180 



PRESSURE. 



of the mean monthly pressnre from year to year are larger in the summer than in the 

 winter. 



An interesting point with regard to the variation of the monthly pressure is the great 

 change in mean pressure from October to November 1911. The monthly means at Cape 

 Evans were October 28-825 and November 29-630 inches. Thus the mean pressure rose -805 

 inch during two consecutive months. The rise was even greater at Framheim, -88 inch, and 

 only slightly less at Cape Adare, -80 inch. 



I called attention to this rapid change in pressure in a letter to ' Nature ' (April 13, 

 1913) and asked for information of other similar large changes. Mr. R. C. Mossman in reply 

 very kindly sent me the following list of large changes at Stykkisholm in Iceland. 



Table 106. 

 Large changes of jnesstire in Iceland. 



Thus similar large pressure changes occur in Iceland about once in ten years. The region 

 near Iceland has the largest pressure changes of any part of the globe previously known, 

 it now appears that changes of the same magnitude occur in the Ross Sea area, thus these 

 two regions may be linked as the northern and southern regions of unstable barometer. It 

 may be mentioned that the changes in these two areas are apx^arently unrelated, the correla- 

 tion between the monthly pressure in the two regions during the four years for which 

 observations are available in McMurdo Sound being +-02 with a probable error of -10. 



Pressure Waves. 



When the height of the barometer at any place is plotted against time, the resultant 

 curve has the form of waves. The curves for two neighbouring places are similar, but the 

 maxima and minima generally occur at one station before the other. At first sight therefore 

 it appears as though true pressure waves travel through the atmosphere affecting first one 

 and then the other station. We know however that in medium latitudes the pressure changes 

 are not handed from station to station by true air waves, but by a succession of more or 

 less circular areas of high and low pressure, which, while travelling, retain their characteristic 

 pressure distribution. 



The pressure curves for the Antarctic show well developed waves and later we shall have 

 to consider whether they are due to the passage of cyclones and anticyclones, but we shall 

 first make a study of their frequency, length and depth at several Antarctic stations without 

 considering their physical meaning. 



