360 ASSINNIBOINE AND SASKATCHEWAN EXPEDITION. 



still its humidity is greater than that of the plains south 

 of the Missouri, in consequence of its high northern 

 latitude. 



AU the thunder-storms we encountered in 1858 in the 

 valley of Lake Winnipeg, came from the west, south-west, 

 or north-west, with one exception. I do not find a single 

 record of thunder-storms with heavy rain coming from 

 the south. This may have been an exceptional year, but 

 the warmth and dryness, often oppressive, of the south 

 wind, west of the 100th degree of longitude, contrasted 

 strongly with the humidity and coolness of winds from 

 the west. This phenomenon is directly opposed to those 

 which prevail in lower latitudes, and may probably be 

 explained as follows : — 



Warm air from the Pacific, loaded with moisture, 

 passes at certain periods of the year over the whole range 

 of the Eocky Mountains in British America and in the 

 United States. These Pacific winds occasion but a very 

 small precipitation of rain or snow on the eastern flank 

 of the Eocky Mountains, south of the great Missouri 

 Bend. Similar winds from the Pacific do occasion a con- 

 siderable precipitation in the northern part of the Saskat- 

 chewan Valley. Whence, then, this apparent anomaly? 

 It probably arises from the difference in the temperature 

 of the two regions, the direction of the prevailing winds, 

 and the lowness .and comparatively small breadth of the 

 Eocky Mountain ranges in that latitude. In spring and 

 summer, warm westerly winds laden with moisture, in 

 passing over the mountain range south of, say the 46th 

 parallel, are cooled to a certain temperature, and pre- 

 cipitate the greater portion of their moisture in the form 

 of rain or snow upon the mountain ridges. On arriving 

 at the eastern flank of the Eocky Mountains, their tem- 

 perature rises to that of the region over which they pass, 



