Meteorological Features. 133 



the difference of temperature between the warmer and colder points 

 and on their distance, therefore the flow would be the strongest in the 

 first hours of the day till the sun being high enough the two points 

 come nearer and get at the same time nearer the same temperature 

 level, when there comes a slowing down of the movement. This will 

 get slower yet by the gradual motion of the warmer points tow^ards the 

 West. In the night, on the contrary, the faster cooling of the land 

 will gradually increase the flow from the South and bring the velocity 

 to a night maximum, till the difference of sea and land temperature 

 will gradually lessen as the night proceeds, and induce a slackening of 

 the wind before sunrise. 



If we consider the wind frequency (in polar co-ordinates) we see 

 that the mean wind at each hour can be decomposed into the main 

 resultant B O, which we may consider as a fairly constant trade wind 

 and the variable A B, and if we incline its major axis so as to have it 

 a little East of North instead of West of North we will have what 

 will in all probability represent about the movement curve. We see 

 there that the variable wind varies both in direction and magnitude 

 according to the theory given above, that the hourly resultants increase 

 in the forenoon that they ought to decrease sharply some time about 

 the middle of the day or earlier as the variable component has a 

 different sign from B O, the other component. They should pass 

 through a minimum before sunset and then increase again, but less 

 rapidly than before to pass through a maximum and decrease again 

 but less rapidly than in the afternoon, B O and A B being now of 

 same sign, or even be nearly constant. Now this is certainly very 

 much what happens here. The inrush from the North a§ the strong 

 North components at 8, 10, 11 hours show is often very marked. The 

 calms fall mostly in the afternoon, the passage of the vane from W. 

 to S. is often replaced by a calm. The nights are rarely quite calm, 

 and as a rule I may say that the afternoon increase of strength in 

 the trade wind which has been observed elsewhere * in the tropics 

 does not appear at all in the records ; the afternoons are generally 

 relatively calm, t 



The remarks in this paper are, I need not say, only indications of 

 problems that present themselves in the meteorology of Southern 

 Rhodesia. Hard and fast conclusions are premature, and would 

 require a more refined discussion of the available data. One thing, 

 however, may be suggested by the periodic appearance of the various 

 records. I would feel inclined to believe that we are permanently 

 under the influence of the Southern high pressure ridge, or, better, of 

 the South Indian Ocean high pressure area. Small shiftings in this 

 area might account for the small oscillations of the barometer. 



* Cf Hann. Lehrbuch der Meteorologie, Vol. i., p. 288 et seq. 



t The theory given above would certainly be strengthened if higher up 

 in the tropics we found that the rotation is through the North 

 at one time and through the South at another, following the 

 changes in the Sun's declination. I have no documents at hand 

 to investigate the point. 



