264 



GENEEAL AIR CIRCULATION. 



(c) Observations made in McMurdo Houiid showed that the height of the alto-cirrus and 

 alto-stratus clouds was a little lower than the top of Erebus, so that their mean 

 height may be taken as about 3,000 metres. This enables us to use the direction 

 of these clouds observed at the Gauss Station, and at the South Orkneys. The 

 motion of these clouds over McMurdo Sound was affected by the surrounding 

 land and therefore cannot be used for this purpose. 

 The following table contains all the available data : — 



Table 132. ! 



Air motion af approximaieh/ 3.000 metres. 



These directions of air motion have been shown on figure 82 by means of arrows flying 

 with the wind. It ^vill be seen at once that all the arrows are approximately parallel to the 

 isobars. At the three last stations, however, the direction of motion is inclined somewhat 

 towards the high instead of towards the low pressure. It must, however, be remembered that 

 each of these directions has been found from cloud observations and the presence of cloud 

 itself indicates that the weather is disturbed and if clouds are generally associated with one 

 type of weather their motion cannot be accepted as shomng the mean direction. If allowance 

 .is made for these considerations, it must be admitted that the air motion is in excellent 

 a<Teement with the pressure distribution deduced for this height from consideration of pressure 

 alone. 



If we compare the pressure distribution at 3,000 metres shown on figure 82 with that 

 deduced by Meinardus for 4,000 metres and shown on figure 77, it will be seen at once that 

 they are radically different. While we show a cyclone and an anticyclone over the region, 

 Meinardus shows only a cyclone and there is no indication of an anticyclone on his diagram. 

 One naturally enquires bow is it that the wind observations, on which Meinardus admittedly 

 based his pressure distribution, can agree with such totally different pressure distributions. 

 The answer is that some of the upper air motions shown on Meinardus's diagi'am are not 

 those which should have been used. Over the Gauss Station Meinardus shows a north-north- 

 easterly wind at 4,000 metres. This is because he has used the mean direction of the cirrus 

 clouds. The observations made in McMurdo Sound show conclusively that the cirrus clouds 

 are much higher than 4,000 metres and move in an entirely digerent direction from the wind 

 on the plateau or from the direction of Erebus smoke. We have used the motion of the 



* Observations of medium cloud are not available, but there is every indication that at this station the high 

 and medium cloud motions are nearly parallel. 



