Canadian Weather Glossary: A-Z of Weather Terms and Definitions

A

Air Mass

A large body of air with approximately uniform temperature and humidity throughout. Canadian weather is largely determined by the clash of different air masses — Arctic air from the north, Pacific maritime air from the west, and tropical maritime air from the south.

Arctic Oscillation (AO)

A large-scale atmospheric circulation pattern that influences weather across the Northern Hemisphere. In its negative phase, it allows cold Arctic air to spill southward into Canada and the United States — creating the "polar vortex" events that periodically send extreme cold across the continent.

B

Blizzard

In Canada, a blizzard is defined by Environment Canada as a weather event with sustained winds or frequent gusts of at least 40 km/h, blowing or falling snow reducing visibility to 400 m or less, and these conditions persisting for at least 4 hours. Wind chill values in blizzards are typically extreme.

Bright Band

A feature visible on weather radar caused by the melting of snowflakes. As snowflakes fall and begin to melt, they temporarily appear larger and produce a stronger radar return, creating a bright horizontal band on radar displays at the altitude where freezing occurs.

F

Flash Flood

A rapid, sudden flood caused by very heavy rainfall in a short time period. Flash floods typically occur within 6 hours of the causative rainfall event. They are particularly dangerous because they provide little warning time and can produce extremely powerful currents.

Freezing Rain

Rain that falls through a layer of air below 0°C and freezes upon contact with cold surfaces, coating them in ice. Canada is one of the most freezing-rain-prone countries in the world, particularly Ontario and Quebec. Accumulations of just a few millimetres can make roads impassable and bring down power lines.

H

Humidex

A uniquely Canadian measure that combines air temperature and humidity to reflect the perceived temperature on human skin. A Humidex of 30-39 is uncomfortable; 40-45 is considered dangerous; above 45 is very dangerous. Environment Canada issues heat warnings when Humidex values are expected to reach 40 or higher.

P

Polar Vortex

A large area of cold, low-pressure air surrounding the Earth's poles. When the polar vortex weakens or becomes displaced, cold Arctic air can spill southward into Canada and the United States, bringing extreme cold events to populated areas far south of the Arctic Circle.

Precipitation

Any form of water (liquid or solid) that falls from clouds and reaches the ground. In Canada, precipitation takes many forms depending on the season and temperature profile: rain, drizzle, snow, sleet, freezing rain, and hail.

W

Wind Chill

A measure of how cold the air feels on exposed human skin, taking into account both air temperature and wind speed. Environment Canada uses the Wind Chill Index, which was developed in 2001 through a joint Canada-U.S. project. A wind chill of -25 to -44 is dangerous; below -45 is extremely dangerous with frostbite possible in minutes.

Winter Storm Warning

Issued by Environment Canada when two or more of the following conditions are expected: significant snowfall, blowing snow, freezing rain/drizzle, or extreme cold. The warning serves as a signal to avoid unnecessary travel and take winter storm safety precautions.