Mountain ranges along the western coast of Canada force the moisture-laden air up to accentuate precipitation. In the mountains of southwest Canada, this moist flow sustains the southernmost ice fields in North America. Interior Canada from the Arctic tundra to the plains of Alberta and Saskatchewan and east to northern Quebec receives maximum precipitation in the summer. Interior Canada has a strongly continental climate with huge temperature swings between frigid winters and mild, or even warm, summers. The maximum possible amount of water vapor in the air depends strongly on the temperature.
Much more vapor can exist in air at higher temperatures than at lower temperatures.
Summers are wetter simply because the amount of vapor available for precipitation is much higher in the warm season. In the southern provinces east of the Continental Divide, moist air often invades from the south during the summer, setting the stage for thundershowers. Winters tend to be dry in the Canadian interior because the western mountains deplete the supply of moisture that would otherwise invade central Canada. Southeast Canada, near the Great Lakes, along the St. Lawrence Seaway, and bordering the Atlantic Ocean, has a moist climate year-round. In London and Toronto, Ontario, precipitation varies little from month to month.