Air which remains for a long time in one position or travels for a long time over the same kind of surface tends to take on qualities characteristic of its environment. Thus polar air masses tend to be cold, tropical air masses tend to be warm, those conditioned by a long sojourn over land are dry and those remaining for a long time over the sea take on a great deal of moisture. Air masses are affected by seasonal temperature regimes so that in winter a great deal of cold dry air tends to accumulate over northern Canada and at times may spread over a great deal of the continent. On the other hand, in summer a large part of the interior of the continent may become occupied by a mass of warm dry air. The climate of Canada is also greatly influenced by air masses which receive their characteristics properties in areas at a considerable distance. We will, therefore, briefly discus the major air masses which affect the continent of North America.
Polar continental air masses accumulate over northern Canada. In winter they become intensely cold. They are protected by the mountains of the Cordilleran region against the invasion of moist air from the Pacific, hence they are also dry. In summer the lower layers of air become somewhat warmed by contact with the earth but there is always plenty of cold air aloft with a tendency to settle earthward. When air from this region spreads over the country, either in winter or summer it brings lowered temperatures and typically cloudless weather.
Polar Pacific air masses originate over the north Pacific Ocean. It is warmer than polar continental air because of its contact with the water but it is still moderately cool. It is also moist. The west coast is under the influence of this air mass during most of the year.
Polar Atlantic air gets its characteristics from contact with the Atlantic, north of the Gulf Stream drift. It is cold and has less moisture content than other marine air masses. From time to time this air invades Newfoundland and the Maritime Provinces. In spring and summer it is a source of cold air accompanied by cloudy unsettled weather.
Tropical Pacific air masses originate in the warm part of the Pacific Ocean and sometimes invade the continent but seem to have little direct effect on the Canadian climate.
Tropical continental air becomes differentiated in Northern Mexico and the southwestern part of the United States. Being an elevated region, it is cool and dry in winter and its atmosphere is hardly to be differentiated from Polar continental air. In summer it is warm and exceedingly dry and may at times move north as far as the Prairie Provinces.
Tropical Gulf and Tropical Atlantic air masses are of extreme importance in central and eastern North America including the southern part of Canada. Summer is the season of greatest effect for these air masses. The interior of the continent develops a low pressure while the western Atlantic becomes a high pressure area. Warm moist air therefore, invades the eastern part of the continent, becoming even warmer until it reaches the region of the Great Lakes. This extra heat generates thunderstorms which are acccompanied by precipitation. In fact most of the summer precipitation is of this type.
In winter the Tropical Gulf air encounters the polar continental mass in the region between the Gulf of Mexico and the Great Lakes and, being warmer and lighter, tends to override it causing heavy precipitation which is usually rain in the Southern States and snowfall in Southern Canada. The source of most of the moisture of the Humid East of North America, then, is in the great expanse of warm water in the Caribbean Sea and Gulf of Mexico.