DIRECTION. 



107 



As to what this factor is at Cape Evans there can be little doiiljt. 'When the winds 

 are investiffated vnth reference to their direction it is seen that the winds from the north 

 do not show the abnormality, their frequency decreasing quite regularly from light winds to 

 high winds. Tlae southerly winds— the blizzards— are alone responsible for the large excess of 

 winds of an average velocity of about 30 miles an hour. We shall show in chapter VI 

 that blizzards are neither cyclonic nor anticyclonic winds, but owe their origin to changes of 

 pressure brought about by waves of pressure which travel from the south-east and afiect 

 the pressure over the whole Ross Sea area. Thus the presence of blizzards superposed on 

 an anticyclonic pressure distribution produces the peculiar form of the frequency curve at 

 Cape Evans. 



The similarity of the curves for Cape Evans and the Gau.ss Station indicates similar condi- 

 tions in the two localities, and we shall use this evidence later when discussing the conditions 

 at the latter station (see page 247). 



This discussion of the frequency curve has taught us the following : — 



(a) McMurdo Sound, Framheim, th.e Gauss Station, Cape Adare and Snow Hill are all 



under the influence of an anticyclonic pressure distribution. 

 {h) Kergulen and the north polar regions are under the influence of a cyclonic pressure 



distribution. 

 (c) The shape of the curve at McMurdo Sound and to a lesser extent at the Gauss 

 Station, shows the presence of some factor neither cyclonic nor anticylonic which 

 factor at McMurdo Sound is clearly the blizzard, it is therefore likely that bliz- 

 zards having a similar origin exist at the Gauss Station also. 



As the investigation of the frequency curves is worthy of considerably more attention 

 than it is possible to give them here, especially as to certain periods of high winds and 

 seasonal changes, the data for each month used in tabic 60 are given in full in the volume 

 of tables where the method used in treating wind velocities measured on the Beaufort Scale 

 is also described. 



Wind Direction. 



Hourly observations of wind velocity and direction at Cape Evans are available for the 

 period February 6, 1911, to August 31, 1912. 



The following table summarises all the data : — 



Table 61. 

 Wind Directions at Cape Evans. 



