SECTION IV. MOUNTAINS 



CHAPTER I 



THE MOUNTAIN CLIMATE 



i. Rarefaction of the Air. Diminution in the atmospheric pressure with increasing 

 altitude above the sea. Simultaneous diminution in the temperature of the air and 

 increase in the radiation of heat. Unequal temperature in the sun and shade on 

 mountains. Nocturnal radiation. Increased intensity of light. Mountain light richer in 

 highly refrangible rays. 2. Atmospheric Precipitations. Increased rainfall as altitude 

 increases. Level of the greatest rainfall. Reduced rainfall above this level. Perpetual 

 snow. Cloudiness. Reduction in water-vapour at great altitudes. Rapid alternation of 

 moistness and dryness of the air. Great intensity of evaporation in the mountain climate. 



i. RAREFACTION OF THE AIR. 



The most important difference between the lowland climate and that of 

 mountains is the diminution in the atmospheric pressure as the altitude 

 increases, which not only causes a change in climate, but also essentially 

 affects the other climatic factors, heat, light, and atmospheric precipitations. 



On the assumption that the atmospheric pressure at the sea-level is 

 762 mm. and that temperature falls 0-5° C. for every ico meters rise in alti- 

 tude, the following figures give the reduced atmospheric pressure for inter- 

 vals of 500 or 1,000 meters: — 



DIMINUTION OF ATMOSPHERIC PRESSURE WITH INCREASE IN 

 HEIGHT ABOVE SEA-LEVEL. 



Mean pressure in millimeters. 

 (After Hann, Bd. I, p. 221.) 



