278 THE ANDES OF SOUTHERN PERU 



solation. It follows, therefore, that at high altitudes the contrasts 

 between sun and shade temperatures will increase. Frankland 2 

 has shown that the increase may run as high as 500 per cent be- 

 tween 100 to 10,000 feet above the sea. I have noted a fall of tem- 

 perature of 15° F. in six minutes, due to the obscuring of the sun 

 by cloud at an elevation of 16,000 feet above Huichihua in the 

 Central Eanges of Peru. Since the sun shines approximately 

 half the time in the snow-covered portions of the mountains and 

 since the tropical Andes are of necessity snow-covered only at 

 lofty elevations, this contrast between shade and sun tempera- 

 tures is by far the most powerful factor influencing differences in 

 elevation of the snowline in Peru. 



To the drifting of the fallen snow is commonly ascribed a 

 large portion of this contrast. I have yet to see any evidence of 

 its action near the snowline, though I have often observed it, 

 especially under a high wind in the early morning hours at con- 

 siderable elevations above the snowline, as at the summits of lofty 

 peaks. It appears that the lower ranges bearing but a limited 

 amount of snow are not subject to drifting because of the wetness 

 of the snow, and the fact that it is compacted by occasional rains 

 and hail storms. Only the drier snow at higher elevations and 

 under stronger winds can be effectively dislodged. 



The effect of unequal distribution of precipitation on the wind- 

 ward and leeward slopes of a mountain range is in general to de- 

 press the snowline on the windward slopes where the greater 

 amount falls, but this may be offset in high altitudes by tempera- 

 ture contrasts as in the westward trending Cordillera Vilcapampa, 

 where north and south slopes are in opposition. If the Cordillera 

 Vilcapampa ran north and south we should have the windward 

 and leeward slopes equally exposed to the sun and the snowline 

 would lie at a lower elevation on the eastern side. Among all the 

 ranges the slopes have decreasing precipitation to the leeward, 

 that is, westerly. The second and third passes, between Arma and 

 Choquetira, are snow-free (though their elevations equal those of 



3 Haim, Handbook of Climatology, Part 1, trans, by Ward, 1903, p. 232. 



