64 WEATHER FORECASTING. 



winds are in opposition, vertical air currents are also irregular, 

 so that secondaries are common. It, therefore, comes about 

 that where a " Col " exists one must look for local disturbances, 

 such as unsettled weather, rain or thunder, but, of course, the 

 season of the year must be taken into account in making a fore- 

 cast. 



Line squalls, thunderstorms, hail storms, non-isobaric rains, 

 etc., are subjects of which very little is known in South Africa; 

 much investigation will be necessary before we can speak upon 

 these with confidence, and with the meagre data available this is 

 likely to be postponed indefinitely. One or two remarks may, 

 however, be hazarded. Line squalls are not of frequent occur- 

 rence, even when the configuration of the isobars indicates them, 

 in most cases a secondary V or wedge, associated elsewhere with 

 such squalls, merely leads to fog. The " black " south-easter of 

 the Cape Peninsula seems to be often associated with such forms 

 of isobars. The so-called " tornado " at Malmesbury on the 

 morning of the 29th of September, 1905, was a line squall, as 

 the isobars for that morning clearly prove. 



Thunderstorms follow where line squall isobars exist, in 

 " cols," at the apex of a wedge, or where winds are opposed and 

 temperature conditions are suitable. Thus, if during summer 

 a fog wedge shews on a chart, it is safe to say in the forecast 

 that thunder conditions exist and that thunder is probable. 

 Again, if a very narrow " col " is seen, thunder can be forecast, 

 the narrower the " col " and the closer the wedge apices are 

 together the more the probability of thunder. Large open loops 

 are the homes of secondaries, and thunder can be forecast during 

 summer. In forecasting such storms it must be borne in mind 

 that certain parts of the country are more subject to storms than 

 others. 



Where thunderstorms are prevalent hail can be looked for, 

 but so far it is impossible to say whether any storm will be 

 accompanied by hail or not. One thing seems to be clear ; hail 

 storms do not change their position in a depression with 

 reference to the configuration of the isobars, but move with the 

 depression, and as most of these travel from S.W. to N.E. so 

 must of our hail storms follow a similar direction. 



The forecaster must bear in mind that slow changes in 

 existing barometrical conditions prefigure a continuance of 

 existing weather, be it fine or unsettled ; quick pressure changes 

 foretell rapid alterations in weather conditions. Stagnation any- 

 where will charge the atmosphere with moisture and lead to 

 fogs, mists and drizzle rain. 



Pressure increasing rapidly is often followed by a depression ; 

 while a slow increase makes for fine weather. Slow increase 

 after the rain of a depression has begun gives merely drizzles 

 and a slow clear-up. Therefore, bear in mind, — slow rise slow 

 clear, quick rise quick clear. Look towards the ruddy glow in 



