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Secrets of the St. Lawrence

Extra - A to Z

Air mass

A large volume of air with uniform properties of temperature and moisture. Air masses extend over thousands of square kilometres.


A swell wave that has broken into foam.


Irregular motion of waves, in which it is difficult, if not impossible, to find one's bearings. Creates a confused sea. Produced on top of rip by wind blowing against the current.

Deep water

Water depths more than one-half the wavelength of a wave.


Minute water droplets suspended in the air that reduce visibility to less than one nautical mile. Fog is cloud on the ground.


The line of separation between 2 air masses with different temperatures and moisture levels. A warm front is a mass of
warm air displacing a mass of cold air, and vice versa.


Same phenomenon as fog, except that visibility is equal to or greater than 1 nautical mile.


A region of high pressure. Air flows outwards and clockwise around high pressure areas. A high is usually associated with
good weather.


Line on a weather map joining points of equal pressure.


A region of low pressure. Winds flow counterclockwise around the low centre. A low pressure centre is usually a storm centre accompanied by precipitation and strong winds.

Main cabin

On a ship or pleasure boat, the main room in which the crew gathers to plan routes and manoeuvres.


A unit used to measure barometric pressure.

Northwest gyre

Slow gyrating motion of sea water. In the Gulf of St Lawrence, this counterclockwise motion may extend over 200 kilometres. The gyre's axis is located in the Northwestern part of the Gulf, between Anticosti and Pointe-des-Monts. Its speed is almost imperceptible: slightly more than 0.10 knot.


Transcription of a message received by radio, or name of the computer system by means of which such messages are transcribed automatically or are pre-recorded. Radiocopy is an exclusive Weatheradio Weathercopy service.


A message transmitted by radio, in particular by Weatheradio or Coast Guard radio.


An elongated area of high pressure, associated with a high.


Turbulence on the sea's surface that results when 2 currents meet.


Combined wind waves and swell. Cross sea - Confused sea formed when one train of waves moves at an angle to other groups of waves. The sea becomes steep, with short, sharp wave crests. When cross seas combine with an underlying tidal current, the sea surface becomes especially confused and hazardous. Also known as cross swell. Choppy sea - Short, sharp, breaking waves. Typically caused by the reflection effect. Breaking sea - Unstable sea resulting from steep, breaking or near-breaking waves.

Significant wave height

Average height of the highest 33 % of all the waves present.


A brief, violent windstorm, often accompanied by rain or snow. Squalls are generally associated with cumulonimbus clouds. If they accompany a fast-moving cold front, they may be of longer duration.

Strong winds

By convention, combination of forces 4, 5 and 6 on the Beaufort Scale to indicate sustained wind speeds in the range of 20 to 33 knots.


An elongated area of low pressure, associated with a low, often produces a wind shift and showery weather.

Wave steepness

Slope of a wave that may not exceed 14 %. Beyond that point, the crest will break and the wave will tumble.


This is the name of Environment Canada's weather information broadcast network. The network has transmitters in every region. Mariners interested in listening to this network need a receiver which can be purchased from electronic equipment dealers. Many portable models are available. Weatheradio signals warnings of severe weather automatically to receivers equipped with special alarm devices for that purpose.


A soft, warm and gentle breeze. For the Ancients, this was the West wind.

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