The major climatic regions shown in the accompanying map have been drawn up with an eye to the realities of the pattern of natural vegetation and the needs of agriculture. From both points of view the length of the growing period and the availability of soil moisture are the most important criteria.The experiments of botanists with ordinary mid-latitude crops such as cereals have indicated that a temperature of 42° or 43° F. is necessary for growth. The length of the summer growing period is conveniently measured in months, and, with less than six months of growing temperatures the variety of crops which may be grown is strictly limited. This limit also seems to fit the boundary between the hardwood and coniferous forests. With a growing season of less than five months, most agriculture is very difficult if not impossible. The northern limit of forests corresponds very closely to the isopleth of 50° F. for July mean temperature and it is adopted as the southern limit of the Arctic climates. Along a line extending from Lake of the Woods to Churchill a balance is struck between water surplus and water deficiency thus dividing the humid east coast climates from the subhumid interior types. This line continues north to divide the moist arctic from the dry arctic climatic regions. The critical limit in the Thornthwaite system which separates the subhumid from the semiarid also conveniently outlines the driest parts of western Canada.The use of the foregoing criteria enables us to outline ten major climatic regions which, of course, may be much further subdivided in the regional studies.
1. Dry Arctic.
2. Moist Arctic.
3. Boreal Interior.
4. Boreal East Coast.
5. Cool Temperate Interior.
6. Semi Arid.
7. Cool Temperate East Coast with short summers.
8. Cool Temperate East Coast with long summers.
9. Cool Temperate West Coast.
10. Mountain Climates.
Arctic climates are those in which no month has a mean temperature above 50° F. Only a narrow southern border of this region has more than one month above 43° F. and much of the area none at all. There are significant moisture differences between the eastern and western regions, the western Arctic in general having less rainfall and an overall moisture deficiency. The eastern Arctic, parts of which have 15-20 inches of precipitation, has a moisture surplus. In this area too, the higher parts of Baffin Island, Devon Island and Ellesmere Island have permanent ice caps.
Permafrost or permanently frozen ground is encountered everywhere.
Boreal, Subarctic or Cold climates extend from Alaska to Labrador. This region has less than five months with growing temperatures of 43° F. or higher. The southern boundary also corresponds very closely to the July isotherm of 60° F. mean temperature. This line marks the northern limits of commercial forests and is also the limit of successful agriculture. West of Hudson Bay the subhumid interior area gets from 10 inches to 15 inches of precipitation which is not enough to balance evaporation. The humid east coast Boreal climate, on the other hand, receives from 20 inches to 40 inches of precipitation and always has a water surplus.
The Cool Temperate climatic regions stretch across southern Canada from Atlantic to Pacific, interrupted only by the high mountains of the Cordilleran region. For the most Dart these regions have from five to seven months of growing temperatures with the July means ranging from 60° F. to 70° F. It is important to note, however, that although not shown. on the map, small areas in southern B.C., southern Alberta and southern Ontario have midsummer means above 70° F. On the other hand much of the west coast has midsummer mean temperatures between 55° F. and 60° F. The short summer phase and the long summer phase are separated by the line representing six months of growing temperatures. This division occurs only in the east since all stations on the west coast have more than six months growing season.
On the basis of moisture relations four divisions appear: the wet west coast, the subhumid interior, the semi-arid areas of southern B.C. and the southern Prairies, and the humid east coast. A narrow fringe of the Atlantic seaboard might also be classed as wet since it has over 25 inches of surplus water yearly.The tenth division comprises the high mountain areas which are both cool and wet or snowy because of their great elevation.