THE PLANT ECOLOGY OF DRAKENSBEEG RANGE. 521 



great importance in connection witli insolation. A large 

 portion of the main Drakensberg faces the rising sun, but it 

 will be seen from the description given of the topography 

 that there are minor slopes and innumerable steep descents 

 into the deep valleys, that are shaded much longer, some- 

 times throughout the greater part of the day. This fact, 

 taken in conjunction with the distribution of water-vapour 

 and precipitation, has an important effect on the vegetation. 



B. Temperatuke. 



Next in point of regularity to the decrease of pressure 

 is the decrease of air temperature with rise of altitude. 

 This has been taken* to average 0"57°C. for every 100 m. 

 There are slight seasonal variations, of course, but here 

 again the effect of the topography is most marked. At night, 

 •owing to radiation, which, as we have seen, increases greatly 

 with altitude, the ground cools rapidly and the lower atmo- 

 sphere is also cooled. The cold air, being heavier, settles to 

 the lowest point ; it flows fx'om the hillsides down into the 

 valleys, and it follows the course of the main valleys down- 

 ward, exactly like the rivers. We thus have an inversion of 

 temperature, higher altitudes being warmer than lower. This 

 is seen regularly during the winter anticyclone, when calm 

 weather prevails, when also the general temperature is lower. 

 The water-vapour of this colder air is condensed into a fog, 

 which settles into the valleys. In the early morning the 

 mountain-tops rise above the dense white mist-cloud like 

 islands in a sea. The rising sun gradually disperses the 

 fog, and here again the exposure determines whether the 

 sun's influence will soon make itself felt, or whether the cold 

 air and fog Avill linger. The effect of the cold air drainage 

 at night is seen most clearly at points where the current is 

 moi-e rapid, as where a narrow " nek " opens into a broader 

 valley. At such a point the vegetation suffers severely from 

 ifrosts. 



' Hann, J., loc. cit. 



