meteorology, for the Northern Hemisphere, when he says: "On plains or also on highlands the wind shews a tendency to turn with the hands of the clock in the forenoon and the other way in the afternoon." I believe that this particular wind veering is only the beginning of a rotation. For these periods almost invariably are the passages of a low barometer, a high temperature, and a low Relative Humidity to a high barometer, a low temperature and a high Relative Humidity or vice versa. Sometimes they are on a slight rise in a low barometer, sometimes on a slight depression in a high one. whole, comparing the various elements connected with a veering wind period in the table given as an appendix to this paper, we see that these East with North wind periods, differ little from the rotating wind periods. These might also be the passages of secondary cyclones or V depressions, with their usual thunderstorms. Observations from other centres would, however, be wanted to decide the point. On the In order to give a more precise idea of the winds of Bulawayo, I have calculated the North and East components of wind frequency and wind movement for the three months of October, 1903 to 1905. The extreme regularity of our climate shown by the traces of the recording instruments allows me to assume that the computation for one month only and even in the short period of three years will fairly well bring out the general features of the wind movement. October is besides likely to be the best month to give average results, as it is astride on the two seasons of the year. The first three columns of the following Table (A) represent the wind frequency, the next three the combined frequency and movement. The co-efficients for the various directions are in the former the number of times the wind blew in each direction, in the latter the velocity in kilometers per hour for each direction. Fig 1-Wind Frequency Resultants (Polar Co-ordinates) Fig II Wind-Frequency Resultants (V Table A) 950 900 850 From this Table I have constructed the wind frequency curve in polar co-ordinates (the vectors are the resultants of the third column) the two other curves are, the first, the curve of the frequency resultants, the next that of the movement resultants in rectangular co-ordinates. The frequency resultants all remain between N.E. and E.S.E., the general resultant being nearly due E. This brings well out the fact that we are quite in the trade wind zone. The curve in polar co-ordinates for the wind movement would give similar results, throwing only the resultant about 10 degrees South of East, and bringing the major axis of the ellipse due North. I did not construct it, as the minimum shown at 4 hours on the third curve gives a loop that cannot well be smoothed away. This result is very different from those got in Johannesburg and in Cape Colony. In Johannesburg the general resultant is N.N.W., in Kimberley it is W.N.W. It compares very much better with the wind system North-East of us, as found in Boroma, near Tete, on the Portuguese Zambesi* where the main wind is from E. to S. E. The frequency curve constructed as explained above supposes a uniform velocity in the winds in all directions. This is obviously not correct; the two curves in rectangular co-ordinates show the differences introduced by taking into account the variable velocity. The period of time used in the computation of the wind movement is evidently too short to smooth out the irregularities introduced by errors, and by the variations so capricious a variable as the wind velocity. I am therefore unable to state whether the minimum at 4 hours corresponds to a physical fact or not. Mr. Sutton found also two minima for the wind velocity at Kimberley, and Mr. Innes notes that there was a loop in the curve he gives for the wind movement at Johannesburg. This loop corresponds also to a minimum. The two curves, however, give a minimum in the afternoon and a maximum at night, and the movement curve has a second maximum at about 10 a.m. Both the wind variations and the peculiarities we find in the movement curve seem to me to fall in fairly well with the explanation Dechevrens gave for similar but more complicated phenomena at Zikawei (Shanghai). He explains them by something like the Monsoon theory, that is, by the flow of air from the colder to the warmer points. This applied to Bulawayo would work out in the following way. During the day the warmer points are successively E.N. and W. and the wind would veer that way; during night, on account of the wedge shape of South Africa the warmer mass would be that of the ocean South and the wind would veer through South to the East again by sunrise. The force of the wind would depend on The winds of Kimberley. J. R. Sutton. Trans. of the S.A. Phil. Soc., Vol. XI S.I. Transvaal Meteor. Department. * Cfr 1) 2) R. T. Innes. Annual Reports. 1904-5. 3) The Meteorology of South Africa. Ch. Stewart. Von L. Manyhart, (Kalocsa) Heft VII. 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 towards 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 as 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. † 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. APPENDIX. As an illustration of the remarks put down in this paper, I give the two following extracts from the complete tables. One is taken in the rainy season, from December 1 to 19, 1904; the other in the dry season, July 22 to August 28, 1904 :— RAINY SEASON, December 1-19, 1904. Mean December Cloudiness, 63. |