14 Transactions of the South African Philosophical Society. 



moisture in the air ; and clouds, when they do form, must float 

 at a much higher level. 



Table 24 gives the mean annual complements of the dewpoint 

 for each hour of the day, and the corresponding computed altitude 

 of the first plane of condensation. According to this the diurnal 

 variation ranges from about 2,000 feet at VI. to 7,000 feet at XV. 

 The minimum altitude here may be somewhat too high, while 

 the maximum is probably quite 400 feet too low. The Blue Hill 

 •observations show that up to about noon the computed and observed 

 altitudes of cumulus, strato-cumulus, stratus, and nimbus, are not 

 appreciably unlike ; but after noon the observed altitudes become 

 greater than the computed, reaching a maximum excess of 500 feet 

 between XV. and XVI." The explanation seems to be that the 

 upward impetus of the warm air currents continues for some time 

 after the temperature has begun to fall at the surface of the earth. 



Table 24 gives also the monthly variation of the computed 

 .altitudes of the first plane of condensation at VI. and at XV. 

 We see from this that the VI. -curve is the flatter, its amplitude 

 being less by some 1,200 feet, and the actual minimum curve 

 is flatter still. The greatest monthly- mean computed altitude at 

 either of the two selected hours in the five years considered w T as 

 in November, 1900, with 10,250 feet, the least in April, 1899, with 

 690 feet, the former of course at XV. and the latter at VI. It may 

 not be out of place to remark here that since the cloudiness of the 

 sky only averages about 29 per cent., and the duration of sunshine 

 is upwards of 76 per cent, of the optimum, the warm air currents 

 do not in the majority of instances rise to the condensation level. 



We have now to consider the position of Kimberley in the general 

 scheme of South African rainfall. The principal published informa- 

 tion suitable for this purpose will be found in the Annual Eeports 

 of the Meteorological Commission, where monthly totals and the 

 maximum fall in each month only are given, derived from 

 300 (odd) gauges distributed throughout South Africa. The 

 quality of the material is not of the best, although it has un- 

 doubtedly improved of recent years. Buchan has made a useful 

 summary of the whole of the monthly totals printed in the reports 

 of the ten years 1885-1894.1 He deals with 278 stations, of which 

 about one-half have a complete record ; the other half having a 

 shorter history of two to nine years. 



Monthly averages for 160 stations having, generally speaking, com- 



* "Blue Hill Met. Obs.," in the Harvard Annals, vol. xlii., Part 1, p. 124. 

 The discussion explains it the other way about. 



f " A Discussion of the Rainfall of South Africa," 1897. 



