Weather Glossary

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Acid rain

Acid rain is more properly called acid precipitation. It occurs when sulphur dioxide and nitrous oxide emissions convert into such pollutants as sulphuric acid and nitric acid. Both dissolve easily in air-born water droplets.


Means actual or expected weather conditions may cause general inconvenience or concern, but do not pose a serious enough threat to warrant a weather warning.  Examples of advisories include Air Quality Advisory, Humidex Advisory, Dust Storm Advisory, and Cold Wave Advisory.  An advisory may also be used when conditions show signs of becoming favourable for severe weather when the situation is not definite enough or too far in the future to justify a warning.

Air mass

An air mass is a large body of air, sometimes extending thousands of kilometres, which has relatively uniform characteristics of temperature and moisture.

Air pressure

Air pressure or atmospheric pressure is the force exerted on an object or person by the weight of the air above. The internationally recognized unit for measuring air pressure is the kilopascal. (See Kilopascals)


At a blacksmith's, the iron block on which the blacksmith hammers out the horseshoes is called an anvil. In weather, mature thunderstorm clouds extend so far up into the atmosphere that high-level winds cause the tops of the clouds to spread out in the shape of an anvil.


The atmosphere is the envelope of air which surrounds the earth and is bound to it by gravity.

Aurora Borealis

The Aurora Borealis or Northern Lights are the shimmering, pulsating lights often seen on clear winter nights in a variety of shapes and colours and usually over countries such as Canada in the middle to higher latitudes. The Aurora Borealis results from the collision between fast-moving particles from the sun and rarefied gases in the upper atmosphere. Green, the most common colour, is caused by the electrons striking very cold oxygen. Pink is produced when nitrogen is hit.


Backing is the name given to a counter-clockwise change in wind direction, such as from the southwest to the south. (See Veering)

Beaufort Wind Scale

This scale for estimating the speed of wind while at sea was originally based on the effect which the winds have on ships and the surface of the water. The scale was later adapted so it could be used on land.

Black ice

Black ice is a thin layer of new ice on a road or on the water's surface. The ice appears dark because it is transparent.


A blizzard is a severe winter storm with high winds, bitter cold, and low visibility because of blowing snow. Across much of southern Canada, Environment Canada calls a severe winter storm a blizzard when the following conditions are expected to last 4 hours or more:  sustained wind speeds or gusts of 40 kilometres an hour or more, widespread reduction of visibility to less than 1 kilometre due to snow and/or blowing snow, and temperatures colder than 0°C.

Blowing snow

Meteorologists use this term when winds lift snow from the ground to a height of 2 metres or more -- which is high enough to affect the visibility.


A breeze is a light wind.


A Chinook is a native Canadian word meaning snow-eater used to describe a class of gusty winds that become warm and dry as they flow down the slopes of a mountain range. In Canada, Chinooks are strongest in southwestern Alberta where they flow through Crowsnest Pass and fan out over the southern part of the province. A Chinook can raise the temperature by as much as 22°C in a few hours, melting snow and bringing spring like weather in the darkest months of winter.


It is the long-term average that describes the kind of weather or characteristic meteorological conditions you can expect in an area, region, province, or country. The climate is the synthesis of day-to-day weather conditions in a particular area and is represented by the collection of statistics over a period of time -- often 30 years or more.


A cloud is a visible cluster of tiny water droplets or ice particles above the earth's surface.

Cold Front

When a cold air mass is coming in and pushing out a warmer air mass, the boundary between the two air masses is called a cold front.


Condensation is the physical process through which water vapour becomes a liquid.


Convection is the process in which air rises because it is warmer and therefore lighter than the air around it.

Coriolis force

The Coriolis force is named after the French scientist Gaspard Gustave de Coriolis, who, in 1835, discovered that the earth's daily rotation on its axis exerted a force on all free-moving objects such as wind. In the Northern Hemisphere, the earth's rotation deflects the winds to the right and in the Southern Hemisphere, to the left.


The word cyclone comes from the Greek word kyllon which means cycle, circle or coil of a snake. In the Northern Hemisphere, the counter-clockwise movement of air around and into any low pressure system is called cyclonic circulation. A low which intensifies in the tropics is called a Tropical Cyclone; if the storm's winds reach 120 kilometres per hour or more, the storm is called a hurricane. In the Arabian Sea, Indian Ocean and around Australia, hurricanes are called cyclones (See Hurricane, and Typhoon)

Deep low

The term is used to describe a low pressure system with much lower air pressure at the centre than in the surrounding air mass. Because of the steep rate at which the pressure drops as you move in towards the centre of a deep low, the winds rushing towards the centre are strong.


A depression is an area of low pressure.

Developing low

The term is used to describe a low pressure area in which the air pressure at the centre continues to drop. This causes the winds around it to increase in speed.

Dew-point temperature

The temperature at which the air, when cooled, becomes saturated and the water vapour in it condenses into water droplets or, if the temperature is cold enough, into ice crystals. This last process is called sublimation. (See Condensation and Sublimation).

Dog days

The dog days of summer are those days when the heat is the greatest, usually between mid-July and early September. They are named for Sirius, the Dog Star, which is the brightest star in the sky. Sirius is hotter and more massive even than our sun. The ancient Greeks and Romans believed that Sirius caused summer, as its hottest days coincide with the emergence or rising of this star after being lost in the sun's luminance.

Doppler weather radar

The Doppler weather radar is named after the Austrian physicist J.C. Doppler who hypothesized that the frequency of acoustic or sound waves from a moving source -- such as a train -- would increase as they approached a listener and decrease as they moved away. He was right.

As a train comes towards the listener, the whistle's pitch climbs as the motion of the train compresses the sound wave. When the train speeds away, the pitch drops because the sound wave is no longer being compressed by the motion. This process is called a frequency shift and is known as the Doppler effect.

Doppler weather radar uses the same principle to track the size, concentration, and relative motion of precipitation in storm systems. This helps meteorologists identify tornadoes in the early stages of development


This is a column of comparatively cooler air that sinks rapidly toward the ground. Downdrafts are often accompanied by precipitation. (See Updraft)


When precipitation droplets are less than 0.5 millimetres in diameter, they are called drizzle.


A drought is an extended period of dry weather which lasts longer than normal and leads to measurable losses in businesses such as farming.

Dust devil

A dust devil is a small and vigorous but usually short-lived whirlwind. You can see it because, as its name suggests, a dust devil picks up dust and debris as it moves across the countryside. Unlike tornadoes, dust devils do not drop from thunderclouds but are caused by intense heating of the ground. Dust devils have an average height of 200 metres and diameters which range from about three metres to 30 metres.

El Nino

El Nino is Spanish for little boy and it is what local South American fisherman call a warmer than usual current along the western coast of that continent at Christmas time. Most years, the strong and prevailing trade winds blow westward dragging the warmest surface waters across the Pacific to Australia and Indonesia. But every 2 to 7 years, these trade winds weaken or change direction. This allows the warm waters to change direction and head toward the coast of South America, increasing water temperatures there as much as 5°C. This causes changes in atmospheric pressure which, in turn, trigger a shift in global weather patterns. (See La Nina)


This is the name given to the moment when the sun passes directly above the equator with the result that day and night are the same length all over the world. The vernal equinox occurs in the spring, usually around March 21. The autumnal equinox occurs in the fall, around September 22.


This is the change of state of a liquid, such as water, into a vapour.

Eye of the storm

In a severe tropical storm such as a hurricane, there is a roughly circular area right in the centre, between six and 60 kilometres in diameter, where the winds are comparatively light and the weather is fair. This is called the eye of the storm. (see Hurricane)


Flurries or snow flurries are winter's equivalent of rain showers. They start and stop suddenly and change quickly in intensity from light to heavy.


Essentially, fog is a ground-level cloud. It consists of tiny water droplets which are suspended in the air and reduce visibility to less than one kilometre.


A forecast provides a description of the most likely weather conditions one is likely to encounter in the near future. The public forecast includes information about the temperature and probability of precipitation, and may also include cloud cover, wind speed, and other weather phenomena.

Freezing rain or drizzle

Freezing rain or drizzle occurs when very cold rain falls and hits objects, such as tree branches or sidewalks, which have temperatures of below 0°C. When this happens, the cold rain freezes on contact with the colder sidewalk, for instance, and forms a coat of ice.


This is the boundary or transition zone between two different air masses.


The term frost is used to describe water vapour which changes directly into a solid -- without becoming a liquid -- when it touches an object, such as a window pane. For this to happen, the temperature of the window pane must be lower than the surrounding air, which itself must be below 0°C. A killing frost is severe enough to end the growing season. (See Sublimation

Fujita Scale

This scale describes the intensity of tornadoes based on the damage that they cause. The scale ranges from F0 where damage is light to F5 where damage is catastrophic. The scale was developed by American scientist Tetsuya (Ted) Fujita, a pioneer in tornado research. As of April 1, 2013, this scale has been replaced by the Enhanced Fujita Scale in Canada.

Enhanced Fujita Scale

            This scale is an updated and improved version of the Fujita Scale. While the levels of intensity, ranging from EF0 to EF5, have the same relationship to damage as the original F-scale, the associated wind speeds have been made more accurate. It has been in use in Canada since April 1, 2013.

Funnel cloud

A funnel cloud is the preliminary stage of a tornado. At this point the funnel does not touch the ground. All tornadoes begin as funnel clouds, but not all funnel clouds become tornadoes.


Funneling is the term used to describe what happens when wind is forced to funnel through a narrow opening with the result that the speed of the wind increases, in some cases dramatically. This process is well known in cities where the streets downtown are lined with skyscrapers.


Meteorologists call a strong wind a gale when its speed ranges from 63 to 87 kilometres an hour.

Greenhouse Effect

This term describes the warming of the lower atmosphere caused by atmospheric gases such as water vapour, carbon dioxide and methane containing the heat from the earth's surface. The natural greenhouse effect keeps the surface temperature of the earth warmer than it would be if all the radiation from the sun escaped immediately.

Groundhog Day

February 2 is the day that famous groundhogs across Canada are supposed to pop put of their burrows and forecast the number of days until spring. According to legend, if the area's groundhog sees his shadow and scurries back down into his burrow then winter will continue for six more weeks, which is not bad by Canada's standards. If he does not see his shadow and stays outside to sniff around, then winter is almost over.

Gulf Stream

A warm and relatively swift and narrow ocean current that runs up the east coast of North America, curves south of Newfoundland, and heads out into the North Atlantic toward Europe.


A gust is a sudden increase in wind speed which usually lasts for less than 20 seconds.


A hail stone is a lump of ice which forms during a thunderstorm when water droplets have been tossed high into the freezing reaches of cumulonimbus or thunder clouds. In Canada, hail stones generally range from the size of peas to the size of oranges but have been as large as 114 millimetres or the size of a grapefruit.


Halos are the coloured or white rings or arcs around the sun or moon. They are caused by light shining through ice crystals in the atmosphere or high, thin cirrus clouds.


The term haze is used when pollutants, fine dust or salt particles in the air reduce visibility. Haze usually has a yellow or bluish tinge.

Heat wave

A heat wave is three or more consecutive days when the maximum temperature is 32°C or more.


In the Northern Hemisphere, this is an area of high pressure characterized by winds which move clockwise out of it. A high is sometimes called an anti-cyclone because its winds move in the opposite direction of a cyclone's winds. (See Low)


This is a way of expressing what hot, humid weather really feels like. The air of a given temperature and humidity is equated in terms of comfort to air with a higher temperature and low humidity. Some people are uncomfortable when the humidex is 30°C. Most people are uncomfortable when the humidex is above 40°C or 45°C.


The humidity is the measure of how much water vapour the air contains.


In the Atlantic and eastern Pacific oceans intense tropical storms with wind speeds of 120 kilometres per hour or more are called hurricanes. They are called typhoons in the western Pacific and cyclones in the Arabian Sea, Indian Ocean and around Australia. Whatever their name, these tropical storms can extend up to thousands of square kilometres in area and last for several days.

In the North Atlantic, the hurricane season starts June 1 but most occur during August, September and October. On average, hurricanes hit the east coast of Canada less than once a year. The most famous hurricane to strike Ontario was Hurricane Hazel on October 15, 1954. In less than 18 hours, more than 178 millimetres of rain fell causing flash floods in creeks and rivers and killing 80 people. (see Eye of the storm)

Ice fog

Ice fog is also known as ice-crystal fog, frozen fog, and frost fog. What ever the name, this fog is composed of the suspended particles of ice that occur at very low temperatures -- usually below -30°C. This type of fog happens in clear, calm weather in high latitudes such as Northern Canada. The sun is usually visible, sometimes with a halo.

Ice pellets

This is the term Canadians use to describe frozen rain drops which are five millimetres or less in diameter and bounce when they hit a hard surface. Americans call this sleet.

Indian summer

This term is more than 200 years old and probably originates from a time when native people in North America relied on these warm days to harvest their crops. Today, Indian summer refers to the sunny, dry and unusually warm days which occur in the autumn and are usually preceded by a snowfall, hard frost or spell of cool, windy and stormy weather. During an Indian summer, the winds are either light or calm and the air is often hazy blue or smoky.


In the troposphere, temperatures usually decrease the higher you go. With an inversion the temperature increases with altitude.


Isobars are those lines on weather maps which connect points of equal pressure.

Jet stream

This is an undulating band of strong winds at high altitudes which may extend for hundreds of kilometres. The jet stream may have wind speeds exceeding 400 kilometres per hour. In Canada, the jet stream travels mostly from west to east and at altitudes of 9,000 metres or 9 kilometres.

Killing frost

This frost is severe enough to end the growing season, usually when the temperature falls below -2°C. (See Frost


A kilopascal is the internationally recognized unit for measuring air pressure. The unit is named after Blaise Pascal, a seventeenth-century French scientist who proved that air pressure decreased with altitude. (See Air Pressure)


A knot is a unit of speed equal to one nautical mile per hour or 1.9 kilometres per hour.

La Nina

Every four to five years or so, a pool of cooler than normal water replaces the warmer than normal El Nino current off the west coast of South America. This pool of water is called La Nina or girl child and may be as much as 2°C lower than the average sea surface temperature of 28°C. In contrast to El Nino, La Nina brings colder winters to western Canada and Alaska and drier, warmer weather to the American south-east. (See El Nino)

Lake breeze

This is the breeze which blows from the sea or large lake to the land. A lake 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 is replaced by the cooler air from over the water. (Also called sea breeze)

Lake effect

This is term used to describe the modifying effect large lakes, such as the Great Lakes, have on the weather along their shores and for some distance down wind.

Land breeze

This coastal breeze blows from the land to the sea or lake 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.


The term used to describe an area of low pressure, a depression or a cyclone. In the Northern Hemisphere, the winds blow counter-clockwise around and into a low pressure area. (See High and Cyclone)

Mean temperature

The mean temperature is the average temperature of the air during a specific period of time, usually a day, month or year. The mean temperature for any given day, for example, is an average of the highest and lowest recorded during that 24-hour period.


The study of the atmosphere and the changes which take place within it.


Mist looks like a thin gray veil but it is actually microscopic water droplets which are suspended in the air. Mist reduces visibility to a lesser extent than fog, usually 1 to 10 kilometres.


Meteorologists use the term normal to refer to the average value of a meteorological element such as temperature or precipitation for a certain area and over a fixed number of years -- usually 30 years.

Northern Lights

See Aurora Borealis


The outlook is a term used by meteorologists to refer to the anticipated trend in the weather for the period immediately following the current forecast.


Ozone is a pungent-smelling, slightly bluish gas which is a close chemical cousin to oxygen. About 90 per cent of the earth's ozone is located in a natural layer high above the surface of the globe in region of the atmosphere called the stratosphere. Here, it protects the earth and all that lies within it from the harmful effects of the sun's ultra-violet radiation by absorbing much of it.


Parhelion are more commonly called sundogs. They are the luminous spots which appear on either side of sun when its rays shine through the ice crystals floating in the air during the cold winter months.

Polar night

A polar night is the length of time -- longer than 24 hours -- that the sun stays below the horizon. For example, at Resolute in the Northwest Territories, the polar night lasts from November 9 to February 2.


Precipitation is any form of water -- liquid or solid -- that falls from the atmosphere and reaches the earth. Forms of precipitation include snow, ice pellets, freezing rain, freezing drizzle, rain and drizzle.

Prevailing winds

This is the direction from which the winds blow most frequently during a given period of time. For example, the prevailing winds in Canada come from the west.

Probability of Precipitation or POP

The probability of precipitation is a subjective estimate of the likelihood that a measurable amount of precipitation - that is 0.2 millimetres or more -- will fall sometime during the day at any given spot in the forecast area. For example, a 40 per cent probability of showers means there are four chances in 10 of getting wet at your school.


The term radar is short for RAdio Detection And Ranging. This technology was developed just prior to the Second World War as a method to detect and locate hostile aircraft and was later refined to track storm systems. Microwave energy is transmitted in bursts by the antenna -- the amount of energy received back is directly proportional to the size and concentration of the precipitation that reflected it.


Rainbows form when sunlight is refracted, then reflected by raindrops, breaking the white light into the colours of the spectrum. On occasion, the coloured light is reflected from both the front and the back of the raindrops and 2 rainbows are visible. When this happens the bands of colour in the second rainbow are displayed in the opposite order of those in the primary rainbow which displays red on the outside and violet on the inside. To see a rainbow, you must have your back to the sun and face the rain shower.

Relative humidity

The relative humidity is the ratio of water vapour which is actually in the air to the maximum amount of water vapour which could exist in the air at that temperature. The ratio is usually expressed as a percentage. (See Humidity)


The term refers to an elongated area of high pressure (as in a ridge of high pressure) which extends from the centre of an area of high pressure. A ridge is the opposite of trough.

Sea breeze

(See Lake breeze)

Snow squall

A snow squall is a moderate to heavy snow flurry which is driven by strong gusty winds. Visibility during snow squalls is usually poor. (See Flurry)


This is the area where prevailing onshore winds are responsible for heavy snowfall, usually downwind of the open water.


The winter and summer solstices are the 2 times of the year when the sun at noon is farthest from the equator. In Canada, the shortest day of the year occurs at the winter solstice -- around December 21 -- when the sun is closer to the Tropic of Capricorn in the Southern Hemisphere. The longest day of the year in Canada occurs at the summer solstice around June 21 when the sun is closer to the Tropic of Cancer in the Northern Hemisphere.


A squall is a sudden, strong wind which generally lasts only a few minutes. Squalls are usually but not always associated with severe thunderstorms.

Squall line

A term used to describe a solid or almost solid band of thunderstorms.

Storm surge

This is the abnormal rise in the level of water along the shoreline as a result of strong winds associated with a storm.


This is the direct change of state from a gas, such as water vapour, to a solid, such as ice, and vice versa.


These are the relatively dark spots on the sun's surface.


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


Tornadoes or twisters are violently rotating columns of air that are usually visible hanging from the dark base of thunderstorm clouds. Tornadoes are one of the least extensive of all storms, but in violence, one of the most destructive.

Trade winds

The trade winds are the belts of winds on either side of the equator. In the Northern Hemisphere, the trade winds blow from the north east and in the Southern Hemisphere, they blow from the south east.


A trough is an elongated area of low pressure which extends from a centre of low pressure. (See Ridge)


This the name given to the vertical movement of air -- updrafts and downdrafts -- which is sometimes violent and may cause the up and down movement of an aircraft.


A typhoon is a severe tropical storm in the western Pacific Ocean. (See Hurricane and Cyclone)


An updraft is a current of -- sometimes rapidly -- rising air.

Urban heat island

This phenomenon is caused by the emission of heat from buildings in a city, which leads to higher temperatures there than in the surrounding countryside. The greatest difference in temperature between a city and the adjacent countryside is often on calm, clear winter nights.


This is the term used to describe a clockwise change in wind direction, such as from the southwest to the west. (See Backing)


These are the wisps or streaks of precipitation which fall out of a cloud but evaporate before reaching the ground.

Warm front

This is the boundary or transition zone which separates a warm and cold air mass when the cold air mass is retreating and allowing the warm air mass to move in. (See Front and Cold front)

Watch and Warnings

Environment Canada issues a weather watch when forecasters expect severe and possibility dangerous weather to develop. Forecasters issue weather warnings when severe weather is occurring or about to occur.


A waterspout is a rotating column of vapour and water which extends from thundercloud to the water's surface. A waterspout looks like a tornado but is much smaller and weaker. The diameter of a waterspout ranges from seven to 20 metres and its winds from 40 to 80 kilometres per hour which is strong enough to flip a boat. A waterspout may last up to 10 minutes. Waterspouts, for the most part, form over some of the major lakes of southern Canada during periods of cool, unsettled weather, usually from mid-summer to mid-fall.


Today the term whiteout is often used to describe a blizzard or snowsquall when blowing snow has reduced visibility to a few metres. But this is not a true whiteout. True whiteouts occur mostly in the Arctic and the Antarctic when unbroken snow cover and a uniformly overcast sky combine to produce a solid white glow. People in whiteouts see no shadows or horizon and lose all sense of depth and orientation.

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