696 



ZONES AND REGIONS [Pt. Ill, Sect. IV 



the rainy season. On the other hand, most of the temperate mountains, 

 especially the Alps, during winter have a much brighter sky above them than 

 the low-lying land has, whereas during summer the reverse is the case. 



MEAN CLOUDINESS ON MOUNTAINS AND IN LOWLANDS. 

 (After Hann, Bd. I, p. 284.) 



In temperate zones ' the higher mountain valleys and summits have 

 a brighter autumn, but especially a clearer sky in winter. The great clear- 

 ness of the winter sky in high alpine valleys is one of the most prominent 

 climatic features in its favour ; it, together with the dryness and reduced 

 pressure of the air, causes the unusually intense insolation 1 .' 



' From Central Asia we have the interesting observations of Sewerzow 

 on the Tian-Shan, that admirably show the rise which the region of clouds 

 and rain experiences from summer to winter. The zone of the winter snow- 

 clouds occurs here at an altitude of 2,500-3,000 m. ; this is at the same time the 

 zone of the silver-fir forests, which are wanting at lower altitudes on account 

 of the dryness. The higher regions receive but little winter snow ; on the 

 other hand, they receive richer supplies of rain from the higher summer 

 clouds, and this favours the growth of grass and the presence of good 

 pasture in these altitudinal zones 2 .' 



As the atmosphere increases in rarefaction, its aqueous vapour decreases 

 iu amount, and at a relatively more rapid pace, as is shown by the following 

 table constructed by Hann, under the assumption that the pressure and the 

 amount of water in the air at sea-level be reckoned as 1 : — 



DIMINUTION OF WATER-VAPOUR AND INCREASE OF RAREFACTION 



WITH INCREASE IN ALTITUDE. 



(After Hann, Bd. I, p. 279.) 



1 Hann, op. cit., Bd. I, p. 285. 



2 Id., p. 301. 



