1885.] Influences and Cyclonic Paths of South Africa. 2d 



To the nortli is the assumed low pressure area of Central Africa. The 

 limits or position of this area are, of course, not known, but all the 

 surrounding conditions go to prove that such a depression does exist, 

 because in no other way can our summer storms be accounted for. 



To the south of the two anti-cyclones is the south sea belt of low 

 pressure which completely surrounds the South Pole. 



It is from the area of low pressure to the north of us that the great 

 majority of our summer cyclones come, the motive power of genera- 

 tion being the great South Atlantic anti-cyclone. 



Neither cyclones nor anti-cyclones are ever motionless, but are 

 constantly varying in shape and pressure, the accompanying chart 

 merely giving the mean of a very great number of years. The reason 

 of this is that the two areas, high and low, are constantly reacting on 

 one another. The air in the depression is more rarified at one time 

 than at another, and consequently the quantity of atmosphere flowing 

 onto the neighbouring high pressure areas varies, hence these areas 

 fluctuate both in pressure and size, sometimes almost embracing the 

 low pressure areas. When such a state of relationship exists, there 

 is a great inclination on the part of the low pressure area to form a 

 secondary, which is accelerated if the high pressure forms itself into 

 a wedge. 



Anti-cyclones change very slowly, and cyclones very quickly, so it 

 can be seen how several of these secondaries may form, become 

 veritable cyclones, and pass to the south before the anti-cyclone 

 changes its shape. 



On the same chart, Nos. 1 to 4, I have depicted the formation of 

 an imaginary cyclone, the area of high pressure extending along the 

 coast and almost embracing the depression on the north. On No. 1 

 we have a cyclone passing off the Colony, while a secondary is form- 

 ing to the S.W. of the central depression. In No. 2 the whole 

 system has moved further to the S.W., the secondaiy has increased 

 in proportions, and the permanent area decreased. Low pressure 

 exists at Natal, where the last depression is passing to the eastwards. 

 On No. 3 the secondary has become a veritable cyclone, and is travel- 

 ling to the south, while the permanent area has retreated and assumed 

 its original proportions. Lastly, on No. 4 the cyclone is fairly over 

 the Colony, and has developed a secondary towards the S.W. 

 Another secondary is forming in the permanent low pressure area, 

 and as long as the relative position of anti-cyclone and depression 

 remains unaltered the same sequence of weather will be experienced. 

 The colonial isobars of these diagrams were taken from my synoptic 

 charts for the 17th to 20th January of this year. Of course when the 

 shape and pressure of the anti-cyclone changes, the paths of the 

 cyclones will change also. 



The foregoing diagrams merely illustrated the formation of a 

 cyclone on the S.W. side of the central depression, but I am of 

 opinion that they are formed all round this area, those on the eastern 

 side, if they do notmove away eastwards at once, will revolve round 

 from E. by S. to W., and then if not retarded by an anti-cyclonic wedge, 

 will pass off and be lost in the S.E. trade zone of the Atlantic ; but if 

 the anti-cyclone bars iheir further progress, they will be deflected 

 and travel south, and pass off to the east. 



These I call the summer cyclones, in opposition to the winter ones 

 to which I shall refer later on. 



