28 Mr. Hoioard, An Investigation into the Isobaric [April, 



North-west Quadrant. — Mostly a blue sky, with the exception of 

 occasional patches of detached cumulo- stratus. 



South-east Quadrant. — To the south the usual watery sky with a 

 halo round the moon, while further to the north very gloomy and 

 threatening weather will be experienced. As the trough is approached 

 we will find near the centre drizzle and driving rain, and further to 

 the east a gloomy sky with dense cumulo-stratus. 



North-east Quadrant. — Showers and squalls, followed by detached 

 cumulus, windy cirrus, and finally blue sky. 



The foregoing is the general description and position of the 

 weather in a cyclone, but of course several conditions have a varying 

 tendency, and considerably modify the weather : thus the presence of 

 an anti-cyolone will increase the gradients, and cause more intense 

 weather on that side than on the others. 



Another cause of variation is when a secondary forms, which 

 generally takes place along the prolongation of the trough or against 

 an area of high pressure. The effect of this is to increase the force 

 of the wind on the side furthest from the primary, and to reduce it 

 between them, sometimes causing a dead calm. 



The secondary forms a most important factor in the study of South 

 African Meteorology, as scarcely a cyclone passes without having a 

 secondary developed over the south-western districts. 



Sometimes when a cyclone is in close proximity to two areas of high 

 pressure a V depression is produced, which also modifies the weather. 



An anti-cyclone is in every way the converse of a cyclone, the 

 isobars, in this case, enclosing an area of high pressure, the circula- 

 tion of wind being the reverse of that in a cyclone. Anti-cyclones are 

 of all conceivable shapes, but are generally longer from east to west 

 than from north to south, very often rising into two or more heads. 

 When an offshoot takes the form of the letter V, it is called a wedge, 

 and plays an important part in the formation of secondaries. 



When two anti-cyclones approach close to one another they are 

 generally joined by a neck of comparatively low pressure called a 

 *' Col," a formation of a very treacherous nature, because upon the 

 near approach of a cyclone the tendency is for secondaries to form in 

 this '' Col," and it then becomes a matter of great doubt which way 

 the secondary will travel. 



The only other barometric influence affecting the weather is caused 

 by waves travelling from the low pressure areas towards those of 

 high pressure. In the central areas of low pressure the barometer is 

 seldom stationary, but keeps on gently rising and falling. This 

 oscillation causes a series of ring waves to flow outwards, similar to 

 the wavelets caused by dropping a stone into still water. 



With the foregoing geueral types of isobaric forms we will now 

 devote a short time to the consideration of the storms, &c., of South 

 Africa. 



On Chart C, Nos. 5 and 6, 1 have delineated the mean barometer 

 readings for the months of January and Jul}^ over that portion of 

 Africa and the adjacent seas which affect Cape Colony. 



We will first devote our attention to January's chart, and see how 

 the differences of level influence our weather. 



On either side of South Africa is to be seen an area of high pres- 

 sure. These are called respectively the South Atlantic and Indian 

 Ocean permanent anti-cyclones, and are joined together by a ** Col." 



