526 J. w. BBWS. 



absorbing heat duinng spring, but when snow does fall, 

 its melting supplies moisture to the soil and gives an earlier 

 spring. 



D. Movements of the Air. 



Certain general movements of the air have already been 

 •dealt with in connection with their effect on temperature and 

 •deposition. 



The moisture-laden winds blow in from the Indian Ocean 

 during summer. There are also local winds, due to the 

 rising of the heated air from valleys during the day-time, 

 which blow up the valleys. Cold air drainage leads to a 

 downward flow at night. Dui-ing the winter, when there is 

 snow on the Drakensberg, a cold wind blows down from it 

 even during the daytime. This cold wind, however, becomes 

 warmer by compression as it descends, so that its effect 

 rarely extends to the coast-belt. The descent of air carries 

 water-vapour down and leaves the mountains drier. The 

 ascent of air leads to condensation and deposition. Since 

 the former takes place regularly at niglit, the mornings are 

 clear at the mountain tops, and the latter leads to the after- 

 noon rains and thunderstorms, already noted. 



These general movements of the air are further modified 

 at somewhat irregular intervals by dry, hot winds, which 

 have been variously named in different countries. Their 

 characteristics have been fully studied in Switzerland,^ and 

 they are now commonly known as Foehn winds. They are very 

 hot and very dry, and they descend the valleys with extreme 

 violence. Their high temperature is due to the fact that a 

 mass of air, descending to levels where the pressure is greater, 

 becomes heated by compression. Observations in Switzerland 

 show that the increase is 1*0° C. for every 100 m. (cf. the 

 average decrease of temperature Avith rise in altitude, Avhich 

 is 0'57° 0. for every 100 m.). Their relative dryness follows 

 as a result of their increased temperature. The conditions 



' Hann, J., loc. cit. 



