Weather Glossary

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Adiabatic warming and cooling

Temperature change caused only by air rising (expanding and cooling) or sinking (compressing and warming), with no heat added or removed. It’s the engine behind clouds and chinooks.

Air mass

A huge body of air, often thousands of kilometres across, with fairly even temperature and moisture, such as cold, dry Arctic air or mild, moist Pacific air.

Alberta clipper

A fast-moving low-pressure system that forms on the lee side of the Rockies in Alberta and races east, bringing wind, a burst of snow and a temperature drop.

Altocumulus

Mid-level cloud in patches or rolls, often in rows. Can hint at instability or an approaching system.

Anvil

The flat, spreading top of a mature thunderstorm, where rising air hits the stable layer near the top of the troposphere.

Arctic front

The leading edge of very cold Arctic air. When it passes, the temperature can drop sharply and the wind swings to the north.

Barometer

An instrument that measures air pressure.

Blizzard

Strong wind with snow, falling or blowing, that cuts visibility for several hours.

Chinook

A warm, dry, often gusty west wind that comes down the eastern slopes of the Rockies and warms rapidly as it sinks.

Chinook arch

A long band of cloud over the mountains with clear sky beneath it, a classic sign of a chinook.

Cirrus

High, thin, wispy cloud made of ice crystals. Thickening cirrus can be the first sign of an approaching system.

Cold front

The leading edge of colder air pushing in. Often brings a wind shift, a temperature drop and sometimes a band of showers or snow.

Condensation

Water vapour turning into liquid water droplets, as in clouds, fog and dew.

Coriolis effect

The apparent turning of moving air caused by the Earth’s rotation. In the Northern Hemisphere it turns winds to the right.

Cumulonimbus

The towering thunderstorm cloud. It can bring lightning, heavy rain, hail, strong winds and tornadoes.

Cumulus

Puffy, flat-bottomed cloud formed by rising pockets of warm air. Fair-weather cumulus are small; growing ones can become thunderstorms.

Dew point

The temperature the air would have to cool to for moisture to condense. The closer it is to the temperature, the more humid it feels and the likelier fog or dew becomes.

Doppler radar

Radar that measures whether precipitation is moving toward or away from it, used to spot rotation and wind shifts.

Downslope wind

Wind blowing down a mountain slope. Sinking air warms and dries, which is how a chinook works.

Ensemble forecast

Running a computer model many times with slightly different starting conditions to see how sure the forecast is.

Freezing rain

Rain that falls as liquid but freezes on contact with cold surfaces, coating everything in ice.

Front

The boundary between two different air masses.

Frost

Ice crystals that form directly on cold surfaces when they drop below the frost point.

Graupel

Soft, small pellets of snow coated in frozen droplets. They look like tiny foam balls.

Ground blizzard

Strong wind lifting snow already on the ground, cutting visibility even when no snow is falling.

Gust

A brief, sudden increase in wind speed, usually lasting seconds.

Gust front

The leading edge of cool air rushing out from a thunderstorm, often arriving before the rain.

Hail

Balls or lumps of ice that grow inside strong thunderstorms as they’re carried up and down through freezing air.

hPa (hectopascal)

The unit Canada uses for air pressure. Normal sea-level pressure is about 1013 hPa. Same as a millibar (mb).

Humidex

A Canadian index that combines temperature and humidity to describe how hot it feels.

Humidity (relative)

How much water vapour is in the air compared with the most it could hold at that temperature, shown as a percentage.

Inversion

A layer where temperature increases with height instead of decreasing. It traps cold air, fog and smoke near the ground.

Isobar

A line on a weather map joining places with equal air pressure. Close isobars mean strong wind.

Jet stream

A fast river of wind, usually 9 to 12 km up, that steers weather systems.

Lapse rate

How quickly temperature drops as you go up.

Lee side

The downwind side of a mountain range. Southern Alberta is on the lee side of the Rockies.

Lenticular cloud

A smooth, lens-shaped cloud that forms in the waves of air flowing over mountains. It often seems to stay still.

Lightning

A giant electrical spark inside a cloud or between a cloud and the ground.

METAR

A standard coded weather report from an airport, used by pilots.

Microburst

A small, intense downdraft from a thunderstorm that can cause damaging straight-line winds.

Millibar (mb)

An older name for the hectopascal. 500 mb is a level about 5.5 km up.

Mountain wave

A wave pattern in the air downwind of mountains, sometimes marked by lenticular clouds.

Okta

One-eighth of the sky. Cloud cover is counted from 0 oktas (clear) to 8 oktas (overcast).

Polar vortex

The large, persistent circulation of cold air over the Arctic. When it weakens or bends, Arctic air can spill south.

POP (probability of precipitation)

The chance that measurable precipitation will fall at any given point in the forecast area during the forecast period.

Pressure gradient

How quickly pressure changes over distance. A strong gradient drives strong wind.

Radiational cooling

The ground losing heat to space at night, strongest under clear sky and calm wind.

Rain shadow

The drier area on the downwind side of mountains, where sinking air leaves less moisture.

Relative humidity

See humidity.

Ridge

An area of relatively high pressure, often bringing warmer, settled weather. On upper-air maps, a northward bulge in the jet stream.

Severe thunderstorm

A storm strong enough to bring large hail, damaging wind, heavy rain or a tornado, as defined by Environment Canada’s warning criteria.

Shelf cloud

A low, wedge-shaped cloud along the leading edge of a thunderstorm’s gust front.

Skew-T

A chart showing temperature and dew point from the ground up, used to judge how stable the air is.

Sounding

A measurement of the atmosphere from the ground up, usually by a weather balloon.

Stable air

Air that resists rising. It tends to bring flat cloud, fog and calm conditions.

Station pressure

The actual air pressure measured at a station’s elevation, before it’s adjusted to sea level.

Stratus

Low, grey, featureless layer cloud. Can bring drizzle or light snow.

Sundog

A bright spot beside the sun caused by sunlight bending through flat ice crystals.

Supercell

A long-lived thunderstorm with a rotating updraft. Supercells produce most large hail and strong tornadoes.

Tornado

A violently rotating column of air reaching from a thunderstorm to the ground.

Troposphere

The lowest layer of the atmosphere, roughly the bottom 10 to 12 km over Canada, where almost all weather happens.

Trough

An elongated area of relatively low pressure, often bringing cloud, precipitation and cooler weather.

Unstable air

Air that keeps rising once it starts. It builds tall clouds and thunderstorms.

Updraft

A current of rising air, such as the one that feeds a thunderstorm.

Upslope

Moist air pushed uphill against the foothills by an easterly wind. It cools and can bring heavy snow to southern Alberta.

Virga

Rain or snow falling from a cloud that evaporates before reaching the ground. Common in our dry air.

Warm front

The leading edge of warmer air replacing colder air. Often brings a thickening layer of cloud and steady precipitation.

Warning (Environment Canada)

Severe weather is happening or about to. Take action.

Watch (Environment Canada)

Conditions are favourable for severe weather to develop. Be ready.

Wind chill

How cold it feels on exposed skin when wind is added to the cold, and how quickly frostbite can happen.

Wind direction

The direction the wind is blowing FROM. A west wind blows from the west.

WXSIM

Local forecasting software that runs on a home computer, used by West Lethbridge Weather to make its own forecast for Lethbridge.