48 



TEMPERATURE. 



Typical Case of a Soutlierly Blizzard on September 16 and 17, 1911 (figure 16). — The 

 forenoon of September 15 was cold after a clear, calm night, and therefore we may expect 



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/!Oljrs 1 8 15lH. /tf iV 



■f 8 16lH. 16 20 



4 a 17th. September 1 



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Fia. 16. Wind ami Temperature. 



that a layer of cold air had formed near the ground with an appreciable temperature iuver- 

 .sion above. The temperature of thi.s layer is shown by the temperature curve to have been 

 about — 3.5°F. At 8 km. the sky commenced to cloud over, at first with cirrus clouds but 

 later with heavy low clouds. When such conditions precede a blizzard it indicates that the 

 upper air has commenced to move from the south, but is moving over the lower layers 

 without disturbing them. The condensation of water vapour indicated by the cloud has 

 warmed up still further the upper air, and as radiation to the clear sky has been cut off 

 by the clouds, the temperature of the cold air near the ground commenced to rise. This 

 rise is shown in the temperature curve to have taken place steadily until midnight of the 

 15th. By this time the lower layers had become disturbed and little puffs of wind from 

 the south commenced to be shown on the wind trace, and the temperature curve became 

 irregular as the cold layer was invaded by warmer air from above. Between 8 hours and 

 18 hours on the 16th the surface layer became more and more disturbed as shown both by 

 greater temperature irregularities and the increasing frequency of the fitful light wind gusts ; 

 it is obvious that the moving layer was now coming lower in the atmosphere. At 18 hours 

 the blizzard commenced, the wind suddenly appeared at the ground and swept away the 

 remainder of the surface layer, as is .shown by the sudden rise in wind velocity and the 

 almost instantaneous rise of temperature of l.j degrees. 



The wind continued to increase in velocity until after midnight, and it was accompanied 

 by an increase of temperature. The wind then decreased and disappeared completely at mid- 

 dav on the 17th. It will be noticed that as soon as the wind had reached its maximum 

 the temperature commenced to fall — this is a mo,st characteristic feature of southerly blizzards. 

 In practically all cases the temperature decreased towards the end of the blizzard. 



