524 J. w. BEWS. 



C. Humidity and Precipitation. 



The absolute liumidity decreases rapidly with lessened 

 pressure, and therefore with increased altitude. At a height 

 of 2000-3000 m. (6561-9842 ft.) it is 49-35 per cent, of 

 what it is at sea-level.^ 



Cloudiness and precipitation, however, depend not on 

 absolute but on relative humidity, i.e. the percentage of 

 saturation of the aii', and this does not as a rule necessarily 

 vary with altitude. Very rapid variations and great ex- 

 tremes of atmospheric humidity are characteristic of mountain 

 climate. The ascending currents of air bring up moistui'e 

 which condenses; descending currents of air bring dryness, 

 and these conditions often rapidly alternate. The moisture- 

 laden winds in Natal come in from the sea, and are forced to 

 ascend by the rising land. 



Deposition takes place wherever the land rises steeply — 

 the edge of each successive terrace first of all, and finally 

 the Drakensberg range. The south-eastern exposures which 

 face the rain-clouds receive the maximum amount of 

 deposition. 



During the winter dry season, when anticy clonic conditions 

 prevail over Natal, the mountains remain for the most part 

 clear of fog. Cold mists deposit a certain amount of 

 moisture in the valleys, owing to the inversion of tempera- 

 tures already described. In the summer rainy season, mists 

 prevail on the mountains. The mist-belt of each mountain 

 varies somewhat in position, the amount of mist usually, 

 however, increasing with altitude, the base of the mountain 

 being comparatively free from it. The rainfall behaves 

 similarly. The zone of maximum deposition is somewhat 

 difficult to determine, no exact observations having been 

 made, but the vegetation itself may be safely used as an 

 indicator. 



The larger valleys remain comparatively dry during the 



' Hann, J., loc. cit. 



