The map of mean annual precipitation shows Canada, on the whole, to be a country of little rainfall. Most of the country has less than 20 inches per annum. Narrow wet belts appear along the western mountain ranges while a broader area with an adequate rainfall is found in the southeast from the Great Lakes to the Atlantic Ocean. The highest rainfall is found on the west coast of Vancouver Island where there are stations which have records of over 200 inches per year while 100 inches seems to be a general average for the coastal strip. The eastern margin of the island is a rain shadow where some stations have an average of less than 30 inches. The pattern is repeated on the coast of the mainland where most stations have over 60 inches per year and one has over 100 inches. The interior of B.C. has less than 15 inches and there are some stations with less than 10 inches per year on the average. The western slopes of Selkirks and Rocky Mountains are wet but not so rainy as the Coast Ranges. The Prairie areas have an average of less than 15 inches of rain per year, constituting a broad "rain shadow" of the Rocky Mountains, wherein a few stations receive even less than 10 inches. A belt in the central part of the Prairie Provinces receives a little over 15 inches per year while north of this lies about half of the area of Canada with less than 15 inches. There is a large area in the Arctic Archipelago and adjoining mainland which gets less than 10 inches. The record for least precipitation is held by Eureka (80° N), the most northerly weather station in Canada, which receives less than 2 inches per year.
In a country such as Canada, with great temperature differences between summer and winter, the seasonal distribution of rainfall is highly important. In the high rainfall areas of the west coast the summers are relatively dry while the winters are wet. Many places on Vancouver Island and the north coastal area of B.C. receive more rain in a single winter month than Prairie stations get in a whole year. On the other hand the rainfall of the Prairies comes in the summer, some stations getting half the year's total in three summer months. The low rainfall of the interior of B.C., on the other hand, tends to be rather uniformly distributed but the winter maximum is again in evidence on the western slopes of the Rocky Mountains. Southeastern Canada is a region of uniform rainfall, with little difference from month to month except on the Atlantic coast where there is a tendency for a winter maximum. In the low rainfall regions of the north the greatest precipitation occurs in late summer and autumn with very little in the winter.
Another result of seasonal temperature differences is that considerable precipitation occurs as snow. As might be expected, the areas of greatest snowfall lie in areas of great total precipitation but the agreement between the two maps is by no means complete. Vancouver Island, which has the greatest total rainfall, is too warm to receive much snow. The highest snowfall records in Western Canada are at Anyox on the border of Alaska (203 inches) and Glacier in the Rocky Mountains (390 inches per year). Much the largest area of land affected by heavy snowfall is located in Eastern Canada, in a great wedge extending from a base on the Newfoundland-Labrador coast to the district surrounding Georgian Bay. All of this region receives over 100 inches of snow per year, with some areas exceeding 120 inches. The record is held by Harrington Harbour in the eastern part of Quebec with 204 inches per year. The position of this wedge is, of course, very closely related to the usual tracks of winter cyclonic storms.