124 THE ANDES OF SOUTHERN PERU 



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 cool- 

 ing process tends constantly to bring the warm, moist, ascend- 

 ing 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 moun- 

 tains 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 con- 

 trasts 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 differ- 

 ence in the rainfall — what we call, for want of a better name, a 

 "wet" and a "dry" season. 



On the western or seaward slopes of the Peruvian Andes the 

 trade winds descend, and the process of rain-making is reversed 

 to one of rain-taking. The descending air currents are com- 

 pressed as they reach lower levels where there are progressively 

 higher atmospheric pressures. The energy expended in the proc- 

 ess is expressed in the air as heat, whence the descending air gains 

 steadily in temperature and capacity for water vapor, and there- 

 fore is a drying wind. Thus the leeward, western slopes of the 

 mountains receive little rain and the lowlands on that side are 

 desert. 



