GLACIERS OF THE CANADIAN ROCKIES AND SELKIRKS. 13 



breeze to a steady gale. Though its temperature rarely exceeds 5o°F, yet coming 

 as it does when one is accustomed to a low temperature, it seems warm by con- 

 trast with the preceding weather. The thermometer often rises from below 

 the zero point to 40° or 45° in a few hours, and the maximum temperatures of 

 the winter months in the Rocky Mountain region nearly always are coincident 

 with the occurrence of a chinook. 



"The sky is usually clear while the warm wind blows, though observers 

 often note a few leaden-colored clouds of a kind seen only during the chinook. 

 These clouds are described as pancake-shaped, with peculiarly smooth, rounded 

 edges, and stand apparently motionless, high above the mountain ranges. 



"The continuance of a chinook is as uncertain as its coming. It may 

 last a few hours or for several clays. With a change of wind the temperature 

 falls rapidly, and winter weather once more sets in. 



"The chinook wind possesses to a remarkable degree the power of melting 

 snow, for it is not only warm, but appears to be dry. Although a foot or more 

 of snow may lie on the ground at the beginning of a chinook, it disappears 

 within a very few hours, often seeming rather to evaporate than to melt. For 

 this reason the chinook is most welcome to the cattlemen on the plains of 

 Montana and Wyoming. In fact, without it, stock-raising would be almost 

 impossible, as the dried grasses of the plains, on which the cattle subsist, would 

 otherwise be buried the greater part of the winter. To a few, however, this 

 wind, instead of being hailed with delight as a break in the cold of the winter, 

 is a source of much discomfort. " ' 



The scientific explanation of this type of wind was first given, in part, by 

 the American meteorologist Espy,^ and later completed by Helmholz, Tyndall, 

 and Hann. Simply stated and adapted to the region under discussion, this 

 explanation may be of interest to many into whose hands this report may fall. 

 The presence of an area of low barometric pressure to the north gives rise to a 

 system of rotary air currents, moving counter-clockwise about the center and 

 constituting a great "whirl." The interposition of mountain barriers, such as 

 those already described, with a north to south trend, compels the winds, which are 

 westerly in the southern portion of the cyclonic area, to mount these obstrtictions, 

 from the crests and through the passes of which the air is again drawn to lower 

 levels through the suction of the great rotating mass. As the air rises upon the 

 windward slope of the mountain range, it experiences less pressure from the 

 suiTounding air, is peiTnitted to expand, and is, in consequence, cooled at the 

 rate of 1° F. for each 180 feet of ascent. This cooling reduces the ability of the 

 air to hold moisture and when the dew-point is exceeded precipitation must 

 result. These facts account for the relatively large precipitation upon the 

 western slopes of the Coast, Gold, and Selkirk ranges. When vapor is condensed 

 there is liberated the so-called latent heat, which disappeared when the water 

 was originally evaporated. It so results that while the air is being cooled by its 

 own expansion, the consequent condensation of its moisture is supplying it 

 with heat, and the actual cooling experienced after condensation begins may be 

 1° for every 300 to 500 feet of ascent. The amount of heat thus supplied to the 



' H. M. Ballou: "The Chinook Wind," American Meteorological Journal, IX, 1892-93, pp. 541-547. 

 2 Fourth Meteorological Report, Washington, 1857, p. 147. 



