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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 () 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. 1., p. 288 et seq. + 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.

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 :

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July 22, 23

24, 25

26-Aug. 6 Aug. 6

54

25.804

.743
.863
771
.843
.791
.831
764
•794
•759

55.9 57:1 54:4 60.0 60.2 63:1 59.5 63.7 62.9 66.5

124.5 136.0 125.7 129.5 130.0 133:1 130.0 136.0 1340 140.0

E.
E. with N. & veering
E.
Veering
E.
Veering
E., E.S.E.
Veering
E.
Veering

7-10 10-14 14-18 18-24 24-27

39 61 53 47 34

»

28 ..

AFRICA

By J. R. SUTTON M.A., F.R. MET.S.

It has been pointed out before* that in expressing the terms representing the climate of a place, it is necessary to give, in addition to the elements of maximum and minimum, mean, and range of temperature, what is known as the mean inter-diurnal variability of temperature, that is to say, the mean difference between the temperatures of one day to the next. The present paper is a contribution to this work for South Africa, giving the variability for three typical stations, namely, for Durban, East London, and Kenilworth (Kimberley), of the maximum and minimum temperatures for each month for each station, from ten years' observations, together with the variability of dew-point and relative humidity at Kenilworth at the hours 2 a.m. and 2 p.m.

The mean monthly values of the inter-diurnal variability for Durban are:TABLE 1.—Variability of Temperature at Durban,

1895 to 1904.

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It appears from this that the variability of maximum temperature at Durban is greatest in the early summer, at least at mid-winter. This result is brought about chiefly by the frequent warm and occasional very hot (foehn) winds of September to December. Generally speaking, the temperature of one day at Durban is pretty much like the next; but a hot wind means in nearly every case a sudden rise from the normal maximum one day and an equally rapid fall the next, such pairs of values of great variability making a considerable addition to what would otherwise be the mean variability of the month. It is quite a mistake, though a common one, to *See inter alia, Hann, Lehrbuch der Met., 1901, p 115; Ward, “Suggestions as suppose that these hot winds last for days: as a matter of fact they seldom last more than a few hours, and are usually confined to the daylight hours. They seldom blow during the night, or affect the minimum temperatures to any great extent, although they may on rare occasions recur the second day. The variability of minimum temperature at Durban is least in the autumn and greatest in the spring. Thus the day temperature variability is falling during the first half of the year, while the night temperature variability is rising. The greatest variability of the minimum temperature nearly coincides with that of the maximum ; but, as we shall see later, this result is not brought about by the extension of the hot winds into the night, but, on the contrary, is caused by temporary cold winds which, in their turn, have no great influence over the day temperatures. In fact, while the hot winds are essentially cyclonic, the cold winds are as essentially anti-cyclonic, both resembling their prototypes of continental Europe, but differing from them to some extent because of the special geographical conditions of South Africa. Naturally, however, the more frequent the occurrence of cyclonic conditions, the more frequently will anti-cyclonic conditions alternate with them, and therefore, so far, the hot and cold spells may be said to be related to each other. The greatest monthly mean variability of maximum temperature at Durban during the ten years discussed appears in January, 1900, with 7.9°, the least in July, 1901, with 2.6°; the greatest monthly mean variability of minimum appears in October, 1900, with 4.7°, the least in March, 1896, with 1.4° Large monthly mean values of the maximum temperature variability occur in any of the summer months, while fairly large values of the monthly means of minimum temperature variability occur all through the year.

to a more Rational Climatology,Report of the Eighth Geographic Congress, 1905, p. 280.

The mean monthly values of the inter-diurnal variability for East London are :TABLE 2.-Variability of Temperature at East London,

1895 to 1904.

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At East London the variability of minimum temperature follows the same rule as at Durban, being least in autumn and greatest in spring ; the spring values are, however, greater at East London than at Durban, and the average variability for the year is also greater by nearly half a degree. The variability of maximum temperature at East London is, however, of quite a different character from that at Durban, for it is least in summer and greatest in winter. Here, again, we have a result due to the hot winds, which are frequent at East London during the winter season. The average variability of maximum temperature for the year at East London 4.1°, i.e., nearly half a degree less than that of Durban. The greatest monthly value of maximum temperature variability at East London during the ten years 1895-1904 is found in July, 1898, with 8.5°, August, 1897, coming next with 8.4°; the least is in March, 1895, with 1.9o. The greatest monthly value of minimum temperature variability is found in August, 1897, with 5.6°, the least in February, 1897, with 1.7

At Kimberley the minimum temperature variability follows the same rule as at Durban and East London, being greatest in October and least in February and March. But the Kimberley maximum

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TABLE 3.—Variability of Temperature at Kenilworth

(Kimberley), 1896-1905.

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temperature variability differs from both. As at Durban, it is greatest in October ; as at East London, it shews a drop in December ; whereas its least value comes in April, that is, later than it does at East London, but earlier than it does at Durban. Thus

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