The seasons dictate the look of the land: according to whether the natural environment is in a state of dormancy or growth, Canadians may be alpine skiing...or water skiing.

While seasonal change signals fluctuations in temperature and the number of hours of sunshine, the shifting position of air masses also plays a part. The usual air flow from west to east is often disrupted in winter when cold, dry air moves down from the Arctic, and in summer when warm, tropical air moves up from the southeast. Added to these factors are the effects of mountain ranges, plains and large bodies of water.

The West Coast
The coast of British Columbia has the most temperate climate in Canada, thanks to warm, moist Pacific Ocean airstreams. The province's most populous cities, Vancouver and Victoria, enjoy comfortable and relatively dry summers and mild, wet winters. Snow seldom falls in low-lying areas, and when it does, it usually melts the same day.

The Cordilleran mountain system, which includes the Coastal Range and the Rocky Mountains, alters the warm, moist Pacific air from the interior plains of the Prairie provinces. As the moist air is forced to rise over the mountains, it cools and falls on the western slopes in heavy amounts of precipitation, as rain at lower altitudes and snow at higher ones. The valleys between the mountain ranges receive much less precipitation and experience warm, even scorching, summers.

Spring rains and dry autumn conditions have helped make the Prairies one of the top grain-growing areas of the world. Farming is not without its risks, however, in the form of wind erosion, drought, floods, thunderstorms and hailstorms and unseasonably early autumn frosts.

Among the most remarkable features of the Alberta winter is the "chinook," a warm, usually dry winter wind that affects much of southern Alberta. The chinook sweeps down from the Rocky Mountains and has been known to raise temperatures as much as 10°C in one hour.

Atlantic Canada
The combined influence of continental air masses with air currents off the ocean give this region one of the most rugged and most variable climates anywhere in the country. In winter, mean temperatures can vary markedly as Arctic air is replaced by maritime air from passing storms. Snowfall in winter is relatively heavy, and fog is common in spring and early summer. The warmest month is July, when mean temperatures are in the 16 to 18°C range, except near coastal areas where August is often warmer.

The North
Spanning the entire country north of the Prairies and the populated Great Lakes-St. Lawrence Region is the boreal forest. This area is usually snow-covered more than half the year; its "summer" - the frost-free period - lasts barely two months. Precipitation is light, except along the coast of Labrador where the influence of Atlantic storms is felt.

Farther north, above the tree-line, lies the Arctic. Here, temperatures rise above freezing only a few weeks a year. Just a metre below the delicate but tenacious vegetation that grows in summer, the ground remains permanently frozen.

Data from Canadian Climate Normals/Normales Climatiques au Canada 1961-1999, published by the Atmospheric Environment Service, Environment Canada, 2002.