PRESSURE WAVES AND WINDS. 217 



There can therefore be little doubt that we have to do with waves of pressure travelling 

 outwards from the centre of the Antarctic Continent along directions parallel to a line joining 

 Cape Adare and Framheim. It is interesting to notice that this line produced passes very 

 near to the position 80° S., 120° W., about which the pressure surges were found to have 

 their maximum intensity (see page 195). 



Pressure Waves and Winds. b 



It becomes necessary now to examine the pressure waves to see if there is any intimate 

 relationship between them and the winds at Cape Evans, Framheim and on the Barrier. 

 The conditions at Cape Adare will be considered separately later. 



If one examines the wind arrows placed along the pressure curves for Cape Evans the 

 periods of blizzards, calms and northerly winds are clearly seen. If, however, one examines 

 the pressure curve for Cape Evans alone it is difficult to find any certain relationship between 

 the pressure changes at that station and the accompanying wind conditions. To fix our 

 attention we will examine the pres-sure and winds on plate 17. The fir.st blizzard on this 

 sheet occurred on October 10th with a nearly steady barometer at Cape Evans. The second 

 commenced on the evening of the 12th just when the barometer commenced to rise sharply ; 

 but the blizzard stopped, for no apparent reason, while the barometer was still rising rapidly. 

 The third occurred on the night of the 15th-16th while the barometer was rapidly falling. 

 It ended at midday on the 16th by the wind suddenly turning completely round and blowing 

 strongly from the north. When this occurred there was no apparent change in the pressure 

 curve such as is usually associated with a complete reversal of the wind. The two remaining 

 blizzards on this plate are associated with the crests of two small pressure waves, while the 

 trough between the two waves passed during calm weather with an occasional light wind 

 from the north. This is exactly the reverse of what happened when the large wave passed 

 on the 14:th, for then the crest of the wave was accompanied by a calm and the blizzards 

 occurred before and after the crest passed. The same will be found on all the plates, 

 blizzards start and stop at all phases of the actual pressure waves and for no obvious reason 

 so far as the pressure alone is concerned. Take as another example plate 7. Here we see 

 a large pressure wave between a trough on .Tune 16th and a trough on the 22nd. During 

 the passage of this wave there is first a period of calm, then a period of high northerly 

 winds which suddenly changed into a southerly blizzard. Then a period of calm with an 

 occasional wind from the north followed by a blizzard, then another high wind from the 

 north. All these changes took place while the crest of a single wave passed over the station. 

 On such evidence as this one is tempted to say that the winds are entirely independent 

 of the pressure waves and occur under the influence of some other motive force. The ca.se, 

 however, takes on a different light when instead of the actual pressure at Cape Evans one 

 examines the difference in pressure between Cape Evans and Framheim. The bottom curve 

 on each plate shows the pressure difierence Cape Evans — Framheim, plotted for convenience 

 to twice the scale used for the pressure curves. On this curve are repeated the winds at 

 Cape Evans. Looking now at the lower curve corresponding to the last example, June 16th 

 to 21st, we see that the wind changes which occurred so capriciously when compared with 

 the actual pressure at Cape Evans are very closely related to the changes in the pressure 

 difierence between Cape Evans and Framheim. The blizzards all occur when the pressure 

 difference curve is rising, and the calms and northerly winds when the curve is falling. The 

 wind changes direction almost exactly at the moment the lower curve starts to rise or fall. 

 Turning back to plate 17 we see that there was no blizzard when the large crest passed on 

 the 14th because this wave produced little or no effect on the pressure difference, on the 

 other hand each of the small waves on the 19th and 21st was accompanied b> large rises 



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