Weather and Meteorology - Glossary
Included in this glossary you'll find terms related to our public forecasting program, weather observations and unique terminology that is of Environment Canada's hurricane centre and ice service products.
Reduction of the water equivalent of a snow cover by melting, evaporation, wind and avalanches.
- Acid Rain
A generic term used for precipitation that contains an abnormally high concentration of sulphuric and nitric acid. These acids form in the atmosphere when industrial gas emissions combine with water, and have negative impacts on the environment and human health.
- Advection Fog
Fog which forms when a relatively moist and warm air mass moves over a colder water or land surface.
A type of alert from Environment Canada’s Meteorological Service (MSC), where a certain weather or environmental hazard (for example air quality, humidex, and tsunami) is either occurring, imminent or is expected to occur.
- Aged Ridge
Sea-ice terminology that describes a ridge which has undergone considerable weathering. These ridges are best described as undulations or waves.
- Agglomerated Brash
Canadian sea-ice terminology that is not part of the World Meteorological Organization’s (WMO) terminology. An area of brash ices that is not compacted.
- Air Mass
An extensive body of the atmosphere with comparable temperature and humidity. It may extend over an area of several million square kilometres and over a depth of several kilometres.
A transmitted signal that is used to heighten awareness and/or initiate preparation for action. Alerts are issued by Environment Canada’s Meteorological Service (MSC) for weather or environmental hazard events that are either occurring, imminent, or forecast to develop. Alerts are currently issued as special weather statements, advisories, watches and warnings.
- Anchor Ice
Sea-ice terminology that describes submerged ice that is attached or anchored to the bottom of the sea, irrespective of the nature of its formation.
A high pressure system with an anticyclonic circulation. It is generally associated with fair weather.
Rotation of an air mass that is clockwise in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere. It is the opposite of cyclonic.
- Apogee (lunar)
The point in the moon’s orbit that is farthest from the Earth. It is the opposite of perigee.
- Arctic Sea Smoke (also called Steam Fog)
A type of fog that forms when an outbreak of cold Arctic air settles over an expanse of open, relatively warmer water.
- Arrangement of the Ice
Sea-ice terminology, that includes the terms:
1) Drift Ice/Pack Ice
2) Ice Cover
4) Ice Distribution
5) Openings in the Ice
6) Ice Edge
7) Ice Boundary
8) Iceberg Limit
The mass of air held close to the earth by gravity. The atmosphere is divided into four sections: the troposphere reaches an altitude of about 10 km from the earth's surface; the stratosphere which is at 10 km to 50 km from the earth’s surface; the mesosphere which is at 50 km to 80 km from the earth’s surface; and lastly the thermosphere which is anything beyond 80 km from the earth’s surface.
- Atmospheric Pressure
The pressure exerted by the weight of air above a given point, sometimes expressed in millibars (mb) or inches of mercury (Hg). The internationally recognized unit for measuring atmospheric pressure is the kilopascal (kPa).
- Aurora Borealis
The luminous, radiant emission from the upper atmosphere that appears over middle and high latitudes, and is centered on the earth's magnetic poles. These silent fireworks are often seen on clear winter nights in a variety of shapes and colours, and are also referred to as the Northern Lights.
- Backing Wind
A counter-clockwise change in wind direction. For example, from southwest to southeast, through south. It is the opposite of veering wind.
- Batture Floes
Canadian sea-ice terminology that is not a part of the World Meteorological Organization’s (WMO) terminology. It defines large, thick, uneven and discoloured ice floes that form on the upstream side of shoals and islets in rivers, when cold weather precedes or accompanies neap tides. It is composed of ice of different thicknesses formed under pressure during ebb tide, the whole mass freezes together and gradually increasing in size, with each successive tide. As the range increases between the neap and spring tides, large sections of grounded ice break away and drift down river causing the floes.
- Beaufort Wind Force Scale
A scale numbered from 0 to 12 which is used to estimate the force of marine wind, based on the observed effects of the wind on sea state.
Sea-ice terminology meaning a large area of pack/drift ice that is longer than it is wide. It can be from 1 km to more than 100 km in width.
- Bergy Bit
Sea-ice terminology that describes a piece of glacier ice, generally showing at 1m to less than 5m above sea level; with a length of 5m to less than 15m. They normally have an area of 100-300 sq. m.
- Bergy water
Sea-ice terminology that describes an area of freely navigable water, in which ice of land origin is present. Other ice types may be present, although the total concentration of all other ice is less than 1/10.
Sea-ice terminology meaning a situation in which a vessel is surrounded by ice and unable to move.
Sea-ice terminology that describes an extensive crescent-shaped indentation in the ice edge, formed by either wind or current.
A severe weather condition characterized by reduced visibility from falling and/or blowing snow and strong winds that may be accompanied by low temperatures.
- Blizzard Warning
A warning issued by Environment Canada’s Meteorological Service (MSC) for hazardous weather conditions characterized by high winds, and a widespread reduction in visibility due to falling and/or blowing snow. Blizzard conditions may persist for a period of time on their own or be part of an intense winter storm. In the latter case, a blizzard warning is issued instead of a winter storm or snowfall warning. Blizzard conditions may be accompanied by a severe wind chill making it even more dangerous.
- Blowing Dust
Dust, raised by the wind to moderated heights above the ground. The visibility at eye level is sensibly reduced.
- Blowing Snow Warning
A warning issued by Environment Canada’s Meteorological Service (MSC) for wind-driven snow that reduces visibility to near-whiteout conditions. A blowing snow event is less extreme than a blizzard in duration and/or visibility.
- Brash Ice
Sea-ice terminology that describes the accumulation of floating ice that is made up of fragments not more than 2 metres across. It is the result of the wreckage of other forms of ice.
- Building Codes
Standards and guidelines for the construction of buildings, to ensure a minimum level of safety for the occupants.
Sea-ice terminology that describes a downward projection from the underside of sea ice; the submariner's counterpart of a hummock
Sea-ice terminology that describes the breaking away of a mass of ice from an ice wall, ice front or iceberg.
- Cape Verde-type Hurricanes
Atlantic basin tropical cyclones fairly close (less then 1000 kilometres) to the Cape Verde Islands, off the west coast of Africa, developing into hurricanes before reaching the Caribbean. Typically, these occur in August and September.
- Chance of Precipitation (COP)
The chance that measurable precipitation (0.2 mm of rain or 0.2 cm of snow) will fall on any random point of the forecast region during the forecast period.
Chinooks occur when mountain ranges are exposed to strong prevailing crosswinds. Moist air is forced up the mountains bringing both cloud and precipitation to the windward side. The descending air becomes warmer and drier as it is forced down the leeward (sheltered) side of the mountains. The relatively warm, dry, gusty winds that occasionally occur to the leeward side of mountain ranges around the world are known by many names. In Canada and the northern United States, they are referred to as Chinooks. In the southern states, they are known as Santa Ana and in parts of Europe, foehn winds.
White patches of cloud composed of ice crystals, found at altitudes of 6,000 metres or higher. Fine and delicate in appearance, their shape and texture often resemble a mare’s tail.
The prevalent or characteristic weather conditions of a place or region over a period of years.
- Close Pack/Drift
Sea-ice terminology that describes floating ice that has a concentration of 7/10 to 8/10. It is composed mostly of ice floes that are in contact with one another.
- Cloud-to-cloud Lightning
A lightning discharge that occurs between areas of the same cloud (intra cloud) or from one cloud to another cloud (inter cloud). Cloud-to-cloud flashes are much more common than cloud to ground flashes (on average there are three to five cloud-to-cloud flashes, for every one cloud-to-ground flash).
- Cloud-to-ground Lightning
A lightning discharge that occurs between a cumulonimbus cloud (thunderstorm) and the ground.
- Compact Ice
Sea-ice terminology that describes floating ice that has a concentration of 10/10, and no water is visible.
- Compacted Ice Edge
Sea-ice terminology that describes a clear-cut ice edge that is compacted by wind or current, usually on the windward side of an area of ice.
Sea-ice terminology that describes an increase in ice concentration and/or a stress which may result in ice deformation (i.e. the ice becoming warped).
Sea-ice terminology, for a ratio (expressed in tenths) which describes the area of the water surface covered by ice, as a fraction of the whole area. Total concentration includes all stages of development that are present. Partial concentration refers to the amount of a particular stage or of a particular form of ice, and represents only a part of the total.
- Concentration Boundary
Sea-ice terminology, describing a line that marks the transition between two areas of floating ice with different concentrations.
The physical process through which water vapour becomes a liquid
- Consolidated Ice
Sea-ice terminology, meaning floating ice in which the concentration is 10/10, and the floes are frozen together.
- Coriolis Force
An effect of the earth’s rotation that deflects the direction of any large moving object (including the wind) to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere. The Coriolis Force is responsible for giving a cyclone its spin.
Sea-ice terminology that describes any fracture of fast ice, consolidated ice or a single floe which may have been followed by separation ranging from a few centimetres to 1 metre.
- Criteria threshold(s)
A set of values associated to weather or environmental factors that are used to issue an alert.
- Cumulonimbus Clouds
Large, dense cloud that frequently has an anvil-shaped top. These clouds produce heavy rain showers, lightning, thunder and sometimes hail or tornadoes. The entire cloud can only be seen from a distance. (See thunderstorm)
- Cumulus Clouds
Bright clouds that appear in fair weather, that have broad, horizontal bases, producing no precipitation and rarely covering more than one-half of the sky.
A low pressure system with a cyclonic circulation. It is also called a depression, and is generally associated with poor or stormy weather. The point of lowest atmospheric pressure marks the centre of the cyclone.
Rotation that is counter-clockwise in the Northern Hemisphere, and clockwise in the Southern Hemisphere. It is the opposite of anticyclonic.
- Dark Nilas
Sea-ice terminology which describes a nilas of up to 5 cm in thickness, which is very dark in colour.
- Deformed Ice
Sea-ice terminology. It is a general term for ice which has been squeezed together, and in places, forced upwards and downwards. The subdivisions of deformed ice are known as rafted ice, ridged ice, and hummocked ice.
- Dew Point (also know as Dewpoint)
The temperature to which air must be cooled, in order to become saturated by the water vapour already present in the air.
- Difficult Area
Sea-ice terminology. A general qualitative expression that indicates that the relative severity of prevailing ice conditions in a particular area are such that navigation will be difficult.
- Diffuse Ice Edge
Sea-ice terminology, meaning a poorly defined ice edge which limits an area of dispersed ice, usually on the leeward side of an area of ice.
Sea-ice terminology that describes ice fields or floes in an area, that move in opposite directions, reducing ice concentration and/or relieving stresses in the ice.
A strong convective downdraft resulting in an outward burst of often damaging winds at or near the surface. The affected area may be from less than 1 km to more than 100 km in horizontal dimension.
A small, gusty current associated with the abrupt vertical movement of air.
- Dried Ice
Sea-ice terminology that describes an ice surface from which water has disappeared after the formation of cracks and thaw holes; during the period of drying the surface whitens.
- Drift Ice/Pack Ice
Sea-ice terminology. This term is used in a wide sense to include any area of ice, other than fast ice, no matter what form it takes or how it gets there. When concentrations are high (7/10 or more) the term pack ice is normally used. When concentrations are 6/10 or less the term drift ice is normally used.
- Drifting Snow
Snow that is raised from the earth's surface by the wind to a height of less than 2 metres.
Precipitation from stratus clouds, consisting of minute, fine water droplets which appear to float.
- Dry Lightning
Cloud-to-ground lightning that comes from a thunderstorm and that produces little, if any, rain.
- Drydocked Iceberg
Sea-ice terminology, which describes an iceberg that has eroded in such a way that a U-shaped slot is formed near or at water level, with twin columns or pinnacles.
- Dust Storm Warning
A warning issued by Environment Canada’s Meteorological Service (MSC) when a prolonged period of reduced visibility caused by blowing dust (of one hour or more) is expected to occur, is imminent, or is occurring.
- Easterly Wave
Also known as tropical wave or African Easterly Waves, they are a type of atmospheric low pressure trough, oriented north to south, which moves from east to west across the tropics, causing areas of cloudiness and thunderstorms. An easterly wave or tropical wave can develop into a tropical cyclone.
- Easy Area
Sea-ice terminology, meaning a general qualitative expression that indicates that prevailing ice conditions in an area are such that navigation is not difficult.
The Enhanced Fujita Scale, or EF-scale, is an intensity scale used to rate wind damage. The scale ranges from EF0 (weakest) to EF5 (strongest). It was adopted in Canada in April of 2013, replacing the original Fujita Scale, or F-scale. The EF-scale is more sophisticated than the original F-scale, having a better correlation between wind damage and wind speed and a much greater variety of indicators with which to rate damage.
- El Niño
El Niño can be distinguished when the surface waters in the eastern tropical Pacific extending westward from Ecuador become warmer than average. The changing pattern of the Pacific Ocean causes a shift in the atmospheric circulation, which then impacts weather patterns across much of the earth. El Nino is like La Niña's brother, the totally opposite.
An individual occurrence of a weather or environmental hazard that meets hazard criteria values.
- Event Time
The time at which the criteria value is first met for an event.
- Extratropical Cyclone
A generic term for the class of frontal, low pressure systems coming from mid-to-low level latitudes (tropics).
- Extratropical Transition (ET)
The transformation of a tropical cyclone into an extratropical cyclone. More than 40 per cent of Atlantic tropical cyclones undergo such a transformation at the end of their tropical existence.
- Eye of the Storm
The roughly circular area of fairly light winds found at the centre of a hurricane and surrounded by the eyewall. Within the eye, the skies are often clear, despite the fact that winds and clouds continue to rage around the edge of the eye in the eyewall. The diameter of a hurricane eye can range from 10 km to more than 100 km.
The ring of thunderstorms that surrounds a storm's eye. The heaviest rain, strongest winds, and worst turbulence is found in the eyewall.
- Fast Ice
Sea-ice terminology, describing ice which forms and remains fast along the coast. It may be attached to the shore, to an ice wall, to an ice front, or between shoals or grounded icebergs. It can extend between a few metres to several hundred kilometres from the coast. It may be more than one year old, in which case it may be attached to the appropriate age category (old, second year or multi-year). If higher than 2 m above sea level, it is called an ice shelf.
- Fast Ice Boundary
Sea-ice terminology. The ice boundary, which at any given time is found between fast ice and the pack/drift ice.
- Fast Ice Edge
Sea-ice terminology, describing the differentiation (at any given time) between fast ice and open water.
The distance wind, usually of uniform speed and direction, which blows over the water surface. Fetch length is related to the height of the wind-generated wave heights. The longer the fetch length, the higher the wind-generated wave heights.
- Finger Rafted Ice
Sea-ice terminology, meaning a type of rafted ice in which floes overlap each other in “fingers.” This is common in nilas.
Sea-ice terminology that describes old snow which has re-crystallized into a dense material. Unlike ordinary snow, particles are (to some extent) joined together; but, unlike ice, the air spaces in it still connect with each other.
- First-year Ice
Sea-ice terminology, meaning sea-ice that has grown for not more than one winter, developing from young ice. It must also be 30 cm or greater. First-year ice may be subdivided into thin first-year ice (sometimes referred to as white ice), medium first-year ice, and thick first-year ice.
- Flash Freeze Warning
A warning issued by Environment Canada’s Meteorological Service (MSC) for when a rapid drop in temperature to below freezing, results in significant ice on road surfaces. This ice is produced by the freezing of residual water from either melted snow or falling/fallen rain.
Sea-ice terminology, describing a narrow separation zone between floating ice and fast ice, where the pieces of ice are in a chaotic state. Flaws form when ice shears, due to a strong wind or current along the fast ice boundary.
- Flaw Lead
Sea-ice terminology, describing a passageway between ice and fast ice, which is navigable by surface vessels.
- Floating Ice
Sea-ice terminology, meaning any form of ice found floating in water. The principal kinds of floating ice are lake ice, river ice and sea ice, which form by the freezing of water at the surface, and glacier ice which is formed on land or in an ice shelf.
Sea-ice terminology. Describes any relatively flat piece of ice that is 20 m or more across. Floes are subdivided according to their horizontal extent, as follows:
- Small: 20 m - 100 m across
- Medium: 101 m - 500 m across
- Big: 501 m - 2000 m across
- Vast: 2001 m - 10 km across
- Giant: Greater than 10 km across
Sea-ice terminology, describing a massive piece of ice composed of a hummock or a group of hummocks, frozen together and separated from any surrounding ice. They may typically protrude up to 5 m above water level.
- Flooded Ice
Sea-ice terminology. Describes ice which has been flooded and is heavily loaded by water, or water and wet snow.
Precipitation in the form of snow from a convective cumulus-type cloud, are known as flurries. They are characterized by the suddenness with which they start and stop, by their rapid changes in intensity, and usually by rapid changes in the appearance of the sky.
A cloud based at the earth's surface, consisting of tiny water droplets, or under very cold conditions, ice crystals or ice fog. It is generally found in calm or low wind conditions. Under foggy conditions, visibility is reduced to less than 1 kilometre.
- Fog Bank
Fog generally caused by local conditions, which extends over a small area of some hundreds of metres across, and which reduces visibility to less than one-half (1/2) nautical miles (or five eighths (5/8) of a kilometre). Multiple fog banks are sometimes referred to as patchy fog or fog patches.
A statement of expected meteorological and environmental conditions for a specified time or period, and for a specified area or portion of airspace.
- Forms of Ice
1) Pancake Ice
2) Ice Cake
5) Ice Breccia
6) Batture Floes
7) Brash Ice
8) Fast Ice
10) Anchor Ice
11) Grounded Ice
Sea-ice terminology. Describes any break or rupture through very close pack ice, compact ice, consolidated ice, fast ice or a single floe, resulting from the deformation processes. Fractures may contain brash ice and/or be covered with nilas and/or young ice. Their lengths may vary from a few metres to many kilometres long.
- Fracture Zone
Sea-ice terminology. Describes an area which has a great number of fractures. Fractures are subdivided as follows:
- Very Small Fracture: 1 m to 50 m wide
- Small Fracture: 51 m to 200 m wide
- Medium Fracture: 201 m to 500 m wide
- Large Fracture: Greater than 500 m wide
Sea-ice terminology that describes the pressure process whereby ice is permanently deformed, and rupture occurs. This term is most commonly used to describe the breaking across of very close ice, compact ice and consolidated ice.
- Freezing Drizzle
Drizzle that freezes on impact to form a coating of clear ice (glaze) on the ground and on exposed objects.
- Freezing Rain
Rain that freezes on impact to form a coating of clear ice (glaze) on the ground and on exposed objects.
- Freezing Rain Warning
A warning issued by Environment Canada’s Meteorological Service (MSC) for freezing rain.
- Freezing Spray
Freezing spray occurs when a combination of below freezing temperatures and strong winds, causes a wind-generated spray to freeze and accumulate (or build-up) on any marine infrastructure located in or near the vicinity of the water.
- Freezing Spray Warning
A warning issued by Environment Canada’s Meteorological Service (MSC) if freezing spray is forecast or observed to be moderate or severe. Freezing spray is termed moderate if the ice accumulation or build-up rate on marine infrastructure is between 0.7 and 2 cm per hour. It is termed severe if the ice accumulation or build-up rate on marine infrastructure is greater than 2 cm per hour.
- Friendly Ice
Sea-ice terminology that describes an ice canopy containing many large skylights or other features, which permits a submarine to surface. There must be more than 10 such features per 30 nautical miles (56 km) along the submarine’s track.
The boundary between two different air masses. A cold front is the leading edge of an advancing cold air mass, while a warm front is the trailing edge of a retreating cold air mass.
- Frontal Cyclone
Any cyclone associated with a front. It is often associated with extratropical cyclone (as opposed to tropical cyclone, which is non-frontal).
- Frost Smoke
Sea-ice terminology that describes fog-like clouds that are formed when cold air and relatively warm water come in contact with each other. Frost smoke can appear over openings in the ice or leeward of the ice edge, and may persist while ice is forming.
- Funnel Cloud
A funnel-shaped condensation cloud caused by the pressure deficit associated with a violently rotating column of air. It is typically vertically oriented and may extend fully or partially from a cumuliform cloud to the surface. A funnel cloud is often generated by a tornado.
- Gale Warning
A warning issued by Environment Canada’s Meteorological Service (MSC) if wind speed of 34 to 47 knots inclusive (gale force wind) is forecast or observed over a marine area.
- Geostationary Satellite
A satellite that orbits the earth at the same rate that the earth rotates, and as a result remains over a fixed place above the equator.
Sea-ice terminology. Describes a mass of snow and ice that is continuously moving from higher to lower ground or, if afloat, continuously spreading. The principal forms of glaciers are: inland ice sheets, ice shelves, ice streams, ice caps, ice piedmonts, cirque glaciers, and various types of mountain (valley) glaciers.
- Glacier Ice
Sea-ice terminology that describes ice in or originating from a glacier, whether on land or floating on the sea as icebergs, bergy bits, growlers or ice islands.
- Glacier Tongue
Sea-ice terminology that describes the seaward extension of a glacier, usually afloat. In the Antarctic, glacier tongues may extend over many tens of kilometres.
- Grease Ice
Sea-ice terminology which describes a later stage of freezing than frazil ice where the crystals have coagulated to form a soupy layer on the surface. Grease ice reflects little light, giving the water a matte appearance.
- Grey Ice
Sea-ice terminology, describing young ice that is 10 cm to15 cm thick, less elastic than nilas, and which breaks on swell. It usually rafts under pressure.
- Grey-White Ice
Sea-ice terminology, describing young ice that is 15 cm to 30 cm thick. Under pressure it is more likely to ridge than to raft.
- Grounded Hummock
Sea-ice terminology. Defines a hummocked, grounded ice formation. There are single grounded hummocks and lines (or chains) of grounded hummocks.
Sea-ice terminology that describes a piece of ice smaller than a bergy bit and floating less than 1 m above the sea surface. A growler generally appears white but sometimes transparent or blue-green in colour. Extending less than 1 m above the sea surface, and normally occupying an area of about 20 sq. m., Growlers are difficult to distinguish when surrounded by sea ice or in high sea state.
- Gulf Stream
A warm, swift, narrow ocean current flowing parallel to the east coast of North America. The current then turns and flows south of Nova Scotia and Newfoundland and continues on towards Europe.
Gusts are sudden, rapid, and brief changes in the wind speed. They are characterized by the more or less continual fluctuations between the high (peak) and low (lull) speed.
Precipitation in the form of lumps of ice mainly associated with thunderstorms. Hail ranges in size from that of a small pea to the size of cherries, but has been observed as large as grapefruit. Hail in Canada occurs most frequently during the summer when thunderstorm activity is at its peak.
A type of condition (weather or environmental) that has a negative impact on the safety and security of the public.
Consists of fine particles of dust and pollution suspended in the atmosphere, and is distinguished from fog by its bluish or yellowish tinge.
Region of the atmosphere where the pressures are high, relative to those in the surrounding region at the same level. In the Northern Hemisphere, winds around a high move in a clockwise fashion.
- High Water Level
Abnormally high water levels or waves along coastal or shoreline areas.
- High Water Level Warning
A warning issued by Environment Canada’s Meteorological Service (MSC) to warn mariners and coastal populations of potential impacts caused by abnormally high water levels, or waves along coastal or shoreline areas.
A small hill or mound.
- Hostile Ice
Sea-ice terminology that describes an ice canopy containing no large skylights or other features, and as a result submarines are unable to surface.
The scale describing how hot, humid weather feels to the average person. The humidex combines the temperature and humidity into one number to reflect the perceived temperature.
- Humidity (also called Relative Humidity)
Humidity is the measure of water vapour content in the air. Usually, relative humidity is expressed as a percentage of total possible moisture content.
Sea-ice terminology, meaning a hillock of broken ice which has been forced upwards by pressure; it may be fresh or weathered.
- Hummocked Ice
Sea-ice terminology, which describes ice that is piled chaotically, one piece over another, to form an uneven surface. When weathered, hummocked ice has the appearance of smooth hillocks.
Sea-ice terminology. Describes the pressure process by which ice is forced into hummocks. When the floes rotate in the process, it is termed screwing.
Hurricanes are tropical cyclones with maximum sustained surface winds of at least 64 knots (118 km/h). Hurricanes are known as typhoons in the western Pacific, very severe cyclonic storms in the North Indian Ocean, and severe tropical cyclones in Australia. There are five classes of hurricane intensity as outlined by the Saffir-Simpson Scale.
- Hurricane Force Wind
Wind speed of 64 knots (118 km/h) or greater, not necessarily in association with a tropical cyclone, with a Beaufort wind force of 12.
- Hurricane Force Wind Warning
A warning issued by Environment Canada’s Meteorological Service (MSC) for a wind speed of 64 knots (118 km/h) or greater is forecast or observed over a marine area.
- Hurricane Season
Hurricane season in the North Atlantic runs officially from June 1 to November 30 when almost 95 per cent of all tropical cyclones occur. About 85 per cent of land falling tropical cyclones in Canada occur from August through October.
- Hurricane Warning
(a) Average sustained winds of 64 knots (118 km/h) or higher;
(b) Dangerously high water or a combination of dangerously high water and exceptionally high waves, even though winds expected may be less than hurricane force. By nature a hurricane also implies the threat of local flooding from heavy rainfall.
- Hurricane Watch
A watch issued by Environment Canada’s Meteorological Service (MSC) for a specific area that a hurricane or a developing hurricane condition poses a possible threat within 36 hours. This watch does not mean that a hurricane is definitely going to strike; it simply means that everyone in the watch area should be more aware of the potential for a hurricane, and be prepared to act quickly if definite warnings are issued that a hurricane will strike.
- Ice Blink
Sea-ice terminology, describing a whitish glare on low clouds above an accumulation of distant ice.
- Ice Boundary
Sea-ice terminology that describes the differentiation at any given time between fast ice and floating ice, or between areas of ice of different concentrations, types and/or floe sizes.
- Ice Breccia
Sea-ice terminology, describing ice pieces in different stages of development that are frozen together.
- Ice Cake
Sea-ice terminology that describes any relatively flat piece of ice less than 20 m across.
- Ice Canopy
Sea-ice terminology, meaning ice as it is seen from the point of view of the submariner. (i.e. Underwater looking up at the ice)
- Ice Cover
Sea-ice terminology. Describes the ratio of an area of ice to the total area of water surface within a large geographic area. This area may be global, hemispheric, or defined by a specific oceanographic entity such as Baffin Bay or the Barents Sea.
- Ice Deformation Processes
- Ice Distribution
- Ice Edge
Sea-ice terminology, which describes the differentiation at any given time between open water, and sea, lake or river ice – whether fast or drifting.
- Ice Field
Sea-ice terminology that describes an area of floating ice, consisting of floes greater than 10 km across.
- Ice Free
Sea-ice terminology, meaning “no ice present.” If ice of any kind is present, this term will not be used.
- Ice Front
Sea-ice terminology. Describes the vertical cliff that forms the seaward face of an ice shelf or other floating glacier. It varies in height from 2 m to 50 m or more above sea level.
- Ice Island
Sea-ice terminology. Describes a large piece of floating ice protruding about 5 metres above sea level, which has broken away from an Arctic ice shelf. Ice islands have a thickness of 30 m to 50 m, and an area of a few thousand square metres up to 500 sq. km or more. They are usually characterized by a regularly rolling surface giving a ribbed appearance from the air.
- Ice Island Fragment
Sea-ice terminology, describing a piece of an ice island that has broken away from the main mass.
- Ice Jam
Sea-ice terminology that describes an accumulation of broken river ice or sea ice which is not moving, due to a physical restriction and its resistance to pressure.
- Ice Keel
Sea-ice terminology which describes a downward-projecting ridge on the underside of the ice canopy; the submerged counterpart of a ridge. Ice keels may extend to as much as 50 metres below the surface.
- Ice Limit
Sea-ice terminology. A climatology term that refers to the extreme minimum or extreme maximum extent of the ice edge in any given month or period, based on observations over a number of years. This term should always be preceded by minimum or maximum.
- Ice Massif
Sea-ice terminology that describes a variable accumulation of pack or very close pack, covering hundreds of square kilometres and found in the same region every summer.
- Ice Patch
Sea-ice terminology, meaning an area of ice which is less than 10 km across.
- Ice Pellets
Tiny particles of ice that are formed when supercooled raindrops freeze before reaching the ground.
- Ice Rind
Sea-ice terminology. Describes a brittle, shiny crust of ice formed on a quiet surface by direct freezing, or from grease ice, usually in water of low salinity; with a thickness of about 5 centimetres. Easily broken by wind or swell, it commonly breaks into rectangular pieces.
- Ice Shelf
Sea-ice terminology. Describes a floating ice sheet of considerable thickness that is visible 2 metres or more above sea level, and is attached to the coast. They usually have great horizontal extension, and a level or gently rolling surface. Ice shelf growth occurs with annual snow accumulation, and also by the extension of land glaciers over see. Limited areas of the ice shelf may be attached to land. The edge facing the sea is termed as ice front.
- Ice Stream
Sea-ice terminology. Describes the part of an inland ice sheet in which the ice flows more rapidly and not necessarily in the same direction as the surrounding ice.
- Ice Surface Features
- Ice Under Pressure
Sea-ice terminology describing ice in which the deformation processes are actively occurring. This form of ice is also a potential impediment or danger to shipping.
- Ice Wall
Sea-ice terminology that describes an ice cliff forming at the seaward margin of a glacier which is aground.
Sea-ice terminology. A large, massive piece of floating or stranded glacier ice of any shape detached (calved) from the front of a glacier into a body of water. An iceberg extends more than 5 m above sea level and has the greater part of its mass (4/5 to 8/9) below sea level.
- Iceberg Limit
Sea-ice terminology, meaning the limit at any given time between ice of land origin and the open sea or sea ice.
- Iceberg Tongue
Sea-ice terminology describing a major accumulation of icebergs that are projecting toward the coast, held in place by grounding and joined together by fast ice.
Sea-ice terminology. A harbour, inlet, etc. is said to be ice-bound when ships cannot navigate within it due to the ice. In some cases, navigation is possible with the assistance of an icebreaker.
Sea-ice terminology describing a narrow fringe of ice attached to the coast, unmoved by tides and remaining after the fast ice has moved away.
Sea-ice terminology describing a bay of ice, often of a temporary nature, where ships can moor alongside and unload directly onto the ice itself.
- Intermittent Rain
Rain that starts and stops repeatedly, although not as abrupt or as frequent as showers.
A line connecting points of equal pressure on a map.
- Jammed Brash Barrier
Sea-ice terminology. Describes a strip or narrow belt of new, young or brash ice usually 100 m to 5000 m across, formed at the edge of either floating or fast ice, or at the shore. Heavily compacted, mostly due to wind action, a jammed brash barrier may extend 2 m to 20 m below the surface
- Jet Stream
Relatively strong winds, concentrated within a narrow band in the upper atmosphere.
- Killing Frost
A frost severe enough to destroy annual plants and new growth on trees (in the spring), or to end the growing season (in the fall).
The internationally recognized unit used by the Environment Canada’s Meteorological Service (MSC) for measuring atmospheric pressure.
The unit of speed used in the marine environment, which is equal one (1) nautical mile per hour. One (1) knot equals 1.852 kilometres per hour.
- La Niña
An extensive cooling of the waters in the tropical eastern Pacific Ocean. It is the climatic opposite of the El Niño .
- Labrador Current
A cold, strong ocean current flowing parallel to the east coast of Labrador.
- Lake Ice
Sea-ice terminology describing ice formed on a lake, regardless of where it ends up.
- Land Breeze
This coastal breeze blows from the land to the sea, lake or river, and usually occurs at night when the temperature of the water is often warmer than the nearby land. The water heats the air above, which rises and is replaced by cooler air from the land. (See also sea breeze)
When the eye, or physical centre of the hurricane, reaches a coastline it is said to make landfall.
- Large Iceberg
Sea-ice terminology that describes a piece of glacier ice extending 46 m to 75 m above sea level, and with a length of 121 m to 200 m.
- Latent Heat
Heat that is stored in water vapour in the atmosphere. When water vapour rises, cools and condenses into liquid water, it releases this heat into the surrounding atmosphere. This is the driving mechanism for tropical cyclones.
Sea-ice terminology describing any fracture or passageway through ice, which is navigable by surface vessels.
A term that means, “is situated away from the wind;” in other words, downwind and opposite of windward.
- Level Ice
Sea-ice terminology describing ice that is unaffected by deformation.
- Light Nilas
Sea-ice terminology describing a nilas which is more than 5 cm in thickness and lighter in colour than dark nilas.
Generally, any and all of the various forms of visible electrical discharge that are produced by thunderstorms; often seen as a bright flash of light in the sky.
- Lightning Flash
A scientific term used to describe lightning, which consists of more than one individual stroke of lightning (and as many as 20 single lightning strokes within a single flash). This causes the “flickering” effect that is sometimes seen in lightning.
- Lightning Flash Density
The number of lightning flashes detected per square unit of area (usually per square kilometre or mile) and unit of time.
- Lightning Stroke
A scientific term used to describe a single discharge of lightning. There can be many individual strokes of lightning within a single lightning flash.
- Limit of all known Ice
Sea-ice terminology that describes the limit at any given time between icebergs and/or sea-ice infested waters, and ice-free waters.
- Line Squalls
Squalls which occur along a line or within an elongated area, such as an organized area of squalls in the vicinity of a cold front.
Region of the atmosphere where the pressures are low, relative to those in the surrounding region at the same level. In the Northern Hemisphere, winds around a low move in a counter-clockwise fashion.
- Maximum Iceberg Limit
Sea-ice terminology, describing the maximum limit of icebergs, based on observations over a period of years.
- Mean Ice Edge
Sea-ice terminology that describes the average position of an ice edge in any given month or period, based on observations over a number of years. Other terms which may be used are mean maximum ice edge, and mean minimum ice edge.
- Mean Iceberg Limit
Sea-ice terminology. Describes the average position of the limit of icebergs at any given time, based on observations over a number of years.
- Median Ice Edge
Sea-ice terminology describing the position of the ice edge, where its frequency of occurrence is 50 per cent.
- Median Iceberg Limit
Sea-ice terminology describing the position where the historical or statistical frequency of occurrence of the iceberg limit is 50 per cent.
- Medium First-year Ice
Sea-ice terminology meaning ice that is 70 cm to 120 cm thick.
- Medium Iceberg
Sea-ice terminology meaning a piece of glacier ice extending 16 m to 45 m above sea level, and with a length of 61 m to 120 metres.
- Medium Lake Ice
Sea-ice terminology meaning ice that is 15 to 30 cm thick.
Top of the mesosphere situated at about 80-85 km above Earth’s surface. It corresponds to the level on minimum temperature.
Region of the atmosphere, situated between the stratopause and the mesopause, in which the temperature generally decreases with height.
A unit for expressing the atmospheric pressure. Average global sea level pressure is about 1013 millibars (101.3 kilopascals).
- Minimum Iceberg Limit
Sea-ice terminology that describes the minimum limit of icebergs based on observations over a period of years.
A downburst less than 4 km in horizontal dimension. Microbursts tend to have a shorter lifetime and be more intense than larger downbursts and can result in damage intensity similar to that associated with a strong tornado.
Very small to microscopic-sized water droplets that are suspended in the atmosphere, usually in association with precipitation, and causing obstruction to visibility from one-half (1/2) to five (5) nautical miles, inclusive.
- Multi-year Ice
Sea-ice terminology that describes old ice which has survived at least two summer’s worth of melt. Hummocks are smoother on multi-year ice than on second-year ice, and the ice is almost salt-free. Where bare, this ice is usually blue in colour. The melt pattern consists of large, interconnecting, irregular puddles and a well developed drainage system.
- Nautical Mile
The unit of distance used in the marine environment. One (1) nautical mile is equal to 1.852 kilometres.
- New Ice
Sea-ice terminology. This is a general term for recently formed ice which includes frazil ice, grease ice, slush, and shuga. These types of ice are composed of ice crystals which are only weakly frozen together (if at all) and have a definite form only while they are afloat.
- New Lake Ice
Sea-ice terminology describing recently formed ice, which is less than 5 cm thick.
- New Ridge
Sea-ice terminology that describes a ridge with sharp peaks and sides that has a slope of 40 degrees or more.
Sea-ice terminology. Describes a thin, elastic crust of gray-colored ice that is formed on a calm sea, and is easily bent by waves and thrust into a pattern of interlocking fingers (known as finger rafting). Nilas has a matte surface, is up to 10 cm in thickness, and may be subdivided into dark nilas and light nilas.
Sea-ice terminology. Ice is said to nip when it forcibly presses against a ship. When a vessel is caught in this way, though undamaged, it is said to have been “nipped.”
The long-term average value of a climate element for a certain area, averaged over a 30-year period. Elements can include temperature, precipitation, hours of sunshine, etc.
- Normal High
The climatological average high temperature for a given day.
- Normal Low
The climatological average low temperature for a given day.
- Old Ice
Sea-ice terminology describing sea-ice which has survived at least one summer’s melt. The surface of old ice is generally smoother than first-year ice. Old ice may be subdivided into second-year ice and multi-year ice.
- Open Drift
Sea-ice terminology which describes floating ice in which the concentration is 4/10 to 6/10, with many leads and polynyas. Floes are generally not in contact with one another in open drift.
- Open Water
Sea-ice terminology describing a large area of freely navigable water in which ice is present, with a concentration less than 1/10. In open water no ice of land origin is present.
- Openings in the Ice
- Other Surface Feature Definitions
A term used by meteorologists to refer to anticipated weather conditions in the future. Usually, an outlook covers a period of time that goes beyond a normal forecast.
- Pancake Ice
Sea-ice terminology. Describes (predominantly) circular pieces of ice that are 30 cm to 3 m in diameter and up to 10 cm in thickness, which have raised rims due to the pieces striking against one another. Pancake ice may form on a slight swell from grease ice, shuga or slush, or as a result of the breaking of ice rind, nilas or, under severe conditions of swell or waves, from grey ice.
The time between two successive crests or troughs of a wave.
- Pinnacled Iceberg
Sea-ice terminology that describes an iceberg that has a central spire or pyramid. This pyramid can have one or more spires.
- Polar Low
An intense storm system that usually forms in polar regions during outbreaks of very cold air, over relatively warmer ocean waters. Typically spanning from 400 to 800 kilometres across, and usually existing for only one or two days, polar lows can result in severe blizzard-like conditions, with heavy snow and gales force winds over affected marine areas.
- Polar-orbiting Satellite
A satellite whose orbit closely parallels the earth's meridian lines, and thus crosses the polar regions on each orbit.
Sea-ice terminology. Describes any non-linear-shaped opening that is enclosed by ice. May contain brash ice and/or be covered with new ice, nilas or young ice. Submariners refer to polynya’s as “skylights.”
- Post-Tropical Storm
A storm system that used to be tropical but has since lost most of its tropical characteristics. Since the remnant system may or may not be an extratropical cyclone (a term set aside for a specific breed of cyclone), the term post-tropical is a good way of continuing the use of the storm name. This is important because storms with a tropical pedigree seldom lose all of their tropical characteristics, such as intense rainfalls and high wind gusts.
Precipitation is a liquid or solid form of water falling from the atmosphere to the earth's surface. Examples include rain, freezing rain, hail, and snow.
- Predicted Astronomical Tide
The height of tides at various locations and times, based solely on astronomical calculations. These are the values printed in tide tables. These values do not account for effects such as wind, atmospheric pressure, or waves.
Sea-ice terminology that describes an accumulation of water on ice, mainly due to melting snow. However, in the more advanced stages puddles can also be due to the melting of ice.
Radar is an acronym for Radio Detection And Ranging. It is used in the meteorological environment to detect and locate the presence of clouds and precipitation.
- Rafted Ice
Sea-ice terminology, describing a type of deformed ice, formed by one piece of ice overriding another.
Sea-ice terminology describing the pressure process whereby one piece of ice overrides another. Most common in new and young ice.
Moisture condensed from atmospheric vapour that falls to earth in drops.
- Rainfall Warning
A warning issued by Environment Canada’s Meteorological Service (MSC) for a hazardous, prolonged or intense short-duration of rainfall. It may be a major factor in the cause of disasters, such as floods, flash floods, and landslides.
Sea-ice terminology. Describes an underwater protruding piece of ice that has come from an ice wall, ice front, iceberg or floe. Its formation is usually due to a more intensive melting and erosion of the un-submerged part.
- Relative Humidity (also called Humidity)
Relative humidity is the ratio of water vapour in the air at a given temperature, to the maximum amount which could exist at that temperature. It is usually expressed as a percentage.
An elongated atmospheric area of relatively high pressure, extending from the centre of a high pressure region; the opposite of a trough.
- Ridge (Ice)
Sea-ice terminology describing a line or wall of broken ice forced up by pressure that may be fresh or weathered. Also see ice keel.
- Ridged Ice
Sea-ice terminology describing ice that is piled haphazardly, one piece over another in the form of ridges or walls. Usually found in first-year ice.
- Ridged Ice Zone
Sea-ice terminology that describes an area of many ridges with similar characteristics.
- River Ice
Sea-ice terminology that describes ice that is formed on a river, regardless of where it is observed.
- Rotten Ice
Sea-ice terminology that describes ice which has become honeycombed and is in an advanced state of disintegration.
- Saffir-Simpson Hurricane Scale
Scale numbers range from 1 to 5.
Category 1: Maximum sustained winds of at least 64 knots (118 km/h).
Category 2: Maximum sustained winds of at least 83 knots (154 km/h).
Category 3: Maximum sustained winds of at least 96 knots (178 km/h).
Category 4: Maximum sustained winds of at least 114 knots (210 km/h).
Category 5: Maximum sustained winds of at least 136 knots (250 km/h).
Sea-ice terminology. Describes sharp, irregular ridges formed on a snow surface by wind erosion and snow accumulation. On floating ice, the ridges are parallel to the direction of the wind that was present at the time they were formed.
- Sea Breeze
This is the breeze which blows from the sea or a large lake, to the land. The breeze is set off when the temperature of the land is higher than the temperature of the water. The land heats the air above, which rises, and it is then replaced by the cooler air from over the water. (Opposite to the land breeze).
- Sea Fog
Fog which forms in the lower part of a relatively moist and warm air mass as it moves over a relatively cooler water surface. This process cools the water vapour to such an extent that it condenses into the suspended water droplets that create the fog. It reduces visibility to less than one-half (1/2) nautical miles. (See also advection fog).
- Sea Ice
Sea-ice terminology describing any form of ice found at sea, which has originated from the freezing of water.
- Sea Smoke
A fog that forms when an outbreak of cold air settles over an expanse of open, relatively warmer water, reducing visibility to less than 1 kilometre. Also known as steam fog. (See also arctic sea smoke, and advection fog or sea fog)
- Sea State
Overall state of agitation of a large expanse of ocean or sea due to the combined effects of wind-generated waves, swell waves, and surface currents.
- Second-year Ice
Sea-ice terminology. Describes old ice which has survived only one summer’s melt. Thicker than first-year ice, it stands higher out of the water. In contrast to multi-year ice, summer melting of second-year ice produces a regular pattern of numerous small puddles. In this case, bare patches and puddles are usually greenish-blue.
- Severe Thunderstorm Watch
A watch issued by Environment Canada’s Meteorological Service (MSC) that outlines a “watch zone” where conditions are favourable for the development of thunderstorms, some of which have the potential to become severe thunderstorms.
- Severe Thunderstorm Warning
A warning issued by Environment Canada’s Meteorological Service (MSC) when at least one thunderstorm that produces hail large enough to cause damage (at least 2 cm in diameter), heavy rain, and/or damaging winds, is imminent. This is indicated by radar and/or observation reports.
Sea-ice terminology. An area of floating ice is subject to shear when the ice motion varies significantly from its main direction, subjecting the ice to rotational forces. These forces may result in phenomena similar to a flaw.
A place where the depth of water is shallow, especially where the seafloor is visible at low tide.
- Shore Lead
Sea-ice terminology meaning a lead between ice and the shore, or between ice and an ice front.
- Shore Polynya
Sea-ice terminology describing a polynya between ice and the coast, or between ice and an ice front.
Precipitation that is characterized by the suddenness with which it starts and stops, by its rapid changes in intensity, and usually by the rapid changes in the appearance of the sky.
Sea-ice terminology meaning an accumulation of spongy white ice lumps that have a diameter of a few centimeters across. Shuga are formed from grease ice or slush, and sometimes from anchor ice rising to the surface.
Sea-ice terminology. Describes the thin places in an ice canopy, usually less than 1 meter thick, that appear from below as light, translucent patches in dark surroundings. The under surface of skylights are normally flat. They are termed small or large; if they are big enough for a submarine to attempt to surface through them (120 meters wide or more) they qualify as large.
Sea-ice terminology describing snow which is saturated and mixed with water on land, on ice surfaces, or as a thick floating mass in water, after a heavy snowfall.
- Small Iceberg
Sea-ice terminology that describes a piece of glacier ice extending 5 m to 15 m above sea level, and with a length of 15 m to 60 meters.
Suspension in the atmosphere of small particles, which are produced by fire.
- Snow Grains
Minute, white and opaque grains of ice. When they hit hard ground, they do not bounce or shatter. They usually fall in very small quantities, and never in the form of a shower.
- Snow Pellets
Snow pellets are brittle and easily crushed; when they fall on hard ground, they bounce and often break up. They always occur in showers. They are often accompanied by snow flakes or rain drops, when the surface temperature is around zero Celsius.
- Snow Squall
A moderate to heavy snow flurry, which is driven by strong, gusty winds. Visibility during snow squalls is usually poor.
- Snow Squall Warning
A warning issued by Environment Canada’s Meteorological Service (MSC) for localized, limited duration, intense snowfall that reduces visibility significantly and may be accompanied by strong, gusty winds and (in some cases) lightning. These weather conditions are produced by the passage of cold air over an open body of water (open water squall) or the passage of a cold front (frontal snow squall). Local snow accumulations may be significant during this type of event.
Areas where prevailing onshore winds are responsible for heavy snowfall is known as a snowbelt, and usually refers to regions southeast of open water.
Sea-ice terminology. Describes an accumulation of wind-blown snow that is deposited in the lee of an obstacle. A crescent-shaped snowdrift, with ends pointing down-wind, is called a snow barchan.
- Snowfall Warning
A warning issued by Environment Canada’s Meteorological Service (MSC) for an expected snowfall that is heavy enough to cause significant inconvenience and hazardous conditions.
- Special Marine Warning
A warning issued by Environment Canada’s Meteorological Service (MSC) to warn mariners of any undefined marine weather phenomenon that could pose a hazard to marine navigation or safety.
- Special Marine Watch
A watch issued by Environment Canada’s Meteorological Service (MSC) to notify mariners that conditions are favourable for the development of any undefined marine weather phenomenon that could pose a hazard to marine navigation or safety.
- Special Weather Statement
A type of alert from Environment Canada’s Meteorological Service (MSC) where:
A special weather statement can be used to describe any hazard (unlike an advisory). The statements do not have a formal “in effect” status, since there is no requirement to update or end them.
A spike of ice that has needle-like crystals, which is formed during the freezing of water.
An atmospheric phenomenon characterized by an abrupt and large increase of wind speed within a duration of minutes, that suddenly diminishes. Squalls are usually associated with thunderstorms, and as such are often accompanied by heavy showers, thunder, and lightning.
- Squall Warning
A warning issued by Environment Canada’s Meteorological Service (MSC) for forecast or observed wind gusts of 34 knots or greater, that are associated with a line, or an organized area, of thunderstorms. (See also squall)
- Squall Watch
A watch issued by Environment Canada’s Meteorological Service (MSC) to give advance notice of conditions that are favourable for the development of squalls.
- Stages of Development of Lake Ice
- Stages of Development of Sea Ice
- Stages of Melting
- Standing Floe
Sea-ice terminology that describes a separate floe standing vertically or inclined, and enclosed by rather smooth ice.
- Steam Fog (also called Actic Sea Smoke)
A type of fog that forms when an outbreak of cold Arctic air settles over an expanse of open, relatively warmer water (See sea smoke, arctic sea smoke and advection fog, or sea fog)
- Storm Surge
The positive or negative difference in sea level from the predicted astronomical tide, due to the forces of the atmosphere. The two main atmospheric components that contribute to a storm surge are air pressure and wind.
Deep low pressure systems can create a dome of water under the storm (much like the low pressure in a vacuum on a carpet). High winds along a coastline can also elevate the water levels at the shore, depending on the direction of the wind with respect to the coast. For powerful storms like hurricanes, the abnormally high water levels are due mostly to the high winds and high waves at the coast.
- Storm Surge Warning
A warning issued by Environment Canada’s Meteorological Service (MSC) for abnormally high water levels and high waves (storm surge) caused by a storm, expected to be a significant threat to public safety and property because of the potential for coastal flooding.
- Storm Track
The line of movement (known as propagation) of the storm's centre through an area.
- Storm Warning
A warning issued by Environment Canada’s Meteorological Service (MSC) if wind speed of between 48 and 63 knots inclusive, is forecast or observed over a marine area.
- Stranded Ice
Sea-ice terminology describing ice which had been floating and is then deposited on the shore by retreating high water.
A boundary or zone of transition separating the stratosphere and the mesosphere; it marks a reversal of temperature change with altitude. It is located at the height of roughly 50 km.
The region of the atmosphere extending from the top of the troposphere (the tropopause), at height of 10-17 km to the base of the mesosphere (the stratopause), at a height of roughly 50 km.
Generally grey cloud layer with a fairly uniform base, which may produce drizzle, ice prisms or snow grains.
A surface force measured per unit area, for example, the force due to the transport of momentum by turbulent fluid motions.
Sea-ice terminology. Describes a long narrow area of pack/drift ice, about one (1) km or less in width, usually composed of small fragments detached from the main mass of ice, which run together under the influence of wind, swell or current.
- Strong Wind Warning
A warning issued by Environment Canada’s Meteorological Service (MSC) if a wind speed of 20 to 33 knots inclusive, is forecast or observed over coastal or inland water bodies during the recreational boating season.
The process by which solids are transformed directly to the state of vapour or vice versa, without passing through the liquid phase.
- Subtropical Ridge
A large belt of high pressure located around 30 degrees latitude in both hemispheres, and characterized by mostly calm winds. Air flows out from the ridge toward the upper and lower latitudes of each hemisphere, creating both the trade winds and the westerlies.
- Subtropical Storm
A subtropical storm is a cyclone that has characteristics of both a tropical storm and an extratropical cyclone. Subtropical Cyclones can form in waters normally too cool for tropical cyclones, therefore they are not as restricted to the hurricane season months. With time, the subtropical storm can become tropical.
- Suête Winds - Cape Breton Highlands
Les Suête is an Acadian’s expression, local to the west coastal highlands area of Cape Breton Island, and used to describe the very strong Southeast winds common to that area.
To cool a liquid (i.e. water) below its freezing point without solidification (for example, ice).
Undulating movement of the sea surface that persists after the originating cause (wind-generated waves) of the motion has ceased.
- Tabular Iceberg
Sea-ice terminology describing a flat-topped iceberg; most tabular icebergs show horizontal banding.
- Target Lead Time
The target lead time is Environment Canada’s service goal for the timeliness of alerts. The target lead time is intended to provide enough time for the public to take appropriate action when alerted of a predicted event. Other factors, such as the predictability of an event, and the ability of the public and media to receive notice of the message, may influence the actual lead times provided.
In meteorological terms, temperature refers to the degree of heat or cold of the air, as measured by a thermometer.
- Temperature Anomaly
The deviation of temperature in a given region over a specified period from the long-term average value for the same region.
- Terms Related to Submarine Navigation
- Terms Related to Surface Shipping
- Thaw Holes
Sea-ice terminology describing vertical holes in the ice that are formed when surface puddles melt through to the underlying water.
Layer of the Earth's atmosphere, above the mesopause, in which the temperature generally increases with height.
- Thick First-year Ice
Sea-ice terminology which describes ice that is between 30 cm to 70 cm thick.
- Thick Lake Ice
Sea-ice terminology which describes ice that is between 30 cm to 70 cm thick.
- Thin First-year Ice/White Ice - First Stage
Sea-ice terminology which describes ice that is 30 cm to 50 cm thick.
- Thin First-year Ice/White Ice - Second Stage
Sea-ice terminology which describes ice that is 50 cm to 70 cm thick.
- Thin Lake Ice
Sea-ice terminology which describes ice that is 5 cm to 15 cm thick.
The sound that results from the formation of lightning. This burst of lightning expands air around it, producing an effect similar to an explosion, thus creating the noise.
A local storm, usually produced by a cumulonimbus cloud, and always accompanied by thunder and lightning.
Sea-level instability/movement in an approximately daily or twice daily period. The movement is caused by the difference of the gravitational attraction between celestial bodies and the centrifugal acceleration of their rotation, and is periodic because it is related to the motion of the sun, earth, and moon.
- Tide Crack
Sea-ice terminology. Describes the crack at the line of junction between an immovable ice foot or ice wall, and fast ice. The fast ice being subject to rise and fall of the tide.
Sea-ice terminology meaning an extension of the ice edge up to several kilometres in length, caused by wind or current.
A tornado is a violently rotating column of air that extends from a cumuliform cloud to the surface. The pressure deficit in a tornado often results in the formation of a funnel cloud that extends fully or partially from the cumuliform cloud to the surface. A tornado is typically also made visible by rotating debris near the ground or a spray ring near the water surface.
A tornado can be tens to hundreds of metres wide and have a lifespan of minutes or hours. In terms of size and area, it is one of the least extensive of all storms, but in terms of how violent storms can be, it is the world's most severe.
Canada has the second highest tornado occurrence rate in the world, behind the United States. In Canada, tornadoes occur mostly on the Prairies and in southern Ontario with a peak frequency in late June and July.
- Tornado Warning
A warning issued by Environment Canada’s Meteorological Service (MSC), when at least one tornado is imminent, as indicated by observations, reports and/or radar scans.
- Tornado Warning (Marine)
A warning issued by Environment Canada’s Meteorological Service (MSC), when tornadoes are forecast or observed over a localized marine area, or when an existing tornado is moving from land to an adjacent marine area.
- Tornado Watch
A watch issued by Environment Canada’s Meteorological Service (MSC) when severe thunderstorms are forecast, and conditions are favourable for one or more tornadoes to be spawned from those thunderstorms within a defined “watch zone.”
- Tornado Watch (Marine)
A watch issued by Environment Canada’s Meteorological Service (MSC) to give advance notice of conditions that are favourable for the localized development of tornadoes over, or in the vicinity of a marine area.
- Towering Cumulus Clouds
Cauliflower-shaped clouds associated with intermittent-type precipitation (showers). They may appear to be tall and towering, or tall and broad, and can either be isolated, or grow from a group of lower cloud called cumulus.
- Trade Winds (also called Tropical Easterlies)
The belts of wind on either side of the equator, blowing from the northeast in the Northern Hemisphere, and from the southeast in the Southern Hemisphere. In both hemispheres the winds become more easterly the closer they are to the equator.
- Tropical Cyclone
These non-frontal cyclones originate over tropical or sub-tropical waters, and are identified by their organized showers and thundershowers, mixed with a cyclonic surface wind circulation. They are unlike extratropical cyclones in that they are generally symmetric in their temperature, rain and wind patterns.
The rainfall in tropical cyclones is not dependent on the intensity of the storm. The driving mechanism for a tropical cyclone is the release of latent heat.
Tropical cyclone systems typically last a week or more.
- Tropical Cyclone Information Statement
Routine bulletins issued by Environment Canada’s Canadian Hurricane Centre (CHC) when a tropical cyclone could bring high winds or seas to Canadian waters; or high winds, heavy rain, dangerous coastal waves, or coastal storm surges to Canadian territory within the next 72 hours.
- Tropical Depression
A tropical cyclone with maximum sustained surface winds of 37 to 62 km/h. It does not typically have the organization or the spiral shape of the more powerful storms. When a tropical cyclone reaches this stage it is given a number so that it can be better recognized.
- Tropical Disturbance
An organized region of showers and thunderstorms in the tropics, generally 200 km to 600 km in diameter, that maintains its identity for at least 24 hours but does not have a circular wind circulation.
- Tropical Easterlies (also called Trade Winds)
The belts of wind on either side of the equator, blowing from the northeast in the Northern Hemisphere, and from the southeast in the Southern Hemisphere. In both hemispheres the winds become more easterly the closer they are to the equator.
- Tropical Storm
A tropical cyclone with maximum sustained surface winds of 63 to 117 km/h. When a tropical cyclone reaches this stage it is given a name so that it can be better recognized.
- Tropical Storm Warning
A warning issued by Environment Canada’s Meteorological Service (MSC) indicating that tropical storm conditions, including possible sustained winds within the range 63 km-117 km/h, are expected in specified areas within 24 hours or less. By nature, a tropical storm warning also implies the threat of local flooding from heavy rainfall.
- Tropical Storm Watch
A watch issued by Environment Canada’s Meteorological Service (MSC) for a specific area that a tropical storm or a developing tropical storm poses a possible threat within 36 hours.
- Tropical Wave (also called Easterly Wave or African Easterly Wave)
A type of atmospheric trough, oriented from north to south, which moves from east to west across the tropics causing areas of cloudiness and thunderstorms. A tropical wave can develop into a tropical cyclone.
The boundary between the troposphereand the stratosphere, where an abrupt change in lapse rate usually occurs.
The lowermost layer of the atmosphere, in which air temperature falls steadily with increasing altitude. The troposphere begins at ground level and ranges in height from an average of 11 km (at the International Standard Atmosphere) at the poles to 17 km at the equator.
An elongated area of relatively low pressure, extending from the centre of a low pressure region. It is the opposite of a ridge.
A gravitational sea wave produced by any large-scale, short-duration disturbance of the ocean floor. It is often caused by a shallow submarine earthquake but can also be caused by submarine earth movement, subsidence, or volcanic eruption
The vertical motion of the air, at times violent, which can cause the up-and-down movement of a plane.
The name given to hurricanes in the western North Pacific Ocean, west of the International Date Line.
- Ultraviolet (UV) Radiation
Invisible electromagnetic radiation with a frequency between that of visible violet light and x-rays.
Most of the ultraviolet component of sunlight is absorbed by the ozone layer of the atmosphere, however UV-B radiation can cause sunburn and skin cancer, and UV-A radiation can cause photosensitivity reactions and possibly skin cancer.
A turbulent, convective state in the atmosphere, resulting from a rapid decrease in air temperature with the altitude.
UTC is the international abbreviation for Coordinated Universal Time. It is the local time at the Greenwich meridian (0°), situated in United Kingdom. The World Meteorological Organization (WMO) has adopted the UTC as the standard time for use in reporting of all meteorological data. Times in UTC sometimes get the suffix "Z", i.e. 16Z. From the "Z" suffix came the mnemonic "Zulu" (as used in international marine communications standards).
- Veering Wind
A clockwise change in wind direction. For example, from southeast to southwest, through south. It is the opposite of backing wind.
- Vertical Wind Shear
The condition produced by a change in wind velocity (speed and/or direction) with height. Vertical wind shear can weaken or destroy a tropical cyclone, by interfering with its symmetric nature and organization.
- Very Close Pack/Drift
Sea-ice terminology that describes floating ice in which the concentration is from 9/10 to less than 10/10.
- Very Large Iceberg
Sea-ice terminology. Describes a piece of glacier ice that extends more than 75 m above sea level, and has a length of more than 200 metres.
- Very Open Drift
Sea-ice terminology describing ice in which the concentration is 1/10 to 3/10 and water dominates over ice.
- Very Weathered Ridge
Sea-ice terminology describing a ridge with tops very rounded. The slope of its sides is usually from 20 to 30 degrees.
The greatest distance at which a black object of suitable dimensions can be seen and recognized against the horizon sky during daylight. It could also be seen and recognized during the night if the general illumination were raised to the normal daylight level.
A type of alert issued by Environment Canada’s Meteorological Service (MSC), where a hazardous weather or environmental event that poses a significant threat to public safety and property is certain or imminent.
A type of alert issued by Environment Canada’s Meteorological Service (MSC), where conditions are favourable for the development of weather or an environmental hazard that poses a significant threat to public safety and property, but the occurrence, location, and/or timing of the expected hazardous condition(s) is still too uncertain to issue a warning. It is intended to heighten public awareness of the potential impact of the event, and serves as a lead-up to a warning.
- Water Sky
Sea-ice terminology. It describes dark streaks on the underside of low clouds, indicating the presence of water features, in the vicinity of ice.
A tornado that occurs over water. Unlike tornadoes over land, they can occur frequently in the fall months when water temperatures remain warm relative to air temperatures in the lower atmosphere.
- Waterspout Watch
A watch issued by Environment Canada’s Meteorological Service (MSC) to give advance notice when conditions are favourable for the development of waterspouts over a marine area.
- Wave Height
The vertical distance between the trough and crest of a wave.
- Wave Period
The time it takes for two successive wave crests to pass a fixed point.
The distance between two successive crests or troughs of a wave.
- Weather and Meteorology
Meteorology is the science that studies the processes and phenomena of the atmosphere. Weather is the state of the atmosphere at a given time and place with regard to temperature, atmospheric pressure, humidity, wind, cloudiness, and precipitation. The term weather is used mostly for conditions over short periods of time.
- Weather Summary
A bulletin issued by Environment Canada’s Meteorological Service (MSC), which contains a post-event summary of a weather event, especially one that has received significant media attention.
- Weathered Ridge
Sea-ice terminology describing a ridge with peaks slightly rounded, and slope of its sides that is usually 30 to 40 degrees. Individual fragments of ice are not visible.
Sea-ice terminology. Describes the processes of ablation and accumulation, which gradually eliminate irregularities in an ice surface.
- Wedged Iceberg
Sea-ice terminology. Describes an iceberg which is rather flat on top, with steep vertical sides on one end, sloping to less steep sides on the other end.
The dominant west-to-east motion of the atmosphere, centered over the middle latitudes of both hemispheres.
The horizontal movement of air, relative to the earth's surface.
- Wind Chill
Chill that results from a specific combination of wind speed and air temperature, expressed by the loss of body heat in watts per square metre (of skin).
- Wind Chill Index
Index used to determine the relative discomfort resulting from a specific combination of wind speed and air temperature, expressed by the loss of body heat in watts per square metre (of skin).
- Wind Shear
The change of wind speed and/or wind direction over a given distance.
- Wind Warning
A warning issued by Environment Canada’s Meteorological Service (MSC) for sustained winds of speeds that pose a significant threat to public safety and property.
Windward or upwind, is the direction from which the wind is blowing; the opposite of leeward.
- Winter Storm Warning
A warning issued by Environment Canada’s Meteorological Service (MSC) for a major snowfall, or significant snowfall combined with freezing rain, strong winds, blowing snow, and/or extreme wind chill. The mix of these winter weather conditions poses a threat to public safety and property. Winter storm conditions are not necessarily restricted to the winter season, but may occur in the late autumn and early spring as well.
- Winter Storm Watch
A watch issued by Environment Canada’s Meteorological Service (MSC) in advance of a winter storm warning, to alert the public of the possibility of a potential winter weather storm.
- Wreckhouse Winds
Wreckhouse is located between Cape Ray and St Andrew's in Southwestern Newfoundland. This area is, at times, subject to extremely violent southeast winds, which have been strong enough to blow railcars off their tracks.
- Young Coastal Ice
Sea-ice terminology. Describes the initial stage of fast ice formation consisting of nilas or young ice; its width varying from a few metres up to 200 metres from the shoreline.
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