162 THE ANDES OF SOUTHERN PERU 



If we take the means of the diurnal variations by months we 

 have a still more striking curve showing how little change there 

 is between successive days. June and December are marked by 

 humps in the curve. They are the months of extreme weather 

 when for several weeks the temperatures drop to their lowest or 

 climb to their highest levels. Moreover, there is at these lofty 

 stations no pronounced lag of the maximum and minimum tem- 

 peratures for the year behind the times of greatest and least heat- 

 ing such as we have at lower levels in the temperate zone. Thus 

 we have the highest temperature for the year on December 2, 

 70.4° F. (21.3° C), the lowest on June 3, 0.2° F. (—17.7° C). The 

 daily maxima and minima have the same characteristic. Eadiation 

 is active in the thin air of high stations and there is a very 

 direct relation between the times of greatest heat received and 

 greatest heat contained. The process is seen at its best immedi- 

 ately after the sun is obscured by clouds. In five minutes I have 

 observed the temperature drop 20° F. (11.1° C.) at 16,000 feet 

 (4,877 m.) ; and a drop of 10° F. (5.6° C.) is common anywhere 

 above 14,000 feet (4,267 m.). In the curves of daily maximum and 

 minimum temperatures we have clearly brought out the uniform- 

 ity with which the maxima of high-level stations rise to a mean 

 level during the winter months (May- August). Only at long in- 

 tervals is there a short series of cloudy days when the maximum 

 is 10°-12° F. (5.6°-6.7° C.) below the normal and the minimum 

 stands at abnormally high levels. Since clouds form at night 

 in quite variable amounts — in contrast to the nearly cloud- 

 less days — there is a far greater variability among the minimum 

 temperatures. Indeed the variability of the winter minima 

 is greater than that of the summer minima, for at the latter 

 season the nightly cloud cover imposes much more stable atmos- 

 pheric temperatures. The summer maxima have a greater 

 degree of variability. Several clear days in succession allow 

 the temperature to rise from 5°-10° F. (2.8°-5.6° C.) above 

 the winter maxima. But such extremes are rather strictly 

 confined to the height of the summer season — December and 

 January. For the rest of the summer the maxima rise only 



