3— ANTICYCLONES AND THEIR INFLUENCE ON SOUTH 



AFRICAN WEATHER. 



By Colonel H. E. Rawson, C.B., F.R.Met.Soc, F. Ph. See, Etc. 



To anyone holding the clue to South African weather, the study 

 of meteorological problems, not only on this continent but generally over 

 the whole globe, becomes a most fascinating one. South African 

 weather does not present those samples which are so well known in the 

 British Isles, and which are so bewildering by the rapid and 

 inexplicable way they follow one another. Nor are the records which 

 we possess here a mere conglomeration of dry bones, requiring almost 

 a magician's wand to give them life and make them of value. 

 Few as the first-order meteorological stations are in South Africa, they 

 have given us observations of exceptional value and importance, owing 

 to the comparative ease with which they can be interpreted, and an 

 intelligible idea be formed from them of what is going on. We owe 

 a real debt of gratitude to the comparatively large band of volunteers, 

 who supplement our records with local observations, and who help to 

 increase the value of the more precise and accurate data issued by the 

 observatories. 



The object of this paper is to submit this clue to you for your 

 consideration, and to lay before you some of the reasons why it is 

 worthy of your attention. 



Permanent Anticyclonic Systems. 



Before our weather can be intelligently studied, a preliminary 

 knowledge of the seasonal movements of the so-called permanent anti- 

 cyclones in our neighbourhood is indispensable. For a long time it 

 was held that such systems formed persistent girdles, or belts, of high 

 pressure round the world on both sides of the equator, at about Lats. 

 25° to 30°. At certain seasons of the year they were said to decrease 

 in intensity and dissipate, to appear once more when the season 

 returned. Thus in winter in the northern hemisphere two well-defined 

 high-pressure systems were found to prevail for quite two months or 

 more at a time over Eastern Siberia and over North America. They 

 gradually diminished as summer approached, and eventually broke 

 up and gave way to cyclones. On the other hand, during summer 

 high pressure prevailed over certain parts of the Atlantic, and 

 especially in the neighbourhood of the Azores, which completely 

 disappeared in winter and was replaced by low pressure, with its 

 destructive storms. The gradual changes from high to low pressure and 

 back again tojhjgh pressure were described as " reversals," and were 

 looked upon as continuous processes caused by the seasonal increase 

 and decrease of the sun's heat over the areas where these systems were 

 found to prevail. In 1898 I ventured to combat this view, and I put 

 forward the theory * that the permanent anti-cyclonic systems did not 

 dissipate, or disperse, when they disappeared ; but that they gradually 

 moved ofif to localities where the conditions were more congenial. 



*Quarterly Journal of the Royal Meteorological Society, Vol. XXIV., No. 170, 

 July, 1898. Anticyclonic Systems and their Movements. 



