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on the whole it appears that the minimum temperature variability is probably common to the whole country, while the maximum temperature is determined mainly by local conditions. There is a further curious point that the maximum temperature variability is greater than that of the minimum in every month, both at Kimberley and Durban, whereas at East London the minimum values are the greater in the last quarter of the year. In particular, it may be noted that great individual values of the maximum temperature variability do not frequently occur in pairs, on alternate days, at Kimberley, as they do at East London and Durban. The average maximum temperature variability at Kimberley for the year is 4.3°, that for the minimum is 3.7o. Thus the mean variability is greater on the tableland than it is on the coast. The greatest monthly mean maximum temperature variability at Kimberley during the ten years 1896 to 1905 is found in October, 1899, with 8.1°, the least in July, 1905, with 1.7o. The greatest monthly mean minimum temperature variability is found in October, 1904, with 6.5°, the least in April, 1905, with 1.6o. These greatest and least values are not common to the whole country; it by no means follows that a high value of the variability at one station indicates a high value at another at the same time.
The monthly average values of the variability of dew-point and relative humidity, at Kimberley, at 2 a.m. and 2 p.m., are as follows:
TABLE 4.–Variability of Dew-Point at Kenilworth
(Kimberley), 1896 to 1905.
TABLE 5.—Variability of Relative Humidity at Kenilworth
(Kimberley), 1896 to 1905.
appears that both the absolute and relative humidities change more from day to day in the height of summer than they do in the depth of winter ; but that, as distinguished from the temperature variations, the variability of moisture from night to night is considerably greater than that from day to day in respect both to mean and extreme values. This is, to me, a surprising result, considering that the relative humidity has a much wider range by day than by night.
The object of this brief paper is mainly climatological, but it will not, perhaps, be out of place to glance at some of its meteorological aspects.
We have said that the inter-diurnal temperature variability on the coast is much greater than it would be if hot and cold winds did not occur. The hot winds have been dealt with elsewhere. * They are still difficult to account for entirely, but enough is known of them to make it certain that they are associated with a definite type of weather, that they originate on the table-land and blow outwards and downwards, and are quite analogous to the foehn winds of Europe, Greenland, and elsewhere. The cold winds of the coast have not yet been discussed, although they are, from a meterological point of view, at least as interesting as the hot winds.
For the purpose of obtaining a preliminary idea of these coldweather periods on the coast, I have selected 85 typical cases of low minimum temperature registered in any month during the nine years * See Stewart, “The Meteorology of South Africa,” Science in South Africa, 1905, p. 40; Hann, Met. Zeitschrift
, Jan. 1904, p. 42; Sutton, “The Climate of East London,” Trans. S.A. Phil. Soc., Vol. XVI., Part 3.
1896-1904 at East London, and compared them with the simultaneous temperatures at Durban and Kimberley. The following are mean annual results for penthemera, of which the night of a cold wind is the central night, together with the normal means. TABLE 6.-Mean Annual Minimum Temperature of Cold Spells at
East London, compared with the simultaneous Temperatures at Durban and Kimberley.
An examination of these numbers shews that the cold weather is general throughout the country, but that the true epoch of minimum is earliest on the central table-land, and latest at Durban. In fact, the lowest point of the curve of departure from the mean temperature of the five days comes between the second and third day at Kimberley, slightly later than the third day at East London, and between the third and fourth days at Durban. This is a remarkable result, considering that the barometric conditions upon which these low temperatures depend travel from SW. to NE., and are therefore felt earlier at East London than at Kimberley by many hours. If the dependence of the temperatures upon the pressures were absolute we should expect the temperature wave, like the pressure wave, to be felt first at East London, and almost simultaneously, later on, at Durban and Kimberley. It seems to follow, then, that the cold winds in question, like the hot winds, must originate on the tableland, and move downwards to the coast. An analysis of the barometric pressures, which it is unnecessary to give here, supports this view, anti-cyclonic conditions being the rule.
For the purpose of investigating the wind movement I have only had access to the anemometer records of East London for the three years 1898-1900 ; but of the 31 instances of low minimum temperature at East London in those three years 29 occurred with winds blowing off the land, and the other two with wind blowing parallel to the coast, one in December strong from the SW., and the other in October strong from a north-easterly direction. I am inclined to think that in both these exceptional instances there must have been a strong admixture of land wind entering from above. The average velocity of the wind at the time of minimum on the third day in the 31 instances between midnight and 8 a.m. was 19 miles per hour, i.e., about a mile an hour faster than the normal. It is evident from these facts that the cold winds of East London are true bora winds, similar in every respect to those of Dalmatia.
A similar investigation for the occasional low minimum temperature periods at Durban gives 83 typical instances, not, as it happens, generally falling on the same day as those in the above comparison for East London. In most of these there is either calm or light wind, and in that respect they differ from the East London cases. The annual averages are :
TABLE 7.-Mean Annual Minimum Temperature of the Cold Spells
at Durban, compared with the simultaneous Temperatures at East London and Kimberley.
Here we see that while the epochs come in the same order as in Table 6, as might have been expected, the low temperature waves of Durban are not so pronounced at East London and Kimberley as the low temperature waves of East London are at Durban and Kimberley. And an examination of the individual instances shews the
to be that many of the exceptionally low temperatures felt at Durban not accompanied by any
by any great fall of temperature inland, but, surprisingly enough, associated in some way with winds from the sea. Nineteen of the 81 cases of low temperature averaged in Table 7 were associated with winds having a component off the sea, that is to say, blowing from anywhere between S. round by E. to ENE. The following are averages for the three stations for the nineteen occasions upon which the wind at 9 a.m. at Durban came in from the sea
TABLE 8.-Average Minimum Temperatures of the Cold Spells
at Durban, with Sea Winds.
* In eighteen out of the nineteen instances the wind was also blowing off the
sea at 3 p.m, on the previous day:
The shape of the Durban curve is here nearly the same as in Table 7, where all the cold spells are included, but the epoch is somewhat earlier. Also the average is rather greater, but that is accounted for by the circumstance that cold winds from the sea are not met with during the winter months. The East London and Kimberley comparative values are elevated somewhat for the same reason. While these peculiar conditions are in progress, pressure on the table-land is moderately high, the averages at 8 a.m. at Kenilworth being :First Day 26*108 inches.
26'129 inches. Second Day 26*169
26 116 Third Day
26°160 and the wind on the table-land shifting from easterly components to westerly. Thus the low temperatures at Durban coincide with the following sides of moderate barometric crests* and the incidence of westerly winds; also the rise of temperature at Kimberley on the fourth and fifth days of the penthemera corresponds to the falling pressure. The average temperature on the table-land for the five days is probably somewhat greater than the mean minimum temperature of the months in which they mostly occur. Now, there is one very remarkable characteristic of these cold sea winds, and that is their dryness. Not one of them carries the quantity of moisture proper to the time of year, the average for them all being fully 15 per cent.. short of the mean, and, in spite of their low temperature, they appear to be far from humid. One would scarcely expect a cold, dry wind to blow over an ocean whose water is for its latitude among the warmest in the world. Therefore, because of its dryness, it is probably a re-entering land wind which has somewhere moved outwards, but returned after describing a very short path over the ocean. But how, then, is its temperature to be accounted for? If it originate on the central table-land it must have started as a warm wind, and by dynamic heating in its descent should become still warmer, and thus reach the coast as a warm wind. The cause of its low temperature is therefore still uncertain. In this connection it would be interesting to know whether snow is lying on the crests of the Drakensberg while these cold sea winds are blowing. If not, then the only likely place of their origin seems to be in the main westerly atmospheric drift above, from which they slant downwards and inwards in response to the falling pressure.
* The barometric changes at Durban are appreciably the same as those at
Kimberley at these times.