METEOROLOGY 149 



a light, but quite pronounced winter rainfall (Fig. 3) which does not occur in the more northerly parts 

 of South-west Africa. The region of strongest winds also shifts north in winter, and lies in about the 

 latitude of Walvis Bay, where 30% of the trade winds have a velocity of over 16 knots. 



The coastal winds (Fig. 5) 

 Although the inshore region remains under the influence of the South Atlantic anti-cyclone, the winds 

 are also affected by the variations in pressure over the continent, and as this fluctuates diurnally with 

 the heating and cooling of the land, so the coastal winds develop a marked diurnal variation (Fig. 5). 

 During the day, when the land is warmed up, the overlying air mass becomes light, and this causes 

 the cooler, more dense air over the sea to flow inland. This continues until equilibrium is regained, 

 and stable conditions are once more set up. On the whole these coastal winds, while they last, are 

 stronger than those at sea, but when the trade is blowing strongly over the open ocean one generally 

 finds the coastal winds are also stronger. 



In summer (December -February), when the anti-cyclone is in its southern position, south winds are 

 prevalent on the coast as far north as Luderitz Bay (Fig. 5), generally having an easterly component 

 in the morning. This seems to be the effect of the anti-cyclone exerting its influence on the relatively 

 stable air mass over the coastal waters and land during the night and early morning. As the land warms 

 during the day, however, the wind veers round to the south and south-west and intensifies, reaching 

 at Luderitz an average velocity of 22-27 knots. This may occur fairly early in the day, and the sea- 

 breeze then continues until after nightfall when the pressure systems become balanced, and the anti- 

 cyclone once again becomes the dominating influence. It will be remembered, however, that in summer 

 the anti-cyclone has its greatest effect in the Luderitz Bay region, and that north of this its influence 

 steadily decreases, so that at Walvis Bay the stable air mass during the dark hours appears to be little 

 affected by the anti-cyclone, and the conditions experienced there during the night are either complete 

 calms or light northerly winds. As the day progresses, and the temperature effect comes into play, 

 the wind backs to the west and then S.S.W. from which the sea-breeze develops just after noon, and 

 blows with a mean force of 11-16 knots. 



In winter, as might be expected with the northward shift of the pressure systems, the seasonal 

 variation is much more pronounced south of Orange river than on the more northerly part of the coast. 

 With the westerly depressions, the Cape receives winds more or less evenly distributed between north 

 and south from the westerly sector. A marked feature of the winters is the north-westerly gales, which 

 along with south-westerly weather are associated with the passage of the depressions. These features 

 are less prominent at Port Nolloth where the trade and the diurnal variations begin to take effect. 



Occasionally, when the anti-cyclone is not strongly developed near the coast, small depressions form 

 close to the shore, and move northwards. On these occasions the sky remains cloudy, and a light 

 north-west wind sets in with a force of about 7-10 knots. These conditions have a remarkable effect 

 upon the sea temperature, which increases suddenly and has been known to reach as much as 23-9° C. 

 at Walvis Bay. 



At both Luderitz Bay and Walvis Bay there is a marked increase in easterly winds during the 

 mornings in winter time (Fig. 5), but in both cases they are replaced with the much stronger south- 

 westerly breeze in the afternoons. Pressure is high on the continental plateau in winter and, coupled 

 with the lower pressure over the coast, leads to an outward push of air from the plateau. This usually 

 becomes obliterated later in the day, partly by a diurnal variation of the east wind itself further inland. 

 Amidst these easterly winds, when conditions are favourable, the noteworthy ' Berg winds ' occur, 

 which, blowing with considerable force towards the coast, make conditions extremely unpleasant on 

 account of their high temperature compared with the generally cool coastal climate. 



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