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The growth of a precipitation particle by the collision of a frozen particle with a supercooled liquid water droplet which freezes upon impact.

Air Mass
A body of air that has similar properties covering a relatively large area.


Reflectivity; the fraction of radiation striking a surface that is reflected by that surface.


Alberta Clipper
An area of low pressure that moves out of Alberta and can travel across Canada usually during the winter months.


Sunlight can be seen through these clouds. They can cover all or just part of the sky.


These clouds typically have no form to them and appear grey. These clouds can cover all or just part of the sky.


The air that surrounds Earth.

Atmospheric River
Long, narrow regions in the atmosphere that transport water vapour outside of the tropics. When the atmospheric rivers make landfall, they often release this water vapour in the form of rain or snow.



Barometric Pressure
The pressure of the atmosphere.


Beaufort Scale
A wind scale that is used to estimate and report wind speeds when no wind measuring device is available. The scale ranges from zero to twelve.


Bermuda High
A semi-permanent, subtropical area of high pressure in the North Atlantic Ocean off the East Coast of North America that migrates east and west.


Black Ice
Patchy areas of ice on roadways or other transportation surfaces that cannot easily be seen.


Environment and Climate Change Canada define a blizzard as winds of 40 km/h or greater are expected to cause widespread reductions in visibility to 400 metres or less, due to blowing snow or blowing snow in combination with falling snow, for at least 4 hours.


The rapid intensification of a low pressure system with surface pressure is expected to fall by at least 24 millibars in 24 hours.



Convective Available Potential Energy. A measure of the amount of energy available for convection.


The height of the cloud base for the lowest broken or overcast cloud layer.


A warm dry wind that descends from the eastern slopes of the Rocky Mountains, causing a rapid rise in temperature.


Convective Inhibition. A measure of the amount of energy needed in order to initiate convection.


High-level clouds. They appear as white or pale grey with a definite structure, typically as ripples or ribs. The sun is visibly through them.


These clouds can cover all or just part of the sky. They appear as white or pale grey clouds. The sun is visible through these clouds.


High-level clouds that appear as a feather and are always higher up in the sky.


The average or typical weather conditions over a long period of time.


Climate Change
A change in global or regional climate patterns, in particular a change apparent from the mid to late 20th century onwards and attributed largely to the increased levels of atmospheric carbon dioxide produced by the use of fossil fuels.


Cold Front
A zone separating two air masses, of which the cooler, denser mass is advancing and replacing the warmer.


Colorado Low
A low pressure storm system that forms in winter in southeastern Colorado or northeastern New Mexico.


The physical process by which a vapour becomes a liquid or solid.


These are storm clouds and if conditions are right, they can produce sudden violent weather.


This is perhaps the most easily recognizable type of cloud, they appear as puffy clouds and are most common on summer afternoons.



Represents the strength of returned energy to the radar expressed in values of decibels.


Debris Cloud
A rotating "cloud" of dust or debris, near or on the ground, often surrounding the base of a tornado.


A region of low atmospheric pressure that is usually accompanied by low clouds and precipitation.


A fast-moving windstorm associated with convection.


Dew Point
A measure of atmospheric moisture. It is the temperature to which air must be cooled in order to reach saturation.


A strong downdraft current of air from a cumulonimbus cloud, often associated with intense thunderstorms. Downdrafts may produce damaging winds at the surface.


Precipitation consisting of numerous droplets of water less than 0.5 mm in diameter.


Dry Line
A boundary separating moist and dry air masses.


EF Scale
The Enhanced Fujita scale (EF-Scale) rates the intensity of tornadoes in some countries, including the United States and Canada, based on the damage they cause. The Enhanced Fujita scale replaced the decommissioned Fujita scale that was introduced in 1971 by Ted Fujita.


El Niño
A warming of the ocean current along the coasts of Peru and Ecuador that is generally associated with dramatic changes in the weather patterns of the region. It can disrupt weather patterns worldwide.


The process of a liquid changing into a vapour or gas.


The relatively calm center in a hurricane that is more than one half surrounded by wall cloud. The winds are light, the skies are partly cloudy or even clear.



An intermittent light snowfall of short duration


Is water droplets suspended in the air at the Earth's surface.


Freezing Rain
Rain that falls as a liquid but freezes upon contact with a surface.


The formation of thin ice crystals on the ground or other surfaces in the form of scales, needles, feathers, or fans.


Funnel Cloud
A funnel extending from the base of a towering cumulus or cumulonimbus cloud, associated with a rotating column of air that is not in contact with the ground



Precipitation that forms when supercooled water droplets are collected and freeze on falling snowflakes, forming 2–5 mm balls of rime.


Ground Clutter
A pattern of radar echoes from fixed ground targets (buildings, hills, etc.) near the radar. Ground clutter may hide or confuse precipitation echoes near the radar antenna.


Similar to a bergy bit, but smaller, extending less than 1 meter above the sea surface and occupying an area of 20 square meters or less.


A rapid increase in wind speed that doesn’t last.



When tiny ice crystals are moved up and down repeatedly inside a Cumulonimbus cloud. If water droplets hit the ice crystal, it freezes and is solid ice.


Bright circles or arcs centred on the sun or moon


A property of a moving fluid which represents the potential for helical flow to evolve.


High Pressure
A region where the atmospheric pressure at the surface of the planet is greater than its surrounding environment.


Hoar Frost
A deposit of interlocking crystals formed by direct sublimation on objects, usually those of small diameter freely exposed to the air, such as tree branches, plants, wires, poles, etc.


Hook Echo
A radar reflectivity pattern characterized by a hook-shaped extension of a thunderstorm echo, usually in the right-rear part of the storm (relative to its direction of motion). A hook often is associated with a mesocyclone and indicates favourable conditions for tornado development.


Large storm with powerful winds that must have winds of at least 117 km/h. Usually, forms in warm waters.



Ice Pellets
Pellets of ice composed of frozen or mostly frozen raindrops or refrozen partially melted snowflakes. These pellets of ice usually bounce after hitting the ground or other hard surfaces.


A line connecting points of equal pressure.


A line connecting points of equal temperature.



January Thaw
A period of mild weather popularly supposed to recur each year in late January.


Jet Stream
Strong winds concentrated in a narrow stream in the atmosphere, normally referring to horizontal, high-altitude winds. The position and orientation of jet streams vary from day to day.



La Niña
A coupled ocean-atmosphere phenomenon that is the colder counterpart of El Niño, as part of the broader El Niño–Southern Oscillation climate pattern.


Land Breeze
Any wind that blows from a large body of water toward or onto a landmass; it develops due to differences in air pressure created by the differing heat capacities of water and dry land.

The direction downwind from the point of reference. The leeward region of mountains generally remains dry as compared to the windward.


Low Pressure
A low pressure system has lower pressure at its center than the areas around it.



A localized column of sinking air (downdraft) within a thunderstorm. Microbursts can cause extensive damage at the surface, and in some instances, can be life-threatening.



These are dark grey clouds that completely cover the sky and are usually a good indicator of precipitation.


A macro-scale extratropical cyclone in the western North Atlantic Ocean. The name derives from the direction of the winds that blow from the northeast.



Occluded Front
Weather front formed during the process of cyclogenesis, when a cold front overtakes a warm front.



Polar Vortex
An upper-level low-pressure area lying near one of the Earth's poles. There are two polar vortices in the Earth's atmosphere, overlying the North and South Poles.



Consists of a rotating dish protected by a large white dome; this dish sends pulses of energy (the radar beam) into the atmosphere to detect objects like rain or hail. If the radar beam encounters an object, some of the radiation will bounce off of it and return to the radar site.


Moisture condensed from the atmosphere that falls visibly in separate drops.


An arch of colours formed in the sky in certain circumstances, caused by the refraction and dispersion of the sun's light by rain or other water droplets in the atmosphere.


Relative Humidity
The amount of water vapour present in air expressed as a percentage of the amount needed for saturation at the same temperature.


Rex Block
A blocking pattern that in relation to the United States occurs most often in the spring and most often occurs along or near the West Coast. A Rex Block has two adjacent highs and lows.


An elongated area of relatively high pressure extending from the center of a high-pressure region.



Saffir-Simpson Scale
A 1-5 rating based on the hurricane's present intensity.


A type of satellite that is primarily used to monitor the weather and climate of the Earth.


Sea Breeze
Any wind that blows from a large body of water toward or onto a landmass



Atmospheric water vapour is frozen into ice crystals and falls in light white flakes or lying on the ground as a white layer.

Snow Squall
A sudden moderately heavy snowfall with blowing snow and strong, gusty surface winds.

Squall Line
A line of thunderstorms forming along or ahead of a cold front.


Stationary Front
A weather frontal [transition] zone between two air masses, where neither air mass is strong enough to advance into the other.


Storm Relative Helicity
A measure of the potential for cyclonic updraft rotation in right-moving supercells, and is calculated for the lowest 1-km and 3-km layers above ground level.


These clouds cover the sky and have definite bumps and rolls in them. Typically, you cannot see the sun through them.


These clouds are dull and grey. They usually cover the whole sky.


Appears right or left of the sun and only when the sun is low in the sky. Cirrus or Cirrostratus clouds must also be present.


Evaluates the potential for severe weather by combining several parameters into one index.



The degree or intensity of heat present in a substance or object.


A storm with thunder and lightning and typically also heavy rain or hail.


A rapidly rotating column of air that is in contact with both the surface of the Earth and a cloud.


Tropical Depression
A rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain


Tropical Storm
A cyclonic storm that originates from the tropics and has sustained winds ranging between 63 to 117 kilometres per hour. It is characterized by a low-pressure center and by several thunderstorms that create strong winds and heavy rain.


An elongated area of relatively low pressure extending from the center of a region of low pressure.



A measure of the distance at which an object or light can be clearly discerned.



Warm Front
The transition zone where a warm air mass is replacing a cold air mass.


The state of the atmosphere at a given time.


Wet Bulb Temperature
The temperature read by a thermometer covered in water-soaked cloth over which air is passed.


The flow of gases on a large scale. On the surface of the Earth, wind consists of the bulk movement of air.


Wind Shear
A difference in wind speed or direction over a relatively short distance in the atmosphere.


The direction upwind from the point of reference, alternatively the direction from which the wind is coming.

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