Weather Terms

Air Mass - A large body of air with only small horizontal variations of temperature, pressure, and moisture. It may extend over an area of several million square kilometres and over a depth of several kilometres.

Arctic Sea Smoke - A dense and often extensive steam fog occurring over high-latitude ocean areas in winter.

Aurora - A bright and ever changing display of light caused by solar radiation interacting with the upper atmosphere in the region of the poles. It is called aurora borealis in the Northern Hemisphere and aurora australis in the Southern Hemisphere.


Azores High - Name given the subtropical anticyclone when it is situated over the eastern part of the North Atlantic Ocean.


Backing Wind - Counter-clockwise change of wind direction, in either hemisphere.


Beaufort Scale - Wind force scale, originally based on the state of the sea, expressed in numbers from 0 to 12.


Bermuda High - Name given the subtropical high in the North Atlantic during the summer when it is centered near the island of Bermuda.

- A violent and extremely cold wind laden with dry snow picked up from the ground.


Buys Ballot’s Law - With your back to the wind in the Northern Hemisphere, low pressure will be to your left and high pressure to your right. The reverse is true in the Southern Hemisphere.


Ceiling - The height ascribed to the lowest layer of clouds or obscuring phenomena when the sky is reported as broken, overcast, or obscured and the clouds are not classified “thin” or “partail.” The ceiling is termed unlimited when the foregoing conditions are not present.


Cirrus - One of three basic cloud forms; also one of the three high cloud types. They are thin, delicate ice-crystal clouds often appearing as veil-like patches or thin, wispy fibers.

Cloud - A form of condensation best described as a dense concentration of suspended water droplets or tiny ice crystals.


Coriolis Effect - The deflective effect of Earth’s rotation on all free-moving objects, including the atmosphere and oceans. Deflection is to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.


Cumulus - One of three basic cloud forms; also the name given one of the clouds of vertical development. Cumulus are billowy, individual cloud masses that often have flat bases.


Dew Point - The temperature to which air has to be cooled in order to reach saturation.


Fetch - Distance along a large water surface trajectory over which a wind of almost uniform direction and speed blows.


Fog - Suspension of very small, usually microscopic water droplets in the air, generally reducing the horizontal visibility at the Earth's surface to less than 1 km.


Front - The interface or transition zone between air masses of different densities (temperature and humidity).


Gale Force Wind - Wind with a speed between 34 and 47 knots. Beaufort scale wind force 8 or 9.


Gust - Sudden, brief increase of the wind speed over its mean value.


Haze - Suspension in the atmosphere of extremely small, dry particles which are invisible to the naked eye but numerous enough to give the sky an opalescent appearance.


High - Region of the atmosphere where the pressures are high relative to those in the surrounding region at the same level.


Hurricane - Name given to a warm core tropical cyclone with maximum surface winds of 118 km/h (64 knots) or greater in the North Atlantic, the Caribbean, the Gulf of Mexico and in the Eastern North Pacific Ocean.


Knot - Unit of speed equal to one nautical mile per hour. (1.852 km/h)


Land Breeze - Wind of coastal regions, blowing at night from the land towards a large water surface as a result of the nocturnal cooling of the land surface.


Line Squall - Squall which occurs in a line.


Low - Region of the atmosphere in which the pressures are lower then those of the surrounding regions at the same level.


Mist - Suspension in the air of microscopic water droplets which reduce the visibility at the Earth's surface.


Pressure - Force per unit area exerted by the atmosphere on any surface by virtue of its weight; it is equivalent to the weight of a vertical column of air extending above a surface of unit area to the outer limit of the atmosphere.


Ridge - Region of the atmosphere in which the pressure is high relative to the surrounding region at the same level.


Sea Breeze - Wind in coastal regions, blowing by day from a large water surface towards the land as a result of diurnal heating of the land surface.


Sea Fog – Fog which forms in the lower part of a moist air mass moving over a colder surface water.


Sea State - Local state of agitation of the sea due to the combined effects of wind and swell.


Squall - Atmospheric phenomenon characterizes by an abrupt and large increase of wind speed with a duration of the order of minutes which diminishes suddenly. It is often accompanied by showers or thundershowers.


Storm Force Wind - Wind with a wind speed between 48 and 63 knots. Beaufort scale wind force 10 or 11.


Storm Surge - The difference between the actual water level under influence of a meteorological disturbance (storm tide) and the level which would have been attained in the absence of the meteorological disturbance (i.e. astronomical tide).


Suête - Southeasterly winds blowing across the Cape Breton highlands that frequently accelerate into down slope wind storms. Suête events can occur year round but tend to be more frequent and stronger in winter months. Gusts in excess of 90 km/h occur several times a month on winter. Wind gusts of 147 km/h (80 knots), or more, have been recorded at least once in each year for the last 15 years. Gusts in excess of 160 km/h (87 knots) were recorded at least once in 11 of those years and gusts in excess of 200 km/h (109 knots) in 2 of the last 15 years.

Swell - Any system of water waves which has left its generating area.

Thunderstorm - Sudden electrical discharge manifested by a flash of light and a sharp or rumbling sound. Thunderstorms are associated with convective clouds and are, more often, accompanied by precipitation in the form of rain showers, hail, occasionally snow, snow pellets, or ice pellets.


Tropical Cyclone - Generic term for a non-frontal synoptic scale cyclone originating over tropical or sub-tropical waters with organized convection and definite cyclonic surface wind circulation.


Tropical Depression - Wind speed up to 33 knots.


Tropical Disturbance - Light surface winds with indications of cyclonic circulation.


Tropical Storm - Maximum wind speed of 34 to 47 knots.


Trough - An elongated area of relatively low atmospheric pressure.


Veering - Clockwise change of wind direction, in either hemisphere.


Visibility - Greatest distance at which a black object of suitable dimensions can be seen and recognized against the horizon sky during daylight or could be seen and recognized during the night if the general illumination were raised to the normal daylight level.


Waterspout - A phenomenon consisting of an often violent whirlwind revealed by the presence of a cloud column or inverted cloud cone (funnel cloud), protruding from the base of a cumulonimbus, and of a bush composed of water droplets raised from the surface of the sea. Its behaviour is characterized by a tendency to dissipate upon reaching shore.


Wave Height - Vertical distance between the trough and crest of a wave.


Wave Periods - Time between the passage of two successive wave crests past a fixed point.