Diurnal Meteorological Conditions of Kimberley. 161 



to the mean, the deviation increasing from that time to its maximum, 

 in both cases, about ten o'clock a.m. With clear skies the velocity 

 increases from the absolute minimum about V. to the absolute 

 maximum about XIV. : from 4-1 to 6-7 miles per hour; with cloudy 

 skies the corresponding times are not far from V. and XIII. : and 

 the hourly values range from 4-6 to 8' 6. Or, putting it in a different 

 way, the range in the second case is half as much again as it is in 

 the first. This fact is not in concordance with some previous results 

 obtained elsewhere : " Hann has shown for a number of places in 

 Northern Europe, that with a clear sky the velocity is doubled from 

 the minimum to the maximum, with a sky half covered the velocity 

 is three-fourths greater, and with a sky wholly covered the velocity 

 is only a half more. ... At the strictly inland situation of Vienna, 

 with a clear sky the velocity is double, and with a sky half- 

 covered it is two-thirds greater, but with a covered sky the diurnal 

 variation of the wind's velocity becomes irregular and faintly 

 marked." " 



The approach to equality of velocity between night and day under 

 clear skies as compared with the departure therefrom under clouds 

 suggests at first sight either that the temperature is not so great a 

 motive power as might have been supposed and claimed, or else that 

 terrestrial radiation is scarcely less influential than insolation. Both 

 ideas, however, fall short of the truth. For the winds, equally with 

 the clouds, are governed largely by cyclonic and anticyclonic condi- 

 tions ; and consequently the subdivision of wind-movement into 

 clear-day and cloudy-day quantities does not really refer the wind to 

 a preceding cause so much as group it in a manner that may make 

 the reference more promising. The study that can strip the wind- 

 phenomena bare to the mental sense has yet to be completed. We 

 have progressed far enough, nevertheless, even in this discussion, to 

 discover that certain wind-directions do tend to prevail in the two 

 states of the sky ; clouds indicating northerly directions, and clear 

 skies southerly (Table 31) ; the associated shift of the mechanical 

 resultant direction from one to the other being more than a right 

 angle — from the first quadrant into the fourth (Table 35). Now we 

 have already proved the " important fact that for any hour of the 

 day the mean velocity of the wind from any quarter decreases, 

 relatively to the mean diurnal curve, with the deviation of the vane 

 from its normal position." t Therefore, since the normal winds are 

 northerly by day and southerly by night, it must be that a northerly 

 wind by night and a southerly wind by day will lose speed. Thus 



* Buchan, Eiicy. Brit, IX. Ed. Art. " Meteorology," p. 125. 

 t "The Winds of Kimberley," p. 85. 



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