' Islands. When atmospheric gradients move the s _ from the Mackenzie-Peace region to the Type (1) is the wellknown severe cold wave of the Prairies. It brings extremely low temperatures to the Mackenzie Valley on every occasion, very generally affects the Yukon Territory, is less severe in northeastern British Columbia, and only rarely, about once in ten to fifteen years, passes in full severity through British Columbia to the Fraser Valley, Vancouver Island, and the Queen Charlotte Type (2) is the common cold wave of th he Pacific Coast and is felt more severely in British Columbia as far south as Prince George, Quesnel, and Barkerville than in the southern interior. The arrival of its slightly moist, colder upper levels in the Mackenzie and Peace-Athabaska region produces cloud that checks radiation and thus r aises the daily minimum temper. ature when such air passes aloft. urface air away eastward simul taneously with the arrival of the Pacific air, the effect of ‘Pacifie air descending to the surface in the wake of the departing continental air is to produce a very mild spell in the Northwest Territories. Type (3) produces mild weather with snow in northern British Columbia, but very rarely passes eastward except as occluded air at levels far above the ground. It frequently brings cloud and precipitation to the Peace-Athabaska region but not to higher western latitudes. Either types (1) or (2) dominate the weather immediately after the occluded air passes the Rockies. both rain and Type (4) is not commonly encountered west of the Rockies because of the barrier-effect. However, in the case of an extensive polar mass hugging the Rockies as it moves south toward Montana, subsidence on the western side of the anticyclone may bring moderately cold air into eastern British Columbia. This generally produces a quasi-stationary front that at first lies along the Selkirks with overcast skies from coastal air ascending the western face of the subsiding continental air. There is usually another front quickly formed which runs south of the Peace River northwestward to the Alaska Panhandle. This creates cloudy skies and light precipitation over the moderately cold lower air of northern British Columbia and sometimes the southern Yukon. During this period the Mackenzie Valley is very cold. The frequent arrivals of maritime air in British Columbia and very frequent arrivals of new polar air in the Mackenzie Valley with mountain ranges interfering with the free flow of the denser lower levels, naturally make for higher average temperatures in winter in northern British Columbia than in the Mackenzie Valley. There is a strong tem- ) Perature-gradient from the coast to the Stikine and Mac- _kenzie Mountains. East of these ranges the gradient | flattens. If the observed temperatures are reduced to a sea-level basis by assuming an average rise of 3-5 degrees F. for each 1,000 feet of descent, there still remains a fall of 36 degrees F. in passing in January from Prince Rupert to [148 J the headwaters of the Liard River. This gives a rough measure of the predominance of the bitterly cold lower levels of the polar continental air in the Mackenzie Valley and on the eastern slopes of the Stikine and Mackenzie Mountains, with the added effect of elevation, the tem- perature-drop will average over 60 degrees F. between the coast andthe high valleys of the upper Liard tributaries. The extremely high St. Elias Range is also a very effective barrier to Pacific air, producing a drop of 50 to 55 degrees F. in January between Yakutat Bay and the Dawson region. The Arctic air of the Bering Sea enters freely into Alaska north of latitude 60 degrees N., but there is a drop of temperature eastward through Alaska of 25 degrees F. from Nome to Dawson. It is evident, therefore, that the climb of air from Nome to Dawson of little more than 1,000 feet is not the cause of the very low temperatures in the Yukon. After all temperatures in Alaska and the Yukon have been reduced to sea-level, it is found that there is a tongue of very low temperatures which presses into Alaska and the Yukon from the Beaufort Sea between longitudes 130 and 150. The base of this tongue (which roughly lies along the Yukon River) is often the site of a quasi-stationary warm front, the air from the north Pacific or the Aleutian region passing aloft to the northeast over the stationary wedge of polar air lying north of the Nome-Dawson line and extending over the Beaufort Sea. Such a condition may remain practically unchanged for several days. It was the opinion that Pacific air might more easily climb the valley of the Alsek River which descends from the Alaska-Yukon Highway near Campagne to Dry Bay at the foot of Mount Fairweather. Instruments placed at Pine Creek, in the Dezadeash country about 100 miles west of Whitehorse, in the autumn of 1944, do not appear to support this possibility. The temperatures at Pine Creek averaged three or four degrees lower than at Whitehorse during January; February, and March, and five to seven degrees lower than at Carcross. The elevations at Pine Creek and Whitehorse are approximately the same. It appears that this break in the St. Elias Range favours the descent of dense polar air to the sea, but does not generally help Pacific air to appear at the surface in the Dezadeash country. It does apparently favour low cloud, although the one winter's observations are too few for any safe deductions. For the winter months, therefore, it may be concluded that from Lesser Slave Lake northwestward to Atlin and Teslin a line is followed with temperatures averaging as low as at The Pas, Cumberland House, and Norway House in Manitoba. Southwest of this line in the Peace River country of British Columbia to the Finlay River ten- peratures improve to resemble those of the wheat belt between Daysland and Lloydminster in Alberta. The severe winters persist into the region of Stuart Lake and Babine Lake in British Columbia ina southwesterly direction. From Prince George, southward and westward, a country is entered where the valleys have moderately cold winters.