274 SOUTH AFRICAN METEOROLOGY. 



one to the south, and such is the case, although upon many 

 occasions the high pressures follow one another so closely that 

 these areas of low pressure are of little importance, as thev do 

 net develop into veritable lows. 



At about 60° of south latitude true cyclones are constantly 

 moving from west to east, and as these pass along, their northern 

 fringes affect South Africa, surges keep on t-olling northward, 

 and expend their forces against the anticyclone belt. When, 

 however, they approach the west coast, where the Atlantic high 

 has its minimum width, they extend much further north and 

 often join issue with the low, which comes down from the 

 equatorial region as a V-shaped depression. If there be a high 

 pressure over South Africa, or even if one be moving awav 

 eastwards, the low area, a long inverted V, extending in a south- 

 westerly to north-easterly direction, sweeps right across the sub- 

 continent, but, owing to the presence of the equatorial low pres- 

 sure, known as the monsoonal tongue, it swings round, so that 

 when it is on the south coast it extends from south to 

 north, while as it passes away to the east, its axis becomes 

 south-easterly to north-westerly., This sequence is most com- 

 mon during summer. 



During winter there is usually a more or less permanent 

 high pressure over the north-east (Transvaal and east), and this 

 often erctends to the west coast, the whole forming a belt of 

 high pressure with two cores which fluctuate in intensity. 

 Should a trough of low pressure approach from the west and 

 the high be over the north-east only, the low will extend along 

 the we^t coast, being first noticed well to the north, moving down 

 to the south-west. The high, however, will remain firm, and 

 the arm of the low will be slowly withdrawn, the appearance 

 being that of a low moving down the west coast, and along the 

 south to the south-east or east ocean. The high will be but 

 little aft'ected, except that it may be reduced in intensity owing 

 to its having supplied atmosphere to the low in passing. 



In Europe each pressure type is named after the prevailing 

 winds, but in South Africa winds are affected by local conditions 

 to such an extent that it would not be safe to base any group- 

 ings on them. It has, therefore, been deemed more satisfactory 

 to define the types according to the positions of the high pres- 

 sures. By doing this we get six clear types. 



1. The Westerly (IV) Type. — High, over the west coast, 

 sometimes extending to the south. Prevailing winds should be 

 southerly to westerly. 



2. The Easterly (E) Type. — High over the north-east 

 (Transvaal and east), sometimes extending to the southern- 

 coast. Prevailing winds should be northerly. 



