Atmosphere is 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 - from 10 km to 50 km; the mesosphere - from 50 km to 80 km; and the thermosphere - beyond 80 km.

ATMOSPHERIC PRESSURE

Atmospheric pressure is the force exerted by the weight of the atmosphere, also known as barometric pressure. The internationally recognized unit for measuring this pressure is the kilopascal.

AURORA (AURORA BOREALIS)

Aurora is the luminous, radiant emission from the upper atmosphere over middle and high latitudes, and 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

Backing is a counter-clockwise shift in wind direction; for example, from east to west, through north and is the opposite of veering.

BLIZZARD

A blizzard is a winter storm characterized by significant wind chill, strong winds and poor visibility due to blowing snow. The Meteorological Service of Canada uses the following general criteria for issuing blizzard warnings in Canada:

visibility less than 1 km
strong winds
temperature values vary across the country
duration is three to six hours.

BLOWING SNOW

Snow lifted from the earth's surface by the wind to a height of 2 metres or more is called blowing snow; less than 2 metres is referred to as drifting snow.

CHINOOK

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.

CIRRUS

Cirrus are 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 mares' tails. In forecasts they are referred to as high cloud.

CLIMATE

Climate is the prevalent or characteristic weather conditions of a place or region as determined by the temperature and meteorological changes over a period of years.

CLOUD

A cloud is a visible cluster of tiny water and/or ice particles in the atmosphere above the earth's surface.

CONDENSATION

Condensation is the physical process by which vapour becomes liquid or solid; the opposite of evaporation.

CUMULONIMBUS

Cumulonimbus is a large, fairly dense cloud - frequently with 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

Fair-weather clouds with broad horizontal bases, producing no precipitation and rarely covering more than one-half of the sky.

CUMULUS (TOWERING)

Cauliflower-shaped clouds associated with showery-type precipitation. They may appear to be tall and towering or tall and broad, and can be either isolated or grow from lower lines or decks of cloud.

CYCLONE

In the Northern Hemisphere, a cyclone is a closed counter-clockwise movement of air - known as a circulation - around a low pressure centre; usually called a low. The term is frequently misused to describe a tornado.

DEW

As the surface of the earth cools at night, warm moist air near the ground is chilled and water vapour in the air condenses into droplets on the grass and other objects. Dew is more prevalent on clear nights, when the earth cools rapidly. When a blanket of cloud insulates the earth, the cooling rate is slower.

DEW POINT

Dew point is the temperature to which air must be cooled to become saturated by the water vapour already present in the air.

DRAFT

Draft is a small gusty current associated with the abrupt vertical movement of air; for example, updrafts and downdrafts.

DRIFTING SNOW

Snow raised from the earth's surface by the wind to a height of less than 2 metres is called drifting snow; more than 2 metres is referred to as blowing snow.

DRIZZLE

Drizzle is precipitation from stratus clouds consisting of numerous minute, fine water droplets which appear to float. In drizzle, the droplets are much smaller than in rain.

EYE

The roughly circular area of comparatively light winds and fair weather found at the center of a hurricane is usually referred to as the eye of the storm.

FLURRY

Precipitation in the form of snow from a convective cumulus-type cloud is 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.

FOG

A cloud based at the earth's surface consisting of tiny water droplets or, under very cold conditions, ice crystals or ice fog; generally found in calm or low wind conditions. Under foggy conditions, visibility is reduced to less than 1 km.

FOG BANK

A well-defined mass of fog observed in the distance, most commonly at sea. This does not apply to patches of shallow ground fog which usually occur over land at night or early in the morning.

FORECAST

A forecast provides a description of the most significant weather conditions expected during the current and following day. The exact content depends upon the intended user, such as the marine or farm forecast audiences.

FREEZING RAIN

Rain that freezes upon impact and forms a layer of ice on the ground and objects it strikes. A freezing rainwarning is issued if more than four hours of continuous freezing rain, or seven hours of freezing drizzle, are expected.

FREEZING SPRAY

Freezing spray occurs when a combination of low temperatures and strong winds cause sea spray to freeze on a ship's superstructure or on other structures either in the sea or near the water's edge. A freezing spray warning is issued whenever moderate or heavy ship icing is expected.

FRONT

A front is 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.

FROST

Frost is water vapour which deposits directly as a solid on a surface colder than the surrounding air and which has a temperature below freezing. It is not frozen dew. Killing Frost is a frost severe enough to end the growing season.

FROST WARNING

A frost warning is issued when air temperatures are expected to fall to 2 degrees Celsius or lower during the growing season which is approximately May 15 to October 15.

GALE

A gale is a strong wind. Gale warnings are issued for expected winds of 34 to 47 knots (65-100 km/h) over the water.

GULF STREAM

The warm, relatively swift and narrow ocean current that runs up the east coast of North America, south of Newfoundland and into the North Atlantic is known as the Gulf Stream.

GUST

A gust is a sudden, brief increase in wind speed, generally less than 20 seconds.

HAIL

Precipitation in the form of lumps of ice, mainly associated with thunderstorms is known as hail. The size of hail ranges from a small pea to the size of cherries, but has been observed as large as oranges. Hail in Canada occurs most frequently during the summer when thunderstorm activity is at a peak. Extensive damage in the agricultural industry is caused each year by hailstorms.

HALO

In ages past, the huge rings or haloes around the sun or the moon were thought to portend everything from storms to great personal disasters. We now know they are the optical result of the refraction of light from the sun or moon by ice crystals in very high cloud (9,000 meters) called cirrus or cirrostratus.

On occasion, only two bright spots on either side of the sun can be seen. These are known as sun dogs and are caused when the ice crystals occur in a certain uniform arrangement.

HAZE

Haze is fine dust or salt particles dispersed through a portion of the atmosphere that reduces visibility. Haze is distinguished from fog by its bluish or yellowish tinge.

HEAVY RAINFALL WARNING

A heavy rainfall warning is issued when heavy or prolonged rainfall is sufficient to cause local or widespread flooding. Expect 50 mm of rain over a 12 hour period or less, or 80 mm of rain in less than 24 hours. (50 mm of rain over a 24 hours or less in areas North of 60)

HEAVY SNOWFALL WARNING

Expect a snowfall of 10 cm or more in 12 hours or less, or 15 or more cm in 24 hours. Heavy snowfall warnings may be issued for lesser amounts at the beginning or end of the snow season.

HIGH

In the Northern Hemisphere, an area of high atmospheric pressure with a closed, clockwise movement of air (circulation) is called a high and is also known as an anticyclone.

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.

HURRICANE (Typhoon, Tropical Cyclones,Willy-Willies)

A hurricane is a tropical storm with wind speeds of 65 knots (120 km/h) to 240 knots (460 km/h) and can be thousands of square kilometers in size. These systems usually have a lifespan of several days.

In the North Atlantic, the hurricane season is from May to November, but the majority of storms occur in August, September and October. Although the east coast is the area of Canada most frequented by hurricanes, these storms still average less than one per year over the Atlantic provinces and coastal waters. There have been as many as five hurricanes in one year, however, several years can pass with no tropical storms.

A hurricane warning is issued in the Marine Forecast if winds are expected to exceed 64 knots (115 km/h).

INTERMITTENT RAIN

Intermittent rain starts and stops repeatedly, although not as abrupt or as frequent as showers.

ISOBAR

A line on a weather map or chart connecting points of equal pressure is called an isobar. The large concentric lines on television or newspaper weather maps are isobars.

JET STREAM

A jet stream is an undulating band of strong high-altitude winds which may occasionally exceed 400 km/h. In Canada, the jet stream travels mostly from west to east and may extend over hundreds of kilometers. It has an altitude of about 9,000 metres. Pilots flying at high altitudes often search out the jet stream to speed along eastbound flights.

KILOPASCAL

The internationally recognized unit used by the Meteorological Service of Canada for measuring atmospheric pressure is the kilopascal.

LEEWARD

Leeward is situated away from the wind; downwind and opposite of windward.

LIGHTNING

Generally, any and all of the various forms of visible electrical discharge produced by thunderstorms is known as lightning.

LOW

An area of low atmospheric pressure that has a closed counter-clockwise circulation in the Northern Hemisphere is called a low and is also known as a cyclone.

MARINE WIND WARNINGS

Small Craft Warning - issued if winds are forecast to be in the range of 20 to 33 knots (37 km/h to 61 km/h) inclusive
Gale Warning - issued if winds are forecast to be in the range of 34 to 47 knots (63 km/h to 87 km/h) inclusive
Storm Warning - issued if the winds are forecast to be in the range of 48 to 63 knots (89 km/h to 117 km/h) inclusive
Hurricane Force Wind Warning - issued for winds of 64 knots (119 km/h) or greater

MIST

Mist consists of microscopic water droplets suspended in the air that produces a thin gray veil over the landscape. It reduces visibility to a lesser extent than fog.

NORMAL

A normal is the long-term average value of a climate element for a certain area averaged over a 30 year period. For example, "temperatures are normal for this time of year."

OUTLOOK

An outlook is intended to provide, in very general terms, an indication of a trend in the weather for one to three days beyond the current forecast.

OZONE

Ozone is a chemical in our atmosphere with a dual nature: at high altitudes it benefits us by blocking harmful UV radiation; at ground level it is a harmful pollutant. Ozone is not emitted directly, but forms from reactions between other air pollutants in the presence of sunlight. This ground-level ozone can irritate the lungs causing difficulty breathing, especially for people with asthma or other lung diseases. At high concentrations, ozone can damage rubber, plastics and paint, as well as vegetation.

PREVAILING WINDS

The wind direction most frequently observed during a given period is known as prevailing winds.

PROBABILITY OF PRECIPITATION

Probability forecasts are estimates of the chances of receiving measurable precipitation at some time during the forecast period. Measurable means at least 0.2 mm of rain or the water equivalent of snow. For example, a 40 percent probability of rain today means there are four chances in ten that it will rain.

RAINBOW

Rainbows occur when sunlight is reflected by raindrops. The raindrops act like a prism, breaking the light into the colors of a rainbow, with red on the outer edge, and blue on the inner edge. On occasion, the light is reflected from both the front and back of the raindrops and two rainbows are visible. The color bands in the second rainbow are opposite to those in the primary rainbow.

Rainbows can be seen when the sun is shining and the air contains water spray or raindrops. This condition occurs frequently during or immediately following showers. Rainbows are always observed in the portion of the sky opposite the sun. The sun, the observer's eye, and the center of the rainbow arc, always fall in a straight line. The colors of the rainbow are violet, indigo, blue, green, yellow, orange, and red.

RELATIVE 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.

RIDGE

A ridge is an elongated area of relatively high pressure extending from the center of a high pressure region; the opposite of a trough.

SEA BREEZE

Sea breezes occur during the daytime in warm sunny weather when the air over a land area is heated more rapidly than over an adjacent water surface. As a result, the warmer air rises and relatively cool air from the sea flows onshore to replace it.

At night, the air over the land cools faster than over the nearby ocean and causes the air circulation to be in the opposite direction, called a land breeze. Land breezes are usually weaker than sea breezes and have less impact on the temperature.

SHOWER

Showers are precipitation that 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.

SMALL CRAFT WARNING

A Small Craft Warning is issued when winds over the coastal marine areas are expected to reach and maintain speeds of 20 to 33 knots (37 km/h to 61 km/h) inclusive.

SNOW BELT

Areas where prevailing onshore winds are responsible for heavy snowfall is known as a snow belt and usually refers to regions southeast of open water.

SQUALL

A squall is a strong, sudden wind which generally lasts a few minutes then quickly decreases in speed. Squalls are generally associated with severe thunderstorms.

TEMPERATURE

In general, temperature is the degree of hot or cold measured by a thermometer.

THUNDERSTORM

A local storm, usually produced by a cumulonimbus cloud, and always accompanied by thunder and lightning is called a thunderstorm.

TORNADO

Tornadoes are referred to as funnel clouds until they touch the ground. Tornadoes are spawned by severe thunderstorms and are violent funnel-shaped wind vortexes in the lower atmosphere with upward spiraling winds of high speeds. The tornado usually appears from a bulge in the base of a cumulonimbus cloud. A tornado can be tens to hundreds of metres wide and have a lifespan of minutes to hours. In area, it is one of the least extensive of all storms, but in violence, it is the world's most severe. More tornadoes occur in the United States than in any other country. In Canada, tornadoes occur mostly on the Prairies and in southern Ontario.

TROUGH

A trough is an elongated area of relatively low pressure extending from the center of a low pressure region; the opposite of a ridge.

TURBULENCE

Turbulence is the vertical motion of the air, at times violent, which can cause the up-and-down movement of a plane.

VEERING

Veering, opposite of backing,is a clockwise shift in wind direction; for example, from east to west through south.

VIRGA

Virga is wisps or streaks of precipitation that fall out of a cloud and evaporate before reaching the ground.

WARM FRONT

A warm front moves in such a way that the warmer air replaces the colder air.

WAVE

In meteorology, wave is the intersection of warm and cold fronts.

WEATHER

Weather is the state of the atmosphere with respect to heat or cold, wet or dry, calm or storm, clear or cloudy. Also, weather is the meteorological day-to-day variations of the atmosphere and the effects on life and human activity. It includes temperature, pressure, humidity, clouds, wind, precipitation and fog.

WEATHER WARNING

Weather warnings are announcements separate from and supplementary to routine forecasts issued to warn of weather conditions that may endanger lives and property. Warnings are broadcast by the media, Weatheradio Canada and by the Coast Guard. Warnings are issued for snowstorms, blizzards, heavy blowing snow, heavy rains, frost, freezing rain, severe thunderstorms and strong winds - according to thresholds established for local and regional public needs. In Vancouver, a warning might be issued for an expected snowfall of 5 cm or more, while in Halifax, a warning would only be issued for 15 cm or more of snow.

WIND

Wind is the horizontal movement of air relative to the earth's surface.

WIND CHILL

The combined cooling effect of wind and temperature is called wind chill. The wind chill factor is a measure of this cooling effect. The larger the wind chill factor, the faster the rate of cooling. Note, however, that an inanimate object will not be cooled below the actual air temperature, it will cool to the air temperature in less time.

WIND WARNING

Expect winds to blow steadily at 60 km/hr or more, or gusts of 90 km/hr or more, lasting for at least one hour. In the Lethbridge Crowsnest and Cardston regions of Alberta, expect winds to gust from 70 km/h to 120 km/h, lasting for at least one hour.

WINDWARD

Windward or upwind, is the direction from which the wind is blowing; the opposite of leeward.