52 TRANSACTIONS OF THE CANADIAN INSTITUTE. [VoL. V. 



westerly and westerly winds over Canada from Manitoba to the Gulf of 

 St. Lawrence. Along the eastern slope of the Rocky Mountains they are 

 generally northerly, but as we cross to the Pacific Coast the barometric 

 gradient alters its direction in a way to produce a prevalence of easterly 

 to southerly winds near the British Columbia Coast, and as we go still 

 further west we find in crossing the Pacific south of latitude 50° a preva- 

 lence of southwesterly winds blowing from a subtropical belt of high 

 pressure towards the low to the northward. 



Now the normal low pressure area over the North Pacific is a region 

 frequently traversed by barometric depressions or storm centres, many 

 of which move to the coast of California or British Columbia and thence 

 force their way into the continent, temporarily displacing the high 

 pressure there prevalent. It is when one of these areas of depression is 

 forcing its way inland over Northern British Columbia that the winter 

 Chinook is felt in Alberta. Then is Alberta within the southwest 

 barometric gradient in connection with the storm centre and southerly 

 to westerly winds are blowing over British Columbia. These wnnds 

 blowing off the sea are moist, they ascend the mountain slopes and are 

 gradually deprived of their moisture, rain or snow falling on the moun- 

 tain sides. This condensation of moisture sets free latent heat which 

 retards the dynamic cooling due to diminished pressure, and afterwards 

 as the dry air descends the eastern slopes it is heated by compression 

 at the normal adiabatic rate, and the result is a mild dry wind over the 

 Alberta prairies. 



It will easily be understood from this that the character of the seasons 

 in the far West must depend greatly on the track that storm centres take 

 in passing into the continent from the ocean. In some seasons the 

 majority of them pass over Northern British Columbia, and then there is 

 a marked persistence of the mild southwesterly winds in Alberta ; in 

 other years the majority pass into the continent further south, perhaps 

 only a few moving north of the latitude of Calgary, then is Alberta 

 favoured with but a small quantity of balmy breezes. 



When a storm centre passes over Washington or Oregon States and 

 thence perhaps east or southeast, Alberta and the Territories, in lieu of 

 the mild southwester, experience bitter north and northeast winds and 

 perhaps a snowfall, and at times Vancouver Island itself is subjected to- 

 a northerly gale with snow. 



