Precipitation

Canada's land area is not uniformly watered. West Coast residents, for example, must open their umbrellas much more often than other Canadians. Indeed, this 'wet' coast receives 2,500 to 3,500 millimetres of precipitation per year, compared with 1,000 to 1,250 millimetres on the East Coast and 250 to 500 millimetres on the Prairies.

	Autumn forest ©2003 ICN-RCI / Hemera

These differences are due to the eastward flow of air and the presence of the cordillera. After loading up with moisture on contact with the Pacific, air masses bump up against the cordillera. Forced to rise, they cool, causing condensation that takes the form of rain. Since all the water falls on the western slopes of the mountains, the air that blows onto the interior plateaus of British Columbia and the Prairies is dry. This is what meteorologists call a 'rainshadow.' Because of the low moisture content and sometimes suffocating summer temperatures, these areas are subject to drought.

On the East Coast, the maritime influence is more subdued, precisely because of the west-to-east movement of air masses. Atlantic coast residents nevertheless receive more abundant precipitation than inhabitants of the St. Lawrence and Great Lakes lowlands.

Finally, the extreme north receives so little precipitation—no more than 120 to 150 millimetres per year, falling mainly in the form of snow—that it could be likened to a vast cold desert. However, this lack of water has little adverse effect on the vegetation, since the cool summer temperatures diminish the plants' need for moisture.