mountains is kept rain-soaked the whole year round. For the trades here have only the rarest reversals. Generally they blow from the east day after day and repeat a fixed or average type of weather peculiar to that part of the tropics under their steady domination. During the southern summer, when the day-time temperature contrasts between mountains and plains are strongest, the force of the trade wind is greatly increased and likewise the rapidity of the rain-making processes. Hence there is a distinct seasonal difference in the rainfall—what we call, for want of a better name, a “wet” and a “dry” season.