Extratropical cyclone

Acid rain

Cloud or rain droplets containing pollutants, such as oxides of sulfur and nitrogen, to make them acidic (eg. pH < 5.6).

Arctic air

A mass of very cold, dry air that usually originates over the Arctic Ocean north of Canada and Alaska.

Ball lightning

A relatively rarely seen form of lightning, generally consisting of an orange or reddish ball of the order of a few cm to 30cm in diameter and of moderate luminosity, which may move up to 1 m/s horizontally with a lifetime of a second or two.

Barometer

An instrument for measuring atmospheric pressure.

Barometric pressure

The actual pressure value indicated by a pressure sensor.

Bitterly cold

In winter, bitterly cold or very cold, refers to more than seven degrees Celsius below normal.

Black ice

Thin, new ice that forms on fresh water or dew covered surfaces; it is common on roadways during the fall and early winter and appears "black" because of its transparency.

Blizzard

Includes winter storm conditions of sustained winds or frequent gusts of 35 mph or more that cause major blowing and drifting of snow, reducing visibility to less than one-quarter mile for 3 or more hours. Extremely cold temperatures often are associated with dangerous blizzard conditions. In Canada, a blizzard described as a severe storm that lasts three or more hours, and brings low temperatures, strong winds and poor visibility due to blowing snow. In Australia, it is described as a violent and very cold wind which is loaded with snow, some of which has been raised from snow covered ground.

Blowing Dust

Dust that is raised by the wind to moderate heights above the ground to a degree that horizontal visibility decreases to less than seven miles. Visibilities of 1/8 mile or less over a widespread area are criteria for a Blowing Dust Advisory.

Blowing sand

Sand particles picked up from the surface of the earth by the wind to moderate heights above the ground, reducing the reported horizontal visibility to less than seven statute miles.

Blowing snow

Wind driven snow that reduces visibility to six miles or less causing significant drifting. Blowing snow may be snow that is falling and/or loose snow on the ground picked up by the wind. In Canada, it is described as snow that is lifted by the wind from the earth's surface to a height of two meters or more.

Blustery

Descriptive term for gusty winds that accompany cold weather.

Breezy

Wind in the range of 15 miles per hour to 25 mile per hour with mild or warm temperatures.

Brisk

Wind in the range of 15 to 25 miles per hour when the temperature is cold.

Broken clouds

Clouds which cover between 5/8ths and 7/8ths of the sky.

Calm

The absence of apparent motion in the air.

Cirrus cloud

High cloud, delicate, hair-like and feathery looking.

Clear

Sky condition of less than 1/10 cloud coverage. In the United Kingdom, clear is defined as “No cloud”, and in Australia, it is defined as “Virtually cloud-free”.

Climate

The prevalent long term weather conditions in a particular area. Climatic elements include precipitation, temperature, humidity, sunshine and wind velocity and phenomena such as fog, frost, and hail storms. Climate cannot be considered a satisfactory indicator of actual conditions since it is based upon a vast number of elements taken as an average.

Cloudburst

A sudden, intense rainfall that is normally of short duration.

Cloudy

Cloudy means that clouds cover more than 60 percent of the sky.

Cold front

A narrow transition zone separating advancing colder air from retreating warmer air. The air behind a cold front is cooler and typically drier than the air it is replacing.

Condensation

The process by which water vapor becomes a liquid; the opposite of evaporation, which is the conversion of liquid to vapor. In Australia, it is described as a change from a gas to a liquid.

Cumulonimbus Cloud

A vertically developed cloud, often capped by an anvil shaped cloud. Also called a thunderstorm cloud, it is frequently accompanied by heavy showers, lightning, thunder, and sometimes hail or gusty winds. In Australia, it is described as a heavy, puffy, heaped, dark cloud of great vertical depth, often bringing rain. Some have a distinctive anvil shaped head.

Cumulus cloud

A cloud in the shape of individual detached domes, with a flat base and a bulging upper portion resembling cauliflower. In Australia, it is described as a cloud with a woolly, heaped appearance that often produces rain.

Cyclone

An area of low pressure around which winds blow counterclockwise in the Northern Hemisphere. Also the term used for a hurricane in the Indian Ocean and in the Western Pacific Ocean. In Australia, it is described as atmospheric circulations that rotate clockwise in the southern hemisphere, and anti-clockwise in the northern hemisphere. Cyclones are areas of lower pressure and generally associated with stronger winds, unsettled conditions, cloudiness and rainfall.

Dense fog

A fog in which the visibility is less than one-quarter mile.

Dew point

The temperature to which the air must be cooled for water vapor to condense and form fog or clouds.

Draft

A draft is a small gusty air current that moves upward or downward abruptly; hence the terms updraft and downdraft.

Drifting snow

Uneven distribution of snowfall caused by strong surface winds. Drifting snow does not reduce visibility. In Canada, it is defined as snow blown to a height of less than two meters.

Drizzle

Small, slowly falling water droplets, with diameters between.2 and.5 millimeters. In Australia, it is defined as fairly uniform precipitation (rain) composed exclusively of very small water droplets (less than 0.5 mm in diameter) very close to one another.

Drought

Abnormally dry weather in a region over an extended period sufficient to cause a serious hydrological (water cycle) imbalance in the affected area. This can cause such problems as crop damage and water-supply shortage. In Australia, it is defined as the prolonged absence or marked deficiency of precipitation (rain).

Dry

Free from rain. Normally used when preceding weather has also been relatively dry and dry weather is expected to continue for a day or so.

Dust storm

An area where high surface winds have picked up loose dust, reducing visibility to less than one-half mile. In Australia, it is described as a storm which carries large amounts of dust into the atmosphere.

El Niño

A major warming of the equatorial waters in the eastern Pacific Ocean. El Niño events usually occur every three to seven years, and are related to shifts in global weather patterns. (Spanish for the "Christ Child” because it often begins around Christmas.)

Evaporation

The process of a liquid changing into a vapor or gas.

Extended outlook

A basic forecast of general weather conditions three to five days in the future.

Extratropical cyclone

A storm that forms outside the tropics, sometimes as a tropical storm or hurricane changes. See table below for differences between extratropical and tropical cyclones.

Fahrenheit

The standard scale used to measure temperature in the United States; in which the freezing point of water is 32 degrees and the boiling point is 212 degrees.

Fine

No rain or other precipitation (hail, snow etc.). The use of fine is generally avoided in excessively cloudy, windy, foggy or dusty conditions. In particular note that fine means the absence of rain or other precipitation such as hail or snow - not 'good' or 'pleasant' weather.

Flood

A condition that occurs when water overflows the natural or artificial confines of a stream or river; the water also may accumulate by drainage over low-lying areas.

Flurry

A flurry or snow shower is a snowfall that suddenly stops and starts and changes rapidly in intensity; the accumulation and extent of the snow are limited.

Fog

Water that has condensed close to ground level, producing a cloud of very small droplets that reduces visibility to less than one km (three thousand and three hundred feet). In Canada, fog is defined as a cloud at ground level, and occurs when air is cooled to its dew point and below, or when atmospheric moisture increases through evaporation from water that is warmer than the air. In the United Kingdom, fog is described to occur when visibility less than one kilometer. In Australia, it is defined as a dense mass of small water droplets or particles in the lower atmosphere.

Forecast

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

Freeze

Occurs when the surface air temperature is expected to be 32 degrees Fahrenheit or below over a widespread area for a significant period of time. In Australia, the term “freeze” is defined as the change from a liquid to a solid.

Freezing

The change in a substance from a liquid to a solid state.

Freezing drizzle

Drizzle that falls in liquid form and then freezes upon impact with the ground or an item with a temperature of 32 degrees Fahrenheit or less, possibly producing a thin coating of ice. Even in small amounts, freezing drizzle may cause traveling problems.

Freezing fog

A suspension of numerous minute ice crystals in the air, or water droplets at temperatures below zero degrees Celsius, based at the Earth's surface, which reduces horizontal visibility; also called ice fog.

Freezing level

The altitude in the atmosphere where the temperature drops to 32F.

Freezing rain

Rain that freezes on objects such as trees, cars and roads, forming a coating or glaze of ice. Temperatures at higher levels are warm enough for rain to form, but surface temperatures are below 32 degrees Fahrenheit, causing the rain to freeze on impact. In Canada, freezing rain is described to occur when the air temperature is below zero Celsius near the ground but above zero Celsius higher up.

Front

The boundary or transition zone between two different air masses. The basic frontal types are cold fronts, warm fronts and occluded fronts.

Frost

The formation of thin ice crystals on the ground or other surfaces. Frost develops when the temperature of the exposed surface falls below 32 degrees Fahrenheit and water vapor is deposited as a solid.

Frost point

When the temperature to which air must be cooled to in order to be saturated is below freezing.

Fujita scale

System developed by Dr Theodore Fujita to classify tornadoes based on wind damage. Scale is from F0 for weakest to F5 for strongest tornadoes.

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Gale warning

Sustained wind speeds from 34 to 47 knots (39 to 54 mph).

Glaze

a layer or coating of ice that is generally smooth and clear, and forms on exposed objects by the freezing of liquid raindrops.

Global warming

A theory that increased concentrations of greenhouse gases are causing an elevation in the Earth's surface temperature.

Good visibility

More than 10 kilometers.

Greenhouse effect

The warming of the atmosphere by the trapping of longwave radiation (heat) being radiated to space. The gases most responsible for this effect are water vapor and carbon dioxide.

Ground fog

Shallow fog (less than twenty feet deep) produced over the land by the cooling of the lower atmosphere as it comes in contact with the ground. Also known as radiation fog.

Gust

A brief sudden increase in wind speed. Generally the duration is less than 20 seconds and the fluctuation greater than 10 miles per hour. In Canada, gust is defined as a sudden, brief increase in wind speed that generally lasts less than 20 seconds. In Australia, gust is defined as any sudden increase of wind of short duration, usually a few seconds.

Gustnado

Gust front tornado. A small tornado, usually weak and short-lived, that occurs along the gust front of a thunderstorm. Often it is visible only as a debris cloud or dust whirl near the ground.

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Hail

Precipitation in the form of balls or irregular lumps of ice produced by liquid precipitation, freezing and being coated by layers of ice as it is lifted and cooled in strong updrafts of thunderstorms.

Haze

Fine dust or salt particles in the air that reduce visibility. In Canada, haze is defined to consist of fine particles of dust and pollution suspended in the atmosphere, and is distinguished from fog by its bluish or yellowish tinge.

Heat balance

The equilibrium existing between the radiation received and emitted by a planetary system.

Heat index

An index that combines air temperature and humidity to give an apparent temperature (how hot it feels).

Heat wave

A period of abnormally hot weather lasting several days.

Heavy snow

Depending on the region of the USA, this generally means that four or more inches of snow has accumulated in 12 hours, or six or more inches of snow in 24 hours.

High

An area of high pressure, usually accompanied by anticyclonic and outward wind flow. Also known as an anticyclone. In Canada, a high is described as an area of high atmospheric pressure with a closed, clockwise movement of air.

Humidity

The amount of water vapor in the atmosphere. In Canada, it is described as the amount of moisture in the air.

Hurricane

A severe tropical cyclone with sustained winds over 74 miles per hour (64 knots). Normally applied to such storms in the Atlantic Basin and the Pacific Ocean east of the International Date Line.

Hygrometer

An instrument used to measure humidity.

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Ice pellets

Precipitation of transparent or translucent pellets of ice, which are round or irregular, rarely conical, and which have a diameter of 0.2 inch (five millimeters), or less. There are two main types. Hard grains of ice consisting of frozen raindrops and pellets of snow encased in a thin layer of ice.

Ice storm

Liquid rain falling and freezing on contact with cold objects creating ice build-ups of 1/4th inch or more that can cause severe damage.

Indian summer

An unseasonably warm period near the middle of autumn, usually following a substantial period of cool weather.

Insolation

Incoming solar radiation. Solar heating; sunshine.

Instability

A state of the atmosphere in which convection takes place spontaneously, leading to cloud formation and precipitation.

Intermittent rain

Intermittent rain stops and starts repeatedly, although not as abruptly or as frequently as showers.

Knot

A measure of speed. It is one nautical mile per hour (1.15 miles per hour). A nautical mile is one minute of one degree of latitude.

La Niña

A cooling of the equatorial waters in the Pacific Ocean.

Land breeze

A wind that blows from the land towards a body of water. Also known as an offshore breeze. It occurs when the land is cooler than the water.

Lightning

Any form of visible electrical discharges produced by thunderstorms.

Likely

In probability of precipitation statements, the equivalent of a 60 or 70 percent chance.

Mainly cloudy

More than half cloud cover but with some breaks in the cloud. It can also be described as cloudy with some sunny periods.

Mainly sunny

Mainly sunny means sunny with some cloudy periods.

Mild

In winter, mild refers to four to seven degrees Celsius above normal.

Minimum temperature

The lowest temperature during a specified time period.

Mist

Consists of microscopic water droplets suspended in the air, which produces a thin grayish veil over the landscape. It reduces visibility to a lesser extent than fog. In Australia, it is described as similar to fog, but visibility remains more than a kilometer.

Moderate risk

Severe thunderstorms are expected to affect between five and 10 percent of the area.

Moderate visibility

Five to 10 kilometers’ visibility.

Monsoon

A persistent seasonal wind, often responsible for seasonal precipitation regime. It is most commonly used to describe meteorological changes in southern and eastern Asia.

Overcast

Sky condition when greater than 9/10ths of the sky is covered by clouds. In Canada overcast means grey and dull skies, with extensive cloud cover.

Ozone

A form of oxygen in which the molecule is made of three atoms instead of the usual two. Ozone is usually found in the stratosphere, and responsible for filtering out much of the sun's ultraviolet radiation. It is also a primary component of smog.

Ozone hole

A thinning of the ozone layer over Antarctica, which occurs each spring.

Partly cloudy

Sky condition when between 3/10ths and 7/10ths of the sky is covered. Used more frequently at night. In the United Kingdom, it is defined as less than half cloud cover.

Partly sunny

Similar to partly cloudy. Used to emphasize daytime sunshine.

Polar air

A mass of very cold, very dry air that forms in polar regions.

Polar front

The semi-permanent, semi-continuous front that encircles the northern hemisphere separating air masses of tropical and polar origin.

Precipitation

Liquid or solid water that falls from the atmosphere and reaches the ground.

Pressure

The force exerted by the interaction of the atmosphere and gravity. Also known as atmospheric pressure.

Prevailing wind

The direction from which the wind blows most frequently in any location.

Radiation

Energy emitted in the form of electromagnetic waves. Radiation has differing characteristics depending upon the wavelength. Radiation from the Sun has a short wavelength (ultra-violet) while energy re-radiated from the Earth's surface and the atmosphere has a long wavelength (infra-red).

Radiation fog

Fog produced over the land by the cooling of the lower atmosphere as it comes in contact with the ground. Also known as ground fog.

Rain

Liquid water droplets that fall from the atmosphere, having diameters greater than drizzle (0.5 mm). In Canada the term "rain", used alone, means liquid precipitation of significant duration and extent.

Rain showers

Rain showers stop and start suddenly and vary widely in intensity, and are gone in less than an hour.

Rainbow

Optical phenomena when light is refracted and reflected by moisture in the air into concentric arcs of color. Raindrops act like prisms, breaking the light into the colors of a rainbow, with red on the outer, and blue on the inner edge.

Relative humidity

The amount of water vapor in the air, compared to the amount the air could hold if it was totally saturated. It is expressed as a percentage.

Sandstorm

Particles of sand carried aloft by a strong wind. The sand particles are mostly confined to the lowest ten feet, and rarely rise more than fifty feet above the ground.

Saturation

A condition of the atmosphere in which a certain volume of air holds the maximum water vapor it can hold at a specific temperature.

Scud clouds

Small, ragged, low cloud fragments that are unattached to a larger cloud base and often seen with and behind cold fronts and thunderstorm gust fronts. Such clouds generally are associated with cool moist air, such as thunderstorm outflow.

Sea breeze

A wind that blows from a sea or ocean towards a land mass. Also known as an onshore breeze. It occurs when the land is warmer than the water.

Severe thunderstorm

A strong thunderstorm with wind gusts in excess of 58 mph (50 knots) and/or hail with a diameter of 3/4 inch or more.

Shallow fog

Fog in which the visibility at 6 feet above ground level is 5/8ths of a mile or more.

Shower

Precipitation that is intermittent, both in time, space or intensity.

Sky condition

The state of the sky in terms of such parameters as sky cover, layers and associated heights, ceiling, and cloud types.

Sleet

Rain drops that freeze into ice pellets before reaching the ground. Sleet usually bounces when hitting a surface and does not stick to objects. Forms when snow enters a warm layer of air above the surface and melts and then enters a deep layer of sub freezing air near the surface and refreezes. In Australia, sleet refers to a mixture of rain and snow or falling snow that is melting into rain.

Slight chance

In probability of precipitation statements, usually equivalent to a 20 percent chance.