Canadian Geographic
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Extremes of weather

Where our weather begins

Five air masses affect Canada’s weather: continental arctic, maritime arctic, maritime polar, maritime tropical, and continental tropical. Winds carry these great bodies of air across the country. Each air mass, extending hundreds or thousands of kilometres, has uniform temperature and moisture conditions, acquired from the underlying landmass or ocean where they developed. The very cold, dry continental arctic air mass, the source of Canada’s bitter winters, originates over snow-covered barrens. In summer, its cool winds sweep south, bringing a welcome respite from heat waves. The maritime arctic air mass, traveling over large open bodies of water, is mild and moist. The maritime polar air masses of the Pacific and Atlantic soak coastal areas with rain, fog, and snow. The Atlantic maritime tropical air mass from the Gulf of Mexico scorches Eastern Canada with summer heat and humidity. By contrast, the Pacific maritime tropical has a cooling influence. The continental tropical air mass rarely reaches Canada because its hot, dry impact disappears as it moves north. The maps show winter and summer air mass movements. All weather changes are related to the interaction of these air masses along what are called fronts (When air masses collide). The polar jet stream forms perhaps the biggest front, an ever-changing boundary where the cool winds from the north meet the warm winds from the south. In spring, the clashing air masses produce severe thunderstorms and tornadoes.


Air masses This animation alternates between maps showing dominant air masses and flows in Winter and Summers.


On the next page:

When air masses collide

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Quiz :

Where in Canada did Hurricane Juan make landfall on September 28, 2003?

British Columbia
Nova Scotia