l8 .PRESIDENTIAI. ADDRESS SKCTIOX A. 



permanent rains ; as soon as the depression approaches a certain 

 area, high feathery clouds, cirrus clouds, appear, soon to be 

 followed by lower stratified clouds. The sky gets dirtier, the 

 air muggy, as the centre, or rather the central trough, of the 

 depression gets nearer. Rain sets in, persistent rain covering 

 large tracts of land. The cloud and rain areas are roughly con- 

 centric with the pressure area, .\fter the passage of the central 

 trough of low pressure, the rains are more in the shape of 

 showers, and the clouds are broken up by patches of blue. When 

 thie depression has passed fine weather sets in again as a rule. 

 Thunderstorms, according to the season of the year, are asso- 

 ciated with the passage of these depressions, and are generally 

 located in a special quarter — the south-western quarter of Europe. 



Higli-pressure areas or anticyclones bring cloudless and dry 

 n'eather with light winds. They are of a more permanent 

 character than the preceding ones, hanging longer over the same 

 regions. The cyclones as a rule bring warm, the anticyclones 

 cool, weather. The seasons, however, have here to be taken into 

 account. An anticyclone in winter will bring periods of intense 

 cold, nights especially with very low temperatures owing to 

 intense nocturnal radiation under a clear sky. In summer, on 

 the contrary, owing to their permanency over the land and the 

 ]3rotracted absence of clouds, anticyclones may produce intense 

 heat, but in this case it is a dry heat, very different from the 

 oppressive heat of the summer cyclones. A cyclone gives mild 

 or oppressive heat according to the season. There is a distinct 

 difference in the temperature of the dift'erent parts of a cyclone. 

 The front is warmer than the rear. In winter, of course, the 

 north-western quarter in America and the eastern in Europe may 

 be intensely cold, as they draw into the cyclone the air from land 

 areas which may at the time be subject to very low temperatures 



I cannot here describe, the other pressure systems. There 

 is only one to which I will allude, as it has apparently a certain 

 importance in South Africa. It is the one represented by a V- 

 shaped system of isobars, with pressures decreasing inwards. 

 This system sweeps over the land broadways from west to east. 

 Tn South Africa its apex is directed northwards, and reaches to 

 the tropics. I have reason to believe that it occasionally reaches 

 right into Rhodesia. It is characterised by winds of a northerly 

 direction in front, and of a southerly direction in the rear. When 

 the middle line of the trough passes over a region there is a 

 >^U(lden calm, lasting for a comparatively short time. The wind, 

 which had been northerly before, springs up suddenly from a 

 >^outherly direction, bringing with it duststorms, and in summer 

 terrific thunderstorms, with heav\' rains. 



This may be sufficient to give a summary idea of the basis of 

 modern weather forecasting. 



If we now pass to the practice of weather forecasting, we 

 have to deal with a far less satisfactory branch of meteorology. 



