1885.] Influences and Cyclonic Paths of South Africa. 27 



I shall refer as little as possible to tlie Northern Hemisphere, so 



will now state that I have tuken the conditions there as a basis, and 



have proceeded on the assumption that "like conditions give like 



results," reversed, of course, on account of the different hemisphere. 

 ^4 % % ■}:•. n. ^ 



To Professor Buys Ballot of Utrecht belongs the honour of placing 

 on record the law of the relationship between winds and gradients. 



For the Southern Hemisphere the law is, " Stand with your back 

 to the wind and the barometer will be lower on your ri^Jd hand than 

 on your left." Thus when a south-east wind is blowing in Cape 

 Town the area of low pressure must be to the north-east, and the 

 steeper the gradient the stronger will the wind blow. 



The wind, as a rule, neither blows direct to the area of low pres- 

 sure nor parallel to the isobars, but takes an intermediate course, 

 blowing nearly parallel when far removed from the centre of low 

 pressure, aud nearly at right angles to them when close in : this 

 centie is generally an area of dead calm. Hence it is evident that 

 we have to examine the configuration of the isobars to arrive at the 

 direction of the wind. 



Isobars, though they are constantly changing, and form themselves 

 into ahnost every shape, have several recognizable and general forms, 

 the two principal ones being the cyclone and the anticyclone, the 

 relative positions and pressures of which govern the weather of the 

 world. 



A cyclone is an area with the lowest barometer reading at the 

 centre and the highest round the edges, the wind of course circling 

 inwards, according to Buys Ballot's law. A cyclone can be of any 

 size, and although the circular form is generally noticeable, stiU they 

 are often so distorted as to appear of almost any shape. 



To shew the immense size of some of these I may mention that it 

 is no uncommon thing for one of them to rest over the whole North 

 Atlantic, so that New York, Newfoundland, Greenland and Europe 

 are being influenced by the same atmospheric circulation. On the 

 12th of November, 1875, one of these depressions covered the whole 

 of Europe with the exception of Spain. 



Although cyclones are often of such grotesque forms, still they 

 have many points in common, and, as I said before, the circular form 

 is generally noticeable, especially as the central isobars are reached, 

 so that they are easy to be seen on a sjTioptic chart. 



Besides this, there is a great similarity of weather in them, so that 

 by examining one system we have the approximate conditions for 

 them all. 



Let us, therefore, imagine a cyclone to be situated over South Africa, 

 and travelling from north to south. Having plotted it down on a 

 chart, and divided it into four quadrants by lines running from north 

 to south, and from east to west through the centre, the following is 

 the weather we should expect to see. 



South-west Quadrant. — To the south and west thin cirro-stratus, 

 with a pale moon and perhaps a large halo. As the trough is 

 approached the sky becomes overcast and threatening with heavy 

 cumulo-stratus. Further from the centre, cirrus blown from the S.E. 

 will be seen. Just as the trough is reached, if near the centre, a few 

 misty showers will fall, but if away from the centre the cirrus will be 

 clearing away to the south, and a hard blue sky be seen. 



