THE VEGETATION OF NATAL. 287 



winter. June and July are montlis when winds ure iiifrequent, 

 and this fact also has an important effect^ for with a still 

 atmosphere the cold air at night sinks into the valleys, and 

 we get the cold mists and valley frosts already described. 



At the end of winter hot dry winds from the noi'th- 

 west occur. These usually blow for two or three days in 

 succession. The temperature in the shade goes up to over 

 90° F. and a fine dust is carried along-. All vee-etation 

 exposed to the full effect of such a wind becomes scorched 

 and withered. It sweeps down the valleys in full force and 

 the northern slopes are fully exposed to it. These winds are 

 undoubtedly of the utmost importance in determining the 

 ■distribution of the two types of woodland, the isolated 

 umbrella-shaped and extremely xerophytic thorn trees on the 

 one hand, and the close type of bush on the other. 



While hot winds are most frequent from August to 

 November they occur occasionally at other times of the year. 

 Dr. Mann, in his ' Guide to Natal,^ gives the result of obser- 

 vations at Pietermaritzburg extending over eight years. 

 The average annual number of such hot winds was twenty-six, 

 distributed as follows: January, 1"5 ; February, 1'2; March, 

 ■7; April, -9; May, 1-2; June, -9; July, 2-4; August, 3-2; 

 September, S'l ; October, 42; November, 3'0 ; December, 

 1-7. 



Dr. Mann has not attempted to define exactly a\ hat he con- 

 sidered a hot wind, and the figures may be somewhat mis- 

 leading. It is fi'om August to November that the iiot winds 

 are most severe and of longest duration. 



During these months there is a fairly regular succession 

 of events as follows : two or three days of hot dry wind from 

 the north, then cooler air from the sea bringing rain for a day 

 or two, then an interval of cool delightful weather, followed 

 by the same cycle over again. 



I give the detailed meteorological data for Pietermaritzburg 

 during such a hot wind beginning on August 19th, 1911. 



