14 GLACIERS OP THE CANADIAN ROCKIES AND SELKIRKS. 



ascending currents will obviously depend upon the amount of moisture con- 

 densed and the rate at which this condensation takes place. The air will reach 

 the mountain crest at a higher temperature than if no moisture had been 

 condensed; it is capable of retaining more of its moisture in consequence, to be 

 carried to the leeward of the mountain range. In being drawn down the lee- 

 ward slope of the mountain barrier the air is compressed, as it descends, owing 

 to the greater weight of superincumbent air, and is still further warmed at the 

 rate of i F. for every 180 feet of descent. As it is warmed its capacity for hold- 

 ing moisture is thereby increased and it becomes, relatively, more and more 

 dry, although it may actually possess considerable moisture. Such a warm, 

 thirsty wind is our chinook. 



The temperature of the air about the peaks and mountain passes differs 

 less in the winter and spring from that of the valleys and lower plateaus, so that 

 the chinook is a more conspicuous feature during these seasons. It is also 

 during these seasons that the cyclonic disturbances are the most pronounced. 

 In the summer and fall the temperature about the peaks and passes is generally 

 sufficiently below that of the lower levels so that, although the heating effect, 

 due to the condensation of vapor and the compression of the air in its descent, 

 may be actually as great, it becomes much less perceptible. During the most 

 favorable season for the chinooks we may, theoretically, account for a sudden 

 rise in temperature of 20 to 25 F., but occasionally it is much greater than this, 

 sometimes amounting to 50. Mr. E. B. Garriott mentions a rise of 43 in 15 

 minutes occurring at Ft. Assiniboine, Montana, Jan. 19, 1892.' In order to 

 account for such a phenomenon we must postulate a correspondingly high tem- 

 perature about the crests of the mountains, brought about by excessive and 

 rapid condensation upon the windward slope, or by some other agency. As 

 is well known the temperature about the crests of mountains is often considerably 

 higher than that in the adjoining valley, giving rise to what are known as "in- 

 versions of temperature. " Since the establishment of the meteorological station 

 upon Sulphur Mountain, October, 1903, with an elevation of 7,459 feet, there 

 have been some 300 such inversions noted up to the close of June, 1906. The 

 following list of some of the most pronounced cases, with dates, is taken from 

 data kindly supplied by Mr. N. B. Sanson, of the Banff station. The upper 

 station is 2,917 feet above the lower and i-J miles to the south-southwest. Of 

 the 206 instances sent, 163, or 79 per cent, of them, were noted in the morning; 

 26, or 13 per cent., in the evening, and 17, or 8 per cent., at noon. Of this number 

 101, or 49 per cent., occurred during the winter months, giving the most pro- 

 nounced cases of temperature inversion. The spring and fall were each repre- 

 sented by 27 cases, or 13 per cent., while the remaining 51, or 25 per cent., were 

 noted during the summer months. 



' Monthly Weather Review, 1892, p. 23. 



