WEATHER FORECASTING. 57 



As a broad and general rule areas of high pressure bring 

 fine weather and those of low pressure rain or unsettled 

 weather. No hard and fast line of demarcation between high 

 and low can be given ; the 30.00 ins. isobar has been looked upon 

 as a convenient zero for departure, and so it is, but the whole 

 matter of highs versus lows is one of comparison. Still, when 

 extremes are reached we are on surer ground; in South Africa 

 the critical sea-level pressures seem to be 29.80ms. and 30.20ms. 



Summer Months. 



Let us consider areas of low pressure. Leaving out of 

 consideration the Monsoonal tongue and the few Indian Ocean 

 lows, the whole of our depressions come from the west or 

 south-west; they are generally in the form of an inverted V, 

 with one, two, or even several apices ; when one of these is 

 approaching from the south-west, pressure over the south- 

 west decreases rapidly ; the distribution of pressure may be that 

 of a low interior and a pericyclonic ring of high pressure round 

 it , purely summer conditions, wind may be east at Cape 

 Agulhas, south-east at Cape Point, and south at the Observatory, 

 but all over the south-west pressure is yielding, and this can 

 only be caused by the presence of a depression to the south or 

 south-west. In any such case one would be safe in forecasting 

 the approach of a depression which should be .found on the 

 South Coast either the next morning, or, if the movement lags, 

 on the following one. 



The forecast would be, — little change over the south-west, 

 slight fall of barometer, wind west or south, sea mists or fogs. 

 South Coast ; pressure yielding, warmer, winds east, with clouds 

 next morning. A similar forecast would apply to the south 

 east and probably to Natal. 



When the V is seen to extend on to the south coast it will 

 be well to assume that it is the foremost apex of a series and 

 look for following ones ; should pressure be fairly steady over 

 the south-west another V will appear, but if it be increasing 

 rapidly a high pressure area will most likely appear. When a 

 succession of V apices pass the rainfall is spasmodic and very 

 local in distribution, accompanied by a few thunderstorms. 



In the case of steady pressure over the north-west the 

 forecast would be, — continuation of existing conditions over the 

 south-west; westerly winds with unsteady pressure and tempera- 

 ture round the south coast and eastwards with possibly loca 1 

 thunder and rain. Over Natal the wind would change from 

 north-east to south-west with increase of pressure, decrease 

 of temperature, and probably thunderstorms. 



Similar conditions would be repeated until a decided increase 

 of pressure developed over the south-west, when rain might 

 be looked for all along the coast and Natal. 



