62 WEATHER FORECASTING. 



Wherever opposing currents come into contact thunder- 

 storms are likely to occur ; thus, along the low trough of a V, or 

 on a " col," and the more wedge-shaped the adjoining high pres- 

 sure spurs the greater the probability of thunder. Similar 

 conditions during winter will bring snow where the altitude is 

 suitable. 



Winter Months. 



During the winter months the southerly depressions are felt 

 more over the Western Districts and along the Coast. The 

 anti-cyclones are greater in pressure and the generating cyclones 

 of the Antarctic Ocean further north, so that the pressure 

 gradients in a depression must be steeper, and, as a consequence, 

 the wind stronger and the rainfalls heavier. No matter what 

 the distribution of pressure be, if it is yielding over the South- 

 West and North-West winds be blowing there, it prefigures the 

 approach of a depression. The prediction for the South-West 

 would be : fresh to strong N.W. winds to continue, with sky 

 becoming more and more clouded. 



Generally an area of high pressure rests over Namaqualand, 

 tailing down towards Cape Agulhas ; therefore many depressions 

 pass, one after another, while the wind in Cape Town remains 

 N.W. 



If the chart shows a V to the east of the foregoing high, 

 and there be a low area over the western ocean, full winter con- 

 ditions exist. Then forecast stormy weather, with westerly 

 winds and rain all over the South-West and from Cape Town, 

 along the South Coast to Port Elizabeth. Should the wedge be 

 misplaced, and the trend of the isobars be all from S.W. to N.E., 

 predict rains all over, as far as Namaqualand, and possibly to 

 the east of the Cape Province. Such a condition will bring snow 

 on to the mountains. As in summer, when pressure increases 

 rapidly in the rear of a depression, the rain is more plentiful and 

 the winds stronger. 



Rain seldom falls within a very high pressure area, unless it 

 be following in the wake of a depression, or if a secondary loops 

 into it. This should be noted, and possible rain predicted accord- 

 ingly. High pressure areas bring cold nights, so that for 

 stations where frost is prevalent in winter, frosts should be 

 forecast until such time as pressure yields. 



Generally high pressure areas bring fine weather, with tem- 

 perature above the mean during the day, and below the mean 

 during the night. Dews and frosts at night are common, and 

 haze or fog during the day. 



If a high follows a V and covers the whole of the Interior, 

 squeezing the Monsoonal tongue into a narrow strip covering the 

 Orange Free State down to the north-east of the Cape Pro- 

 vince, being bounded by a low isobar, forecast snow on all high 

 lands to the north-east, Basutoland, etc. The probabilities will 

 be greater if pressure is low over the Transkei, or if an Indian 



