Изображения страниц
PDF
EPUB

*

:

...

2.

2.

rarely give an appreciable rain, though the air is unpleasantly damp. Drizzles, lasting two or three days, and yielding anything from zero to one-tenth of an inch per day is the most they give us. During that time the Hygrometer remains very high, and evaporation below the average even with high wind. This kind of weather is a noted featuie of the Rhodesian climate ; no data are, however, available to decide whether it could be traced right to the Indian Ocean or whether it might not be due to rains further South or South-East. These reasons would make me hesitate to accept on the whole for Rhodesia Mr. Sutton's theory of the Cape rains.

It will be of interest to compare the other records with the Anemometer sheets. The Barometer, Thermometer, and Hygrometer show a distinct periodic movement, and a comparison with the winds shows a very close synchronism with the wind periods.

To demonstrate this roughly, I have drawn up the following table of means for each wind period : + Mean of Mercurial Barometer reading at 8 a.m. reduced to 32° Fahr.:

Max. + Min.
Mean Temperature by formula
Mean Black Bulb Thermometer readings:

Max. + Min.
Mean Relative Humidity by the formula

read off a Richard Hair Hygrometer sheet. For all these means I have followed the wind periods except now and then for the hygrometer. I found there that the humidity curves have a different appearance with the different winds, but that the change from one period to another does not always coincide exactly with the changes in the other elements.

There is sometimes a difference of 24 hours, sometimes, on the contrary, the changes in the Hygrometer follow very closely the wind variations. In cases where I found only one day's difference in the periods, I put the day of transition now in one and now in the other period, according to the shape of the curve. The mean relative humidity, calculated in this way, is, of course, liable to be very erroneous in the rainy season ; but I do not want absolute figures, I only want to find the way in which the quantity varies. In the dry season from the beginning of April to the end of October, the mean relative humidity does not differ very much from the mean of the highest and lowest readings of the day.

The general result of this tabulation may be stated as follows: A period of easterly winds corresponds to a high barometer, low temperature (both air and sun temperature) and a high Relative Humidity. A period of veering wind to a low barometer a high temperature and a low Relative Humidity.

Out of 84 periods in which there was a daily veering of the winds round the compass 65 fall between two high barometers, 3 between two low barometers, and the others on a rising or falling barometer. At Boroma, near Tete, on the Portuguese Zambezi, latitude 16° North East

of us, I find that half the rainfall of 1891-92 is attributed to S.E. winds.

I have not yet seen the later rainfall returns. + See Appendix.

[ocr errors]

The Auctuation of the barometer is very small at Bulawayo (Lat. 20° 9'). The mean of the extreme ranges for eight years gives only .222 of an inch. The difference between two consecutive days in the two years under consideration was nearly always under .1, the highest was .158.

The correspondence in even this summary handling of the figures, between barometer and wind changes, points out the importance of relatively small pressure variations in these latitudes.

The air temperature results show on the whole the truth of the statement that a low temperature corresponds to East winds; there are more inversions, however, than in the pressure figures, but the Black Bulb temperatures show a remarkable correlation. Out of 70 periods of daily veering winds which occurred from April, 1904, to November 9th, 1905, during which time the Black Bulb readings were taken, 54 show a higher temperature than in the East Winds that preceded or followed. The differences are sometimes very large. especially in the rainy season. Clouds covering the sky in the middle of the day may partly account for this great difference, but in the rainy months clouds appear after 10 a.m. nearly every day, so that the chances may be said to be equally distributed. But the same sequence of high and low temperatures, though with smaller differences, is observed in the clearer months, for instance in July and August, when the cloudiness comes down to 1.1.

The variations of the Relative Humidity are also very interesting. Even in the rainy season, with a large amount of rain in the low barometer periods, there is a marked lower Relative Humidity than on high barometer and East wind periods. The temperature variations partly account for that, but not completely, I think. The East Wind must be more saturated with moisture, although a large amount of it has often been deposited on the high lands of the Eastern coast. To settle this point satisfactorily, the dew point and absolute humidity figures would be required. It might be said also that the greater relative humidity corresponding then to the East winds may be considered as the effect of the rains that fell in the preceding days, especially as the rains setting in from some other direction with thunderstorms, often continue as slow rains with winds ranging from E. to S. But the same sequence of high relative humidities with easterly winds and low ones with other winds is prevailing with much more marked lifferences in the dry season when no rain falls. The Relative Humidity curves show a peculiarity worth mentioning. During East Winds the amplitude is very great, ranging often from 10 per cent. or lower to 90 per cent. or over, whilst during the other periods the daily curve is very contracted round the mean of the day, the maximum remaining considerably lower than the maximum during the East Winds.

Special mention ought to be made of the periods in which the wind passes from E.S.E. or E. to N. E. or N. in the middle of the day and back, usually some time in the afternoon before sunset. This might correspond to the law which Dr. Sprung* gives in his 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. On the 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.

* Lehrbuch der Meteorologie. 1885 p. 345.

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.

[ocr errors]
[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small]
[merged small][ocr errors][ocr errors][merged small][ocr errors][merged small][ocr errors][merged small][ocr errors][merged small][ocr errors][merged small][ocr errors][ocr errors]

Fig 7 - Wind Frequency Resultants (Polar Coordinates)

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][ocr errors][merged small][ocr errors][ocr errors][merged small][merged small][merged small][merged small][ocr errors][merged small]

850

8501
Figl Wind-Frequency Resultants (V Table A)
Fig II Wind Movement Resultants (V Toblo A)

[ocr errors]

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. 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 à 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 Cfr I) The winds of Kimberley. J. R. Sutton. Trans. of the S.A.

Phil. Soc., Vol. XI S.I.
2) Transvaal Meteor. Department. Annual Reports. 1904-5.

R. T. Innes.
3) The Meteorology of South Africa. Ch. Stewart.
4) Beobachtungen zu Boroma, 1891-2. Von L. Manyhart,

S. J., in Publicat des Haynald observ. (Kalocsa) Heft VII.

[ocr errors][ocr errors]
[ocr errors]
« ПредыдущаяПродолжить »