STRUCTURE OF THE WIND. 125 



high wind, and we have on several occasions refeired to the efficient mixing of the air 

 brought about by high winds. This is without doubt the cause of the decrease of gustiness 

 with increase in wind velocity. Light winds only disturb the layer and do not remove it, 

 hence they are abnormally gusty, while during high winds the layer has disappeared and the 

 winds are leas gusty. 



Other features of the structure of the wind.— hi addition to gustiness the Cape Evans» 

 winds show a considerable amount of squallines.i. If one examines the trace for October 

 15 — 16, figure 41A, p. 120, a series of waves of wind intensity will be noticed ; between 12 hours 

 and 20 hours four distinct periods of maximum wind strength were recorded. For the rest 

 of the day recurrences of similar waves appear. Each one of the increases in wund strength 

 was accompanied by a swing of the wind vane slightly towards the 8., the change in direction 

 only being small, from about E. by S. to E.S.E. The change in wind direction indicates 

 that the waves were really of the nature of squalls. Squalls of this nature are recognisable 

 in nearly all the wind records but with considerable variations in intensity and period. In 

 the example just discussed the period of the squalls was about two hours, similar squalls 

 but with a period of considerably less than an hour will be clearly seen between 1 a.m. 

 and 3 a.m. on the morning of October 21st — figure 41B. 



Squalls occurred in winds from the north and the south. A typical example of squalls 

 with northerly mnds is shown in figure 41C. 



An interesting development of squalls is shown in the three charts reproduced on page 

 122. The first two of these are charts of consecutive days, so that the second is the con- 

 tinuation of the first. The chart for the first diy shows fully developed squalls of great 

 intensity, these died out and for the first eight hours on the second chart the wind was not 

 squally. Just, however, as the wind was dying away a squall was recorded after which there 

 was an almost complete calm broken by recrudescences of wind for short intervals. There 

 appears to be little doubt that the calm was due to a layer of stagnant air above which 

 the southerly wind contiaued to blow, and the recrudescences of wind were the result of 

 squalls which broke through the surface layer for a few minutes at a time. This explanation 

 is supported by the thermograph trace which shows a rapid fall of temperature when the 

 wind ceased at 18 hours on the 27th, and each of the recrudescences of wind was accom- 

 panied by a large rise of temperature, showing that the cold layer was temporarily removed 

 each time the wind reached the surface. 



The third chart on this diagram shows the same effect, the irruptions of wind being 

 obviously due to an upper moving layer of aii- extending down to the ground for short 

 periods. 



The wind traces for April 1, 2, and 3 shown on the opposite page are very remarkable. 

 It will be noticed that at 8 hours on the 1st a fairly high south-easterly wind was blowing, 

 this died down to a calm and at about H hours a northerly wind commenced to blow, 

 and blew until just after 22 hours, this was followed by a calm until at about 3 hours 

 on the 2nd the northerly wind again sprang up and lasted until nearly noon. After this 

 another calm until nearly 16 hours when a south-easterly wind commenced. The variations 

 in the south-easterly wind are interesting, three periods being shown with calms in between. 

 The changes in temperature with the wind are instructive. The temperature during the south- 

 easterly wind at 8 hours on the 1st was -f4°F., during the northerly wind between 14 and 

 20 hours +11°F., thus showing a rise of 7°F. With the next calm the temperature fell 

 again to its previous value and only rose slightly with the succeeding northerly wind. During 

 the calm from 12 to 15 hours on the 2nd the temperature was about+ 7°F. ; with the setting 

 in of the southerly wind at 15 hours there was a sudden drop of temperature to -2°F., 



