STRUCTURE OF THE AVIND. 



123 



actual changes in air temperature owing to the sluggishness of the instrument. Eigure 43 

 shows a very typical example (September 19 — ^22). 



hours /s 19th. 



8 20 th. '6 20 



4 21 ST. 12 '6 20 22 NO Sep. 1911. 



-CALM 



Fig. 43. Wind and tempeiatme. 



During the calm weather of the 19th the cold layer formed near the ground. During 

 the 20th the temperature slowly rose probably due to the presence of the southerly current 

 which was sliding over the cold layer, occasionally it dragged some of the stagnant air vnth. 

 it as shown by the fitful gusts of wind recorded by the anemometer and the rapid changes 

 of temperature. At 16 hours on the 20th the upper current descended to the ground removing 

 the cold layer and causing a rapid rise of temperature of over 20° F. After this the blizzard 

 blew for many hours, and the thermograph trace is very thick — compare the thermograph 

 trace during the 19th and 21st— showing that air masses of very different temperatures 

 were moving past the thermograph. The gustiness of the wind is clearly shomi on the reproduc- 

 tion of the anemograph record. During the 22nd the thermograph trace was particularly dis- 

 turbed and shows very large variations of the temperature. 



We have shown above that the gustineis was much greater during August, September, 

 and October than during the remaining nine months. This lends very great support to the 

 present explanation. On page 76 we discussed the unperiodic temperature changes as shown 

 by the average difference between the readings of the maximum and minimum thermometers 

 and it was shown that these changes were mainly due to the formation and removal of the 

 cold layers which we are now considering. A large average difference of the maximum and 

 minimum temperatures was shown to indicate the prevalence of the cold layer. The values 

 of this difference have been plotte^i on the middle curve of figure 40 for the same months 

 as the values of the gustiness are shown on the to pcurve. It will be seen that the curves 

 are very similar, thus indicating that both phenomena are due to the same cause. The 

 significance of the similarity between these two curves is increased by the fact that no 

 other meteorological factor has a marked maximum or minimum in September, 1911. The 

 curve of mean temperature is added to figure 40 for comparison. 



If our explanation is correct we should expect that conditions which remove the surlace 

 layer would reduce the gustiness. Now no factor is so efficient for removing this layer as a 



