METEOROLOGY 147 



In 31 20' S., n° 20' E., there is what appears to be a sea-mount rising from 4000 m. to within 

 1000 m. of the surface, but this is evidently an isolated feature. 



Soundings taken on the 'William Scoresby's' line of stations are plotted in the respective vertical 

 sections (Figs. 12-18, 22, 23, 25-30). In these diagrams the vertical scale is, of course, very exaggerated 

 (about 320 times). The absence of much shelf, with development of a fairly gradual slope, is clear on the 

 most northerly section, while on the Walvis line there are two shelves about 25 miles broad, the upper 

 just over 100 m. deep, and the lower at 300 m. At Sylvia Hill the coast slopes away steeply to a narrow 

 shelf at 200 m., while on the Orange river line the shelf is much broader, about 50 miles, at 175 m. 

 This latter represents the northerly part of the promontory referred to off the Orange river. 



METEOROLOGY 

 Wind systems 

 The movements of the sea are so closely linked with those of the overlying atmosphere that any account 

 of the oceanographical phenomena of a region must necessarily take into consideration the prevailing 

 meteorological conditions. Fortunately, off South-west Africa these seem to fluctuate so regularly 

 (apart from minor variations) that a normal or ' average ' pattern can be formulated, which serves as 

 a background against which the data relating to the periods of these two surveys can be considered. 

 The general account which follows is based upon data collected by the Meteorological Services of the 

 Royal Navy and the South African Air Force (1944), supplemented by the work of Dr S. P. Jackson 

 (1951) who has long been interested in the weather of South-west Africa. 



The wind system over the South-west African waters depends mainly upon the subtropical high- 

 pressure region which overlies the South Atlantic. The latitudinal axis of this anticyclonic centre is 

 situated between 26 and 30 S. From the centre the pressure gradient decreases gradually to the 

 north, but to the south, where it borders on the zone of the 'westerlies', the gradient is much steeper. 

 Round this high-pressure region the winds blow anti-cyclonically, so that off South-west Africa, which 

 lies in the eastern side of the pressure system, the winds are predominantly south or south easterly. 

 The coastal region, however, lies in the transition belt between the oceanic and continental pressure 

 systems, and so in the proximity of the coast the winds become modified by the fluctuations between 

 the two systems. It is necessary, therefore, to distinguish two separate wind regions — that offshore 

 where the South Atlantic anti-cyclone acts alone, and the other inshore, where both the oceanic and 

 continental pressure systems play a part. 



The trade wind 

 In the summer, when the anti-cyclone lies in about 30 S., strong constant winds are produced, which 

 affect the whole oceanic region northwards from the Cape. About 80-85 % or " these winds are south- 

 easterly, and they blow with an average velocity of 11-21 knots. At the Cape peninsula these 'south- 

 easters' are a prominent feature of the summer climate, but the strongest winds are farther north, 

 between 25 and 30 S. North of 25 S. both the velocity and constancy of the trade winds diminish. 

 In winter time the anticyclone moves northwards and intensifies slightly — generally about 2 milli- 

 bars higher than in summer — so that the centre of highest pressure comes to lie in approximately the 

 latitude of Luderitz Bay (26 S.). Owing to the steep southward pressure gradient the trade winds in 

 30 S. then become intermittent. This northward displacement of the pressure system brings the 

 Cape region under the influence of the depressions of the westerlies, which, travelling eastwards from 

 the south-west Atlantic, bring rain to South Africa, accompanied by the attendant cyclonic variation 

 of the winds. The effect of these depressions extends as far north as Port Nolloth, where they produce 



