234 PRESSURE, WINDS AND WEATHER. 



It will be seen that a complete wave has passed during this period and the maps have 

 gone through the complete sequence shown in the theoretical diagrams. 



A similar complete sequence is seen if the maps from April 13th, 8 p.m., to April 17th, 

 8 A.M., are compared with the theoretical diagrams. 



In fact for every well-marked wave on the lowest cirrve of the plates the maps will 

 be seen to go through the same sequence in a more or less complete manner. 



It will be felt that the paucity of observations over such a large area leaves a great 

 deal of freedom in drawing the isobars, and that if there had been more stations their 

 observations might not have fitted in with the simple scheme of pressure changes here dis- 

 cussed. From October to the end of the year there were parties on the Barrier which pro- 

 vided so many more stations, and one therefore turns with interest to the maps for this 

 period to see whether the sledging observations do or do not confirm the theory. Unfortun- 

 ately during this period the pressure waves are not well marked and for days together the 

 pressure was nearly constant over the whole area (see plates 19 to 23). Thus while the 

 pressure distribution will be found to conform to the main types there are few occasions 

 when the passage of waves makes it possible for us to follow the complete sequence of changes. 

 There are, however, one or two examples which are worth studying. 



During the period October 23rd to October 29th the lowest curve of plate 18 shows 

 that two complete waves passed over the area, and as at this time Captain Amundsen was 

 taking observations on the Barrier the period is suitable for our discussion. Both maps for 

 October 23rd show a distribution similar to that of figure 66 or 67. During the next day, 

 24:th, the pressure changes to the blizzard type and is therefore on the whole similar to 

 figures 70 and 71. At 8 a.m. on October 25th the distribution is the same as that shown on 

 figure 73 except that the gradient is stronger. The high pressure is just south of Cape Evans 

 and is passing northward so that with the next map 8 p.m., October 25th, we have 

 returned to the starting point with the high pressure over the north-west of the Ross Sea, 

 figure 66. 



A similar sequence is repeated during the next four days so that at the end of the 

 sequence, 8 a.m. on October 29th, the pressure distribution is the same as at the beginning, 

 8 P.M. on the 25th. 



These and other examples, which the reader will see on turning over the following maps, 

 on all of which there are observations from the Barrier, show that when there are large 

 changes of pressure the Barrier observations fit in as well as can be expected with the 

 theoretical distributions of pressure which we have deduced on the assumption of travelhng 

 pressure waves. 



There can be no doubt now that the passage of the pressure waves alters the pressure 

 distribution making it at one time favourable for blizzards and at another favourable for 

 northerly winds, but it must not bo forgotten that the first step in our theoretical discussion 

 was to show that in the absence of pressure waves the conditions are favourable for blizzards. 

 We will now examine one or two interesting cases of blizzards in the absence of well-marked 

 pressure waves. A good example is shown on plate 12. 



On and after August 18th there are no large pressure waves shown on this plate. On 

 August 19th, however, a blizzard commenced at Cape Evans. The map for 8 p.m. on August 

 17th shows a nearly complete calm at all stations, the north-westerly wind at Framheim 

 being only a light local breeze of 1'4 miles an hour. From the actual pressure at the three 

 stations it is obvious that the isobars must run from the east to the west as in figure 64o, 

 the Barrier being at a higher pressure than the Ross Sea. Throughout the 18th a light 

 easterly wind was blowing over the ea.st of the Barrier as shown by the wind at Fram- 

 heim, while the air was calm at Cape Evans. It will be noticed from the lowest curve 



