218 PRESSURE, WINDS AND WEATHER. 



on the bottom curve and therefore blizzards occurred while they passed. This relationship 

 which is so clear on these two plates will be found to be the general rule throughout all 

 the plates. There are certainly exceptions, but compared with other meteorological rules this 

 is a very rigid one. We can therefore conclude that a blizzard will occur when the pressure 

 difference between Cape Evans and Framheim increases and high northerly winds, calms or 

 light southerly winds will occur while the pressure difference decreases. 



We have now found that it is difficult to see any close relationship between the winds 

 at Cape Evans and the actual pressure waves, as would be the case if the pressure waves 

 were due to the passage of high and low pressure systems which are maintained by the 

 winds circulating romid them — good examples of which are to be seen in the curves for 

 Melbourne at the top of each plate. On the other hand there is a close relationship between 

 the winds at Cape Evans and the changes in pressure difference between that station and 

 Framheim, 300 miles to the east. 



Theoretical Discussion of the Effects of Pressure Waves. 

 We will now investigate what effect the passage of pressure waves would have on the 

 pres.sure distribution and winds supposing that the waves are imposed on the Ross 

 Sea area from without ; this is reasonable in view of the fact that we have been able to 

 identify the same pressure waves on the surrounding plateau and at the sea-level stations. 

 These waves will be supposed to travel over the area adding their pressure to the existing 

 sea-level pressure and in consequence changing the pressure distribution and air motion. 



We have therefore first to determine the pressure distribution without the waves, and 

 then to impose a series of waves on this distribution and examine the result. 



For simplicity in the following discussion we will assume that the three stations Cape 

 Adare, Cape Evans and Framheim are situated at the corners of an isosceles right angle 

 triangle of which the line joining Cape Adare and Framheim is the hypotenuse. In figure 

 %4m the three stations are represented by the points A, E and F, the line A E B then 

 represents the line of the Western Mountains and the line E F the edge of the Barrier. 



We will first consider what would be the pressure distribution if all air motion could 

 be restrained. In the absence of a difference of temperature between the Ross Sea and the 

 Barrier, pressure would increase slowly from the north to the south owing to the general 

 increase of pressure with latitude which is a common feature of the Antarctic. This would 

 cause the pressure to be higher over the Barrier than over the Sea to the north of it. 



There is, however, a large temperature difference between the Barrier and the Sea and 

 the chief pressure difference would be due to it. 



Both effects then give high pressure over the Barrier and low over the Sea. If there 

 were no air motion the isobars would run something like those shown on figure 64a. That 

 is, they would run approximately east and west and parallel to the edge of the Barrier. 



We have now to examine how this simple pressure distribution will be affected when 

 motion of the air takes place. 



In the southern hemisphere, owing to the influence of the earth's rotation, the air motion 

 takes place along the isobars in such a direction that the high pressure is on the left of 

 the direction of flow and the low pressure on the right. Under the pressure distribution 

 shown on figure 64a the whole air over the Ross Sea area would tend to flow to the west 

 as an easterly wind. This, however, is not possible owing to the Western Mountains which 

 act as a wall running north and south at right angles to the easterly wind induced by the 

 pressure distribution. It is easy to see that the air from the eastern half of the Barrier 

 would flow over to the west and would then be forced, along with the air from the western 

 half, to travel to the northwards paraflel to the line of the Western Mountains. 



