220 



PRESSURE, WINDS AND WEATHER. 



Western Mountains is pressed to the west by the force due to the earth's rotation and this 

 produces a pressure gradient away from the mountains. Both effects raise the pressure and 

 cause the isobars to adjust themselves to the direction of the air movement. 



Going further we see that an increase of the pressure difference between the Barrier 

 and the Ross Sea increases air flow over the west of the Barrier with a greater concentration 

 of the motion near to Cape Evans where the pressure rises. Thus a uniform lowering of the 

 pressure over the Ross Sea or a raising of the pressure over the Barrier increases the 

 southerly wind at Cape Evans and causes the pressure at Cape Evans to rise relatively 

 to the pressure of Framheim. 



We have now to investigate the result of impressing travelling pressure waves on this 

 general distribution. So far we have not considered the magnitude of the pressure differences, 

 the isobars shown on figures 6ia and 646 being only diagi'ammatic. 



The required values are found and the theory substantially supported by noticing that 

 in all essentials the pressure distribution shown on figure 646 which has been derived from 

 first principles is the same as that of the average pressure distribution found from the actual 

 observations and already given in figure 58, page 176. In the latter we have numerical values 

 for the isobars and therefore in the future discussion we \vill use it in place of the 

 approximate diagram of figure 646 



On figure 65a the isobars from the average pressure distribution have been entered to 

 fit our diagrammatic representation of the geography of the Ross Ssa area.* The whole area 



45° 



270* 



45 



(a) 



Fio. 65 (a) Actual pressure distribution. 



(6) 



(6) Imposed pressure waves. 



* Figures 65 to 73 were all prepared before the error described in the footnote to page 166 was detected. Hence 

 the pressures used in these diagrams are all -OS" too low. This, however, is quite immaterial to the present discussion 

 as the pressure distribution is unaffected. 



