130 



Report S.A.A. Advancement of Science. 



suppose that these hot winds last for days : as a matter of fact they 

 seldom last more than a few hours, and are usually confined to the 

 daylight hours. They seldom blow during the night, or affect the 

 minimum temperatures to any great extent, although they may on 

 rare occasions recur the second day. The variability of minimum 

 temperature at Durban is least in the autumn and greatest in the 

 spring. Thus the day temperature variability is falling during the 

 first half of the year, while the night temperature variability is rising. 

 The greatest variability of the minimum temperature nearly coincides 

 with that of the maximum ; but, as we shall see later, this result is 

 not brought about by the extension of the hot winds into the night, 

 but, on the contrary, is caused by temporary cold winds which, in 

 their turn, have no great influence over the day temperatures. In 

 fact, while the hot winds are essentially cyclonic, the cold winds are 

 as essentially anti-cyclonic, both resembling their prototypes of con- 

 tinental Europe, but differing from them to some extent because of the 

 special geographical conditions of South Africa. Naturally, 

 however, the more frequent the occurrence of cyclonic conditions, the 

 more frequently will anti-cyclonic conditions alternate with them, and 

 therefore, so far, the hot and cold spells may be said to be related 

 to each other. The greatest monthly mean variability of maximum 

 temperature at Durban during the ten years discussed appears in 

 January, 1900, with 7.9°, the least in July, 1901, with 2.6°; the 

 greatest monthlv mean variability of minimum appears in October, 

 1900, with 4.7°, the least in March, 1896, with 1.4°. Large monthly 

 mean values of the maximum temperature variability occur in any of 

 the summer months, while fairly large values of the monthly means 

 of minimum temperature variability occur all through the year. 



The mean monthlv values of the inter-diurnal variability for 

 East London are : — 



Table 2. — ^Variability of Temperature at East London, 



1895 to 1904. 



