TEMPERATURE AND WIND. 



49 



In this case we have a cold surface layei of — 35°F., being replaced by a warm current 

 of air of +5°F., a change of 40''F. Thus, if the conditions previous to the blizzard had 

 been something like those shown in figm-e 15, the observed change would have necessitated 

 the thorough mixing of the lower 5,000 metres of the atmosphere. The cirrus clouds with 

 which the disturbance commenced were well above this height, so that the explanation given is 

 well able to account for the observed effect. 



Southerly Blizzard preceded by a Northerly Wind — May 15, 16, and 17 {figure 17). — 

 After midnight on the 14th the sky was clear and the temperature was unusually low, showing 



^i/rs ■c s ISth. /e ?o 



•C S 16™. 16 ^o 



-i(J 



-2tf 



4 8 



-CALM- 



7m. May 191! 



Fig. 17. Wind and Temperature. 



the formation of a cold layer in spite of the thin ice. .Tust before 8 hours on the 15th, 

 when the temperature was — 25°F., a northerly wind sprang up which removed this layer 

 and caused the temperature to rise by 10 degrees. An interesting feature of this northerly 

 wind was that above it there were alto-cumxilus clouds moving from the south. The northerly 

 wind continued until about midday on the 16th, when, after a calm hour, a southerh' blizzard 

 sprang up accompanied by much cloud and drift. During this blizzard the temperature rose 

 slowly until near the end, when it started as usual to decrease before the blizzard came to an 

 end. Three things are interesting in this case^(l) the sudden rise of temperature M'hen 

 the northerly wind swept away the cold surface layer ; (2) there was no sudden rise of tem- 

 perature when the southerly wind commenced, for the layer had already been cleared away ; 

 (3) the fact that the temperature rose much higher with the southerly than the northerly 

 wind. The explanation of the latter fact is clear and supports the explanation already given 

 of the cause of temperature changes under the influence of the wind. It has been stated 

 that over the northerly wind there was a wind from the south ; thus the air must have 

 been moving in sharply marked layers and therefore there could have been little or no vertical 

 mixing, hence as soon as the northerly wind had removed the cold surface layer it could 

 produce no further increase in temperature. On the other hand, the southerly wind must have 

 extended much higher and the production of snow shows that there was considerable ascensional 

 motion, thus a normal temperature gradient must have been established to a considerable height, 

 raising the ground temperature to nearly 0°F. This explanation is further supported by the fact 

 that after the blizzard stopped the wind returned to the noi-th and the temperature of the 

 northerly wind was then not what it had been previously, but what it was during the 

 southerly blow. 



