Meteorology* Tue Crmate or NortHwestern CANADA Over much of the northwestern region no instrumental meteorological records have been made. Missionaries and Hudson’s Bay Company’s employees were supplied with instruments from time to time, and from a few points such as Dawson City, telegraphic daily reports were transmitted during the last half-century or more. Not enough data exist to give great local detail. Yet, from the daily synoptic weather maps of North America, supplemented by obser- vations taken on ships, islands, and on the eastern coast of Asia, an understanding is obtained of what are the major controls of the climate of this region. From these con- siderations it is possible to distinguish unusual years or seasons, arising out of a change from the average atmospheric circulation in those latitudes and to recognize those years that follow more closely the usual pattern. Comparing the observational data at the widely scattered points in this region with the general type of year or season, it is possible to decide approximately the average seasonal values of temperature and precipitation and also to see what has happened when the circulation departs seriously from its normal paths or intensity. There are other matters which require actual observa- tions. Such, for instance, are the differing incidence of late and early frosts between one valley and another, or the differing amounts of precipitation deposited on windward or leeward slopes, varying local heights of the permanent snow-line, etc. Only the populating of this region which will provide observations over a long period of years can answer such questions. In the following paragraphs is described the climate in general terms and with figures from a few typical points: CLIMATIC CONTROLS = The climate of this region is, in the main, determined (a) by the drift of masses of air over the terrain, and (b) by the modifications which these airmasses undergo while passing over the region. Aspect (b) may be further sub- divided by considering (1) the weather which results from the juxtaposition of two airmasses of different character- istics, and (2) the changes in the characteristics of a single airmass induced by the terrain itself. Obviously for a fast travelling airmass there will be insufficient time for any considerable change but for a quasi-stationary airmass modifications may after several hours become appreciable. Canada’s Pacific Coast during the winter months is for the most part under the influence of air moving with the prevailing westerlies of middle latitudes. The trajectory of the air near sea-level is directed from the northwest upon leaving the Pacific Coast of Siberia and Manchuria but turns about longitude 180 and is directed northward *Contributed by the Meteorological Service, Department of Transport. again. Upon approaching the British Columbia Coast, airmasses have a resultant motion from the southwest(2). The inflow of such air is not steady but arrives by irregular pulses. The more northerly the region of egress upon the Pacific and the shorter the trajectory before impingement upon the British Columbia Coast and the Alaska Pan- handle, the more pronounced are its polar characteristics. In the extreme cases its upper levels do not differ from polar air entering the Mackenzie Valley from the Beaufort Sea, although at the very lowest levels there are moderate temperatures as well as considerable moisture picked up during ocean transit. From 300 to 350 milligrams of water-vapour per kilogram of dry air may be carried in the first kilometre (20 to 25 grains per pound in the first three- fifths of a mile). The temperature at the surface of the sea may not be far from the freezing-point, while at an elevation of 3 km. the temperature may be 20 degrees F. at 5 km. 5 degrees below zero, and at 10 km. 65 degrees below zero, in the case of a very cold airmass. An example of polar air in winter entering the Mac- kenzie Valley may be taken from the radiosonde flights at Norman Wells in February, 1945. At the surface we have a temperature of 16 degrees below zero with 30 milligrams of water-vapour per kilogram of dry air (2 grains per pound). At 3 km. the temperature was 22 degrees below, at 5 km. 40 degrees below, at 10 km. 67 degrees below. Up to 5 km. the relative humidity was less than 45 per cent. It appears that the coastal regions have their cold Waves tempered by the travel of polar airmasses over a sea-surface whose temperature averages about the freezing- point near the Kurile Islands and increases slowly to a Sea-temperature of 43 degrees F. in latitude 55 in February. On the other hand, the Mackenzie Valley must take any } invasion by polar air from the Beaufort Sea or the polar archipelago without amelioration. Further, the continued loss of heat by the ground in the interior because of | long-wave radiation through extremely dry air will still | further reduce the temperature of the air immediately | above the ground during the hours of darkness. When the air was still or nearly s0, temperatures 5 feet above the | ground have descended to 40 or 50 degrees below zero. , The lowest officially recorded at Good Hope (near the Arctic Circle) is '784 degrees below zero. So far, consideration has been given only to “cold waves” generated by the egress of air which has tarried over the northern Asiatic region or the polar seas long enough to have acquired definitely polar characteristics before invasion of lower latitudes. It is seen that progress across the highest latitudes of the north Pacific Ocean greatly tempers the cold of the lowest levels and adds appreciable moisture. Consider, now, the case of such air which after outburst () Atlas, Climatic Charts of the Oceans, Washington, 1938. An analysis of ships’ observations covering more than 50 years. [145 ]