Chap. I] MOUNTAIN CLIMATE 697 



This decrease only concerns the absolute humidity of the air, whilst the 

 relative humidity stands in no regular relationship to the altitude. All 

 observations regarding the relative humidity of air at great altitudes showed 

 that it was subject to extraordinarily great and rapid fluctuation, so that 

 very commonly complete saturation with ivater -vapour and greatest dryness 

 succeed one another at short intervals, according as an ascending movement 

 including the aqueous vapour, or a descending movement, or a calm is 

 prevailing. 



A fluctuation of this kind in the tropics is exhibited chiefly during the 

 rainy season, in cooler zones however only in spring and summer ; in winter 

 the air on mountains is characterized by great dryness, which is associated 

 with the scanty occurrence of clouds. 



According to the observations of Junghuhn in Java, the relative humidity on the top 

 of Slamat (10,500 ft. = 3374 m.) in the course of less than 24 hours fluctuated between 

 13 % and 100%, on the top of Semeru (11,480 ft.) in the course of45 hours between 35% 

 and 5 %. Regarding the dryness on the latter summit this observer remarks : 'The 

 air was so dry on this summit of Mount Semeru, the highest in the island of Java, that 

 Javanese mats woven from Pandanus leaves, which on the very same morning had 

 been very flexible at 5,000 feet below the summit, could be broken into the smallest 

 fragments, rubbed with the palms of the hands into dust as fine as flour, and blown 

 into the air.' 



C. Martins found on the Grand Plateau of Mont Blanc (3,930 m.), from 

 August 28th to September 1st, a relative atmospheric humidity averaging 

 38 °/ o (minimum 13 °/ o ), whilst at Chamonix it was 8a °/ o (minimum 50 °/J. 



The daily alternation of winds descending to the valleys and ascending 

 the mountain causes a continuous movement in the mountain air, which 

 attains a great intensity at considerable altitudes and on isolated summits. 

 On the high mountains of the temperate zones winds are far weaker in 

 winter than in spring and summer. 



The rarefaction of the air, the strong insolation, the occasional extraor- 

 dinary dryness of the air, the winds, all these unite in causing the remark- 

 able intensity of the evaporation, which every mountain climber knows only 

 too well. 



' Everything dries much more quickly at great altitudes ; animals that 

 have been shot, or killed by falling, become mummies without undergoing 

 decay, perspiration evaporates rapidly, the skin becomes dry and hard, and 

 one's thirst increases. The amount of evaporation in the mountain climate 

 cannot therefore be estimated from the relative humidity alone ; the reduced 

 atmospheric pressure renders possible a much more rapid dispersal of the 

 aqueous vapour produced, and consequently accelerates evaporation 1 .' 



The two following tables show the difference in atmospheric precipita- 

 tions between two neighbouring points of different altitude on the rainy west 



1 Hann, op. cit., Bd. Ill, p. 283. 



