The broad fact is that there is in winter a mass of very cold air concentrated in the polar regions which is carried, in certain meteorological conditions, southward and south- eastward from the region of the Beaufort Sea and, moving rapidly, may reach normally temperate areas with so little appreciation in heat as to cause severe weather there. While the drift of such air masses causes extreme cold in the Mackenzie Valley and low temperatures over much of Canada and the United States, it may have little effect on most of the area west of the Rockies, where the weather is determined by quite a different set of meteorological conditions. Our Pacific Coast, during the winter months, is, for the most part, under the influence of air moving with the prevailing winds across the ocean from the region of Man- churia and Siberia. These air paths pass in a curved trajectory easterly over the Pacific, turning in mid-ocean northeastward to impinge on the coast of British Columbia and the Alaska Panhandle. The flatter their trajectory, i.e., the shorter and hence more northerly their path from northeast Asia, the colder the air they carry and, in extreme cases, its characteristics at upper levels are little different from those of the polar air passing from the Beaufort Sea. In the more southerly trajectory, temperature is moderated by the comparatively warm Pacific waters from which moisture, picked up during the ocean transit, is deposited on the British Columbia and Alaska coastal area. This airflow is not steady, but arrives in irregular pulses, tempered according to the trajectory of its various paths. Generally, only the drier middle and upper strata of these air masses pass into the interior, although, by moving through the mountain passes and by by-passing the peaks, some of the moist air penetrates the coastal ranges. Residual moisture from the high air currents is deposited mostly on the windward sides of intervening mountains in the interior, limiting precipitation on the plateau, maximum final deposi- tion being on the higher westward slopes of the Rockies. East of the Rockies, the flow of polar air is south- eastward. When low pressure areas develop over the western interior of the continent, the cold air superin- cumbent on the Mackenzie, Peace, and North Saskatchewan Valleys will flow eastward to these low pressure zones. This movement tends not only to draw the milder Pacific air across and through the Rockies, but may promote its occasional rapid descent into the prairies in the wake of the departing cold air. This is the phenomenon of the “chinook”, which is noted as far north as the Liard and in some of the valleys of southern Yukon. The frequent arrivals of maritime air in British Columbia, and the very frequent accessions of polar air in the Mac- kenzie Valley, with mountain ranges interfering with the free flow of the denser lower strata, make for somewhat higher average winter temperatures in northern British Columbia than in the Mackenzie Valley. Summer tem- [ 22 } peratures in the former are in a low range, however, due to the elevation of the interior plateau. The arctic air of the Bering Sea enters Alaska freely north of latitude 60° N., but there is a drop of winter temperature of 25° eastward through Alaska from Nome to Dawson, which could not be accounted for by the effect of elevation. It is caused by a tongue of cold air that presses into Alaska and Yukon from the Beaufort Sea. The base of this tongue, which generally lies along the ~ Yukon River Valley, is often the site of a warm front caused by the penetration of air from the North Pacific, which accounts for the variability of Yukon winter weather as compared with the steady cold of the Mackenzie Valley. In the summer months, the source-region for cool air is restricted to the Arctic Ocean; air flowing from this source crosses land which is strongly heated in the clear or relatively clear summer weather, and the very long summer days make this surface heating very effective, causing periodical high temperatures and promoting rapid growth in the Mackenzie Valley. Thus, there are wide climatic variations over the North Pacific Region. In the Mackenzie Basin, as far south as Great Slave Lake and over nearly all of Yukon, . there are long and severe winters. The season of growth, while short, is stimulated by moderate summer temperatures induced by the long hours of sunlight. The average frost- free period is 70 to 72 days for both Mackenzie Valley and Yukon Territory, but local topography may lower the general average at many places. The average frost-free period for the Peace River country north to Great Slave Lake is about 82 days, or about 10 to 12 days better than Mackenzie and Yukon. Weather data are not available for the extreme northerly sections of British Columbia. While the winter climate may be tempered by maritime air from the Pacific, summer temperatures are affected by the elevation of this plateau country. Average frost-free periods range from 43 days at Yellowhead Pass to 65 days at Prince George. The climate of the coast is mild, with a very high precipitation. The average frost-free period at Prince Rupert is 195 days; 163 days is the average period at Premier, 130 miles north. The coastal influence, as men- tioned, extends for some distance into the interior, where Terrace, on the lower Skeena River, with an average of 128 frost-free days, and Aiyansh, in the Nass Valley, both with a moderate precipitation, enjoy, perhaps, the best- balanced climate in the region. An anomaly is the climate of the Queen Charlotte Islands, where, in the Masset Inlet section on Graham Island, with a rainfall of about 54 inches, the average yearly temperature is 45°, or only 4° less than that of Tunbridge Wells in the south of England. This island, with the lower Skeena, Nass and tributary valleys, and sections of the Peace River country, presents the finest and most agreeable climatic conditions in the North Pacific Region.