694 



ZONES AND REGIONS [Pt. Ill, Sect. IV 



freezing-point, and all water in the vessels which have been placed on a bad 

 conductor in the open air becomes ice. Even on pools of water crusts of ice 

 appear, and all dew is frozen on the plants, especially on pointed grasses, that 

 grow in open places. Under the leaf-canopy of the shrubs this is not the case, for 

 the steady temperature of the soil, at two feet below the surface, is 8° R. ( io° C.).' 



Martins l has made observations on the unequal heating of the soil in low- 

 lauds and on mountains. The stations were Bagneres (551 m.) and the 

 summit of the Pic du Midi (2,877 m.), the horizontal distance between the 

 two points being 14-5 kilometers. The measurements were made on three 

 bright September days and on the same kind of soil, a black mould : — 



COMPARISON OF SOIL-TEMPERATURE AND AIR-TEMPERATURE AT 



DIFFERENT HEIGHTS ABOVE SEA-LEVEL. 



(After Martins, from Hann, Bd. I, p. 234.) 



Not only as regards radiation of heat is there a difference between the 

 lowland and mountain climate, but naturally also in the radiation of light. 

 It is however of special importance that mountain light is richer than that 

 of the lowlands in blue, violet, and ultra-violet rays, as the highly refrangible 

 portion of the spectrum is most strongly absorbed by the atmosphere. 



2. ATMOSPHERIC PRECIPITATIONS. 



Owing to their lower temperature mountains tend to condense the aqueous 

 vapour of the atmosphere, and therefore receive more rain than the neigh- 

 bouring lozvlauds. 



1 Thus the higher plateaux and mountains of the central Sahara have a regular 

 summer rainfall, and on the mountains of the Nubian and Arabian coasts heavy 

 discharges of rain are released by storms. Wherever high mountains rise from the 

 steppes of Central Asia, at a certain altitude tree-growth and forests result from the 

 increased atmospheric precipitations. A similar case occurs in the desert of Western 

 North America V 



Increasing altitude is associated with an increase in the rainfall, but only 

 up to a certain level which varies according to the general climatic conditions 

 and local circumstances ; above this level at which the maximum rainfall 

 occurs the atmospheric precipitations again rapidly diminish. 



1 Martins, op. cit., p. 38. 



2 Hann, op. cit., Bd. I, p. 288. 



