STRUCTURE OF THE WIND. 127 



i.e., a fall of 9°F. The temperature rose again as the wind fell, only to go still lower in 

 the following peroid of wind. 



Tlius the northerly winds were warm and the southerly winds cold. There is little diffi- 

 culty in explaining these wind and temperature changes. On these days there was little pres- 

 sure difierence between the Ross Sea and the Barrier and the wind passed backwards and 

 forwards through McMurdo Sound under local differences of pressure. The cold southerly wind 

 and the warm northerly wind simply brought the temperatures from the Barrier and Ross 

 Sea respectively with them. 



The trace for the next day, figure 440 shows a continued wind from the south but 

 with varying intensities and two periods of calm. After the latter, at 6 hours on April 4, 

 the northerly wind again appeared accompanied by a rise of temperature. The continuation 

 of this wind is shown on figure 45A. This trace is most remarkable, the sudden break in 

 the northerly wind between 11 hours and 12 hours 30 minutes has all the appearance of the 

 instrument having gone out of order. But this was not so, the break in the curve was the 

 result of a real interruption in the northerly wind current and during the interval there were 

 a few gusts of wind from the S.S.E. When the northerly wind commenced again at 12 hours 

 30 minutes it blew exactly as if it had not been interrupted, and the wind trace on each 

 side of the break obviously forms a single continuous curve. Coincident with the break in the 

 wind trace there was a similar break in the temperature curve, during which the temperature 

 fell three degrees. The temperature curve before and after the break is continuous in the 

 same way as the wind trace. It is impossible to believe that a northerly wind of fifteen 

 miles an hour could be stopped and restarted in the sudden way shown on the curve, with 

 a slight wind from the south in the interval. There can be no doubt that the northerly 

 wind was continuous but that during the interval cold air frojn the south was able to force 

 its way under the northerly current and raise the latter above the recording instruments. 



An interesting example of a similar nature but with the break occurring in a southerly 

 wind is showTi in figure 45B. There can be little doubt that the part of the curve between 

 23 and 24 hours is a continuation of the main curve which was broken at 21 hours. The 

 wind at the commencement of the blow was from the E.S.E. and gradually worked to the 

 B. as the wind increased. During the break the wind vane swung first to the south then 

 back through E. to N. and then back to the south. It was then unsteady between S. and 

 B. until the main current again set in soon after 23 hours when it remained steady at E.S.E. 

 until the calm. The temperature curve showed a sudden rise of about 3°F. when the break 

 in the wind curve occurred and the temperature returned to its previous value when the 

 main air current was re-established. There can therefore be little doubt that in this case a 

 flow of air from the north lifted the air current from the south above the instruments, 

 thus it was an exact reversal of the previous case. 



Figure 45C is another example of the same phenomenon. The temperature record is 

 interesting in this case also. At 10 hours a sudden drop took place in the wind velocitv 

 with no appreciable change in wind direction, at the same time the temperature commenced 

 to rise until the calm occurred. Wlien the wind rose again after the calm the temperature 

 fell and reached its original value at the same time that the wind returned to its full 

 strength. 



During the interval shown as a calm on the wind record, the wind vane appears to 

 have made a single swing to the E.N.E. and back again ; otherwise the wind direction re- 

 mained practically constant throughout. 



The lifting of the air current again is the only possible explanation. 



