1. Physical Characteristics of the Union.-An important factor in the climate of South Africa is found in the topography of the country. There are essentially four elevated plateaux; the Coast Flats, with an elevation of 500 to 600 ft. and a variation in width from thirty miles in South-West Africa to three miles or even less in the south-east of the Cape Province; the Little Karroo, a narrow stretch of from fifteen to twenty miles, with an elevation of about 1500 ft.; the Great Karroo at an altitude of from 2000 to 3000 ft., and the Northern Karroo with an elevation of 4000 ft., rising to 6000 ft. in the eastern portions. These plateaux are separated by steep escarpments, rising a considerable height above them.

2. Temperature. One effect of this formation, and the most apparent, is to counteract the ordinary increase of temperature with decrease of latitude giving a remarkable uniformity to the mean annual temperature of the different parts of the Union as shown below:

UNIFORMITY OF MEAN ANNUAL TEMPERATURE AT CERTAIN PLACES IN THE

Temperature is, moreover, affected by the relatively small area of the sub-continent making it possible for the ocean to exert an influence almost, if not entirely, throughout the country, and to impart an equability to the climate which would otherwise be absent. As the western shores of the Union are, however, washed by the cold Benguela current, and the eastern and southern shores by the warm Mozambique current, different temperature effects are produced according to the influence under which a particular part may fall. This is apparent in the following table :

The mean daily range of temperature generally increases from the coast landwards, averaging about 12° F. along the south coast and about 25° F. on the high veld. The extreme shade temperatures recorded are 125° F. at Main Tembuland in January, 1903, and 6° F. at Palmietfontein in the north-east of the Cape Province in June, 1902. Frosts of sufficient severity to freeze standing waters are practically unknown along the coasts; but in the interior between May and the middle of September when, under anticyclonic conditions, clear skies and low humidity facilitate nocturnal radiation, they are fairly frequent. Frosts may occur as early as March and as late as October.

3. Humidity.-Although the amount of water vapour in the air decreases from the coast inland the north-easterly and easterly winds of the summer months convey to the high veld more moisture than is probably present on the same plane at the coast. The curves of Fig. I below illustrate this, and in some measure indicate to what extent the air is deprived of moisture in mounting the plateaux. Fig. 3 represents graphically the mean monthly maximum and minimum temperatures and relative humidity at eight stations.

4. Rainfall. The average annual rainfall over the Union is approximately 19 inches, which is distributed as follows:AREAS SUBJECT ΤΟ AVERAGE ANNUAL RAINFALLS

BETWEEN

CERTAIN

Inches.

Sq. miles. Sq. miles. Sq. miles. Sq. miles. Sq. miles.

35.I

"

50 0.

Total area.....

276,966

50,389 110,450

35,284

473,089

The positions of these areas are shown in Map No. 2, and it is seen that there is a general decrease in amount of precipitation from east to west, excepting in the south where the effect of winter rains is apparent.

With regard to seasonal distribution, Map No. I shows that within the Union are regions of well-defined summer and winter rainfall, precipitation occurring almost entirely during the months October to March in the northwest and during the months April to September in the south-west.

This distribution is related to the movement and action of the permanent anti-cyclones which lie off the west coast of the Cape Province and off the east coast between the Cape Province and Australia. This belt of high pressure migrates northwards and southwards with the sun, and in addition the centres or cores have a lateral displacement from month to month. During April and May that to the east of the Cape Province moves westwards to the African coasts, while that on the west coast moves eastwards. At the same time an important secondary core appears over the land, where barometric pressure increases until June or July. The movement northwards of the anti-cyclonic belt brings the west and south-west coastal regions of the Cape Province under the influence of A-shaped depressions connected with the cyclonic system to the south; and it is the westerly winds associated with the rear of these depressions which are the rain-bearers fo the west and south-west coastal districts of the Cape Province, where over 75 per cent. of the annual precipitation occurs during the winter. As will be seen, the area thus watered is not extensive. Originating in the cold parts of the Atlantic the capacity of these westerly winds for moisture is small, and after condensation, forced by the elevated ground which forms the western boundaries of the plateaux, they soon cease to act as rain-bearers.

In September and October the high pressure moves off the land, merging into the South Indian anti-cyclone, which then returns eastwards to its summer position just off the west coast of Australia, and the South Atlantic anti-cyclone which lies a short distance from the west coast of the Cape Province. The north-easterly and easterly winds associated with the former introduce the moisture which is deposited over the greater part of the Union during the summer months. These winds when leaving the Indian Ocean are warm and their capacity for moisture is great; and although they deposit a considerable amount of moisture in ascending the plateaux (evidence of which is seen in the mist belts of the more easterly parts of the Union), and so decrease their absolute humidity (Fig. 1), they still reach the interior with a comparatively high humidity (Figs. 1 and 3).

Fig. 2 presents graphically the rainfall at the Royal Observatory, Capetown, and the Union Observatory, Johannesburg, respectively situated in typical winter and summer rainfall areas.

As an indication of the great fluctuations in the amount of precipitation at the same place, Table 2, containing the yearly records for nine stations since their inception, may be referred to. It will be noticed that in only one case is the difference between the lowest and highest amounts less than the average annual fall.