CLIMATOLOGY OF THE PERUVIAN ANDES 127 



more water vapor remains in the air after precipitation has 

 ceased. Curve B ultimately rises nearly to the level of A, for 

 with less water vapor in the air of case B the temperature rises 

 more rapidly (a general law). Moreover, the higher the tem- 

 perature the greater the radiation. To summarize, curve A rises 

 more slowly than curve B, (1) because of the greater amount 

 of water vapor it contains, which must have its temperature 

 raised with that of the air, and thus absorbs energy which would 



F 

 90! 



80° 

 70° 

 60 c 

 50 °_ 

 40° 

 301 



BELT OF FALLING TEMPERATURES BELT OF RISING TEMPERATURES, 



CLOUD BANKS.HIGH DEW POINT CLEAR SKIES, LOW DEW POINT, 



ANDMODERATEPRECIPITATION AND ARIDITY 



IX 



SATURATION TEMPERATURE- 



j x ' SATURATION TEMPERATURE - 



i 

 SEASHORE COAST RANGES DESERT 



JO* 



2b° 



iO" 



15° 



10" 

 5° 

 0° 



Fig. 77 — To show progressive lowering of saturation temperature in a desert under 

 the influence of the mixing process whereby dry and cool air from aloft sinks to lower 

 levels thus displacing the warm surface air of the desert. The evaporated moisture of 

 the surface air is thus distributed through a great volume of upper air and rain becomes 

 increasingly rarer. Applied to deserts in general it shows that the effect of any 

 cosmic agent in producing climatic change from moist to dry or dry to moist will 

 be disproportionately increased. The shaded areas C and C represent the fog-covered 

 slopes of the Coast Range of Peru as shown in Fig. 92. X — X' represents the crest of 

 the Coast Eange. 



otherwise go to increase the temperature of the air, and (2) be- 

 cause its loss of heat by radiation is more rapid on account of its 

 higher temperature. We conclude from these principles and de- 

 ductions that under the given conditions a cold current intensi- 

 fies, but does not cause the aridity .of. the west-coast desert. 



Curves a and b represent the rise of temperature in two con- 

 trasted cases of warm and cold sea with the coastal mountains 

 eliminated, so as to simplify the principle applied to A and B. 

 The steeper gradient of b also represents the fact that the lower 

 the initial temperature the dryer will the air become in passing 

 over the warm land. For these two curves the transition line 

 X-X' coincides with the crest of the Coast Eange. It will also be 

 seen that curve a is never so far from the saturation level as 



