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Weather Glossary – Terms & Definitions

Terminology used in weather reports, forecasts, and readings. Names and words used to describe the weather and climate.

Acid rain
Cloud or rain droplets containing pollutants, such as oxides of sulfur and nitrogen, to make them acidic (eg. pH < 5.6).
Noon to sunset.
Arctic air
A mass of very cold, dry air that usually originates over the Arctic Ocean north of Canada and Alaska.
Arctic high
A very cold high pressure that originates over the Arctic Ocean.
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.
Barber pole
A thunderstorm updraft with a visual appearance including cloud striations that are curved in a manner similar to the stripes of a barber pole. The structure typically is most pronounced on the leading edge of the updraft, while drier air from the rear flank downdraft often erodes the clouds on the trailing side of the updraft.
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.
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.
Blocking high
A high pressure area (anticyclone), often aloft, that remains nearly stationary or moves slowly compared to west-to-east motion. It blocks the movement eastward movement of low pressure areas (cyclones) at its latitude.
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.
Blowing spray
Water droplets torn by the wind from a body of water, generally from the crests of waves, and carried up into the air in such quantities that they reduce the reported horizontal visibility to less than seven statute miles.
Descriptive term for gusty winds that accompany cold weather.
Wind in the range of 15 miles per hour to 25 mile per hour with mild or warm temperatures.
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.
The absence of apparent motion in the air.
A temperature scale in which zero is the freezing point of water and one hundred is the boiling point.
A Chinook is a warm, dry, gusty wind that occasionally occurs to the leeward side of a mountain range, particularly the Rocky Mountains.
Cirrus cloud
High cloud, delicate, hair-like and feathery looking.
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”.
Clear slot
A local region of clearing skies or reduced cloud cover, indicating an intrusion of drier air; often seen as a bright area with higher cloud bases on the west or southwest side of a wall cloud.
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.
A sudden, intense rainfall that is normally of short duration.
Cloudy means that clouds cover more than 60 percent of the sky.
Coastal flood warning
Issued when there is widespread coastal flooding expected within 12 hours, more than just a typical overwash.
In the winter, cold refers to four to seven degrees Celsius below normal.
Cold Advection (CAA)
Transport of cold air into a region by horizontal winds.
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.
Cold low
A low pressure system with cold air mass from near the surface to all vertical levels (also called a cold core low).
A funnel cloud or (rarely) a small, relatively weak tornado that can develop from a small shower or thunderstorm when the air aloft is unusually cold (hence the name). They are much less violent than other types of tornadoes.
Collar cloud
Frequently used as a synonym for wall cloud, although it actually is a generally circular ring of cloud surrounding the upper portion of a wall cloud.
Combined seas
The combined height of swell and wind waves.
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.
A pattern of wind flow in which air flows inward toward an axis oriented parallel to the general direction of flow. It is the opposite of difluence. Confluence is not the same as convergence. Winds often accelerate as they enter a confluent zone, resulting in speed divergence which offsets the (apparent) converging effect of the confluent flow.
Congestus (or Cumulus congestus)
A large cumulus cloud with great vertical development, usually with a cauliflower-like appearance, but lacking the characteristic anvil shaped top of a Cb (Cumulonimbus cloud).
Continental air mass
A dry air mass originating over a large land area.
The transfer of heat within the air by its movement. The term is used specifically to describe vertical transport of heat and moisture, especially by updrafts and downdrafts in an unstable atmosphere.
An atmospheric condition that exists when the winds cause a horizontal net inflow of air into a specified region. Divergence is the opposite, where winds cause a horizontal net outflow of air from a specified region.
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.
Cumulus congestus
A large cumulus cloud with great vertical development, usually with a cauliflower-like appearance, but lacking the characteristic anvil shaped top of a Cb (Cumulonimbus cloud).
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.
Sunrise to sunset.
Day of the week (eg. Monday)
Midnight to midnight.
Debris cloud
A rotating "cloud" of dust or debris, near or on the ground, often appearing beneath a condensation funnel and surrounding the base of a tornado.
Dense fog
A fog in which the visibility is less than one-quarter mile.
A region of low atmospheric pressure that is usually accompanied by low clouds and precipitation.
Dew point
The temperature to which the air must be cooled for water vapor to condense and form fog or clouds.
Diamond dust
A fall of non-branched (snow crystals are branched) ice crystals in the form of needles, columns, or plates.
A pattern of wind flow in which air moves outward (in a "fan-out" pattern) away from a central axis that is oriented parallel to the general direction of the flow. It is the opposite of confluence.
A disruption of the atmosphere that usually refers to a low pressure area, cool air and inclement weather.
A strong downdraft resulting in an outward burst of damaging winds on or near the ground. Downburst winds can produce damage similar to a strong tornado.
A column of generally cool air that rapidly sinks to the ground, usually accompanied by precipitation as in a shower or thunderstorm.
Downslope wind
Air that descends an elevated plain and consequently warms and dries. Occurs when prevailing wind direction is from the same direction as the elevated terrain and often produces fair weather conditions.
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.
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.
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).
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.
Until two hours after sunrise.
El Niño
A major warming of theequatorial 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.)
The process of a liquid changing into a vapor or gas.
Sunset to midnight.
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.
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.
Describes weather in which there is less than 4/10ths of opaque cloud cover, no precipitation, and there is no extreme visibility, wind or temperature conditions.
Fall wind
A strong, cold, downslope wind.
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.
Flash flood
A flood that occurs within a few hours (usually less than six) of heavy or excessive rainfall, dam or levee failure or water released from an ice jam.
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.
Flood crest
The highest stage or flow occurring in a flood.
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.
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.
A rainbow that has a white band that appears in fog, and is fringed with red on the outside and blue on the inside.
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.
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.
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.
The boundary or transition zone between two different air masses. The basic frontal types are cold fronts, warm fronts and occluded fronts.
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.
Fujiwhara effect
The Fujiwhara effect describes the rotation of two storms around each other.
Funnel cloud
A rotating, cone-shaped column of air extending downward from the base of a thunderstorm but not touching the ground. When it reaches the ground it is called a tornado.
Gale warning
Sustained wind speeds from 34 to 47 knots (39 to 54 mph).
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.
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.
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.
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.
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 island
A dome of elevated temperatures over an urban area caused by the heat absorbed by structures and pavement.
Heat lightning
Lightning that can be seen, but is too far away for the thunder to be heard.
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.
Heavy snow warning
Older terminology replaced by winter storm warning for heavy snow. Issued when seven or more inches of snow or sleet is expected in the next 24 hours. A warning is used for winter weather conditions posing a threat to life and property.
Heavy surf
the result of large waves breaking on or near the shore resulting from swells or produced by a distant storm.
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.
High cloud
Cirrus, cirrostratus and cirrocumulus.
High risk (of severe thunderstorm)
Severe weather is expected to affect more than 10 percent of the area.
High wind warning
Issued when sustained winds from 40 to 73 mph are expected for at least one hour; or any wind gusts are expected to reach 58 miles per hour or more.
High wind watch
Issued when conditions are favorable for the development of high winds over all of or part of the forecast area but the occurrence is still uncertain. The criteria of a high wind watch are listed under the high wind warning and should include the area affected, the reason for the watch and the potential impact of the winds.
Hot or very warm
In summer, hot or very warm means more than seven degrees Celsius above normal.
Hot spot
Typically large areas of pavement, these "hot spots" are heated much quicker by the sun than surrounding grasses and forests. As a result, air rises upwards from the relatively hot surface of the pavement, reaches its condensation level, condenses, and forms a cloud above the "hot spot".
The amount of water vapor in the atmosphere. In Canada, it is described as the amount of moisture in the air.
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.
An instrument used to measure humidity.
Ice crystals
Ice crystals are tiny sprinkles that sparkle in the sunshine like diamond dust and hang in the air.
Ice 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. Usually occurs at -20 degrees Fahrenheit and below.
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.
Ice storm warning
Older terminology replaced by winter storm warning for severe icing. Issued when 1/2 inch or more of accretion of freezing rain is expected. This may lead to dangerous walking or driving conditions and the pulling down of power lines and trees. A warning is used for winter weather conditions posing a threat to life and property.
Indian summer
An unseasonably warm period near the middle of autumn, usually following a substantial period of cool weather.
Inflow bands (or feeder bands)
Bands of low clouds, arranged parallel to the low-level winds and moving into or toward a thunderstorm.
Incoming solar radiation. Solar heating; sunshine.
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.
Intertropical Convergence Zone (ITCZ)
The region where the northeasterly and southeasterly trade winds converge, forming an often continuous band of clouds or thunderstorms near the equator.
An increase in temperature with height. The reverse of the normal cooling with height in the atmosphere. Temperature inversions trap atmospheric pollutants in the lower troposphere, resulting in higher concentrations of pollutants at ground levels than would usually be experienced.
Brilliant patches of green or pink sometimes seen near the edges of high- or medium-level clouds.
Isentropic lift
Lifting of air that is traveling along an upward-sloping isentropic surface. Situations involving isentropic lift often are characterized by widespread stratiform clouds and precipitation.
Jet stream
Strong winds concentrated within a narrow band in the upper atmosphere. It normally refers to horizontal, high-altitude winds. The jet stream often "steers" surface features such as front and low pressure systems.
Wind blowing down an incline, such as down a hillside; downslope wind.
A front (usually a cold front) at which the warm air descents the frontal surface.
Killing frost
Frost severe enough to end the growing season. This usually occurs at temperatures below 28 degrees Fahrenheit. In Canada, a killing frost is described as a frost severe enough to destroy annual plants and new growth on trees (in the spring) or to end the growing season (in the fall).
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.
From sunset (winter), from two hours before sunset (summer).
Situated away from the wind; downwind - opposite of windward.
Left Mover
A thunderstorm which moves to the left relative to the steering winds and to other nearby thunderstorms; often the northern part of a splitting storm.
The forcing of air in a vertical direction by an upslope in terrain or by the movement of a denser air mass.
Lifting condensation level
The level in the atmosphere where a lifted air parcel reaches its saturation point, and as a result, the water vapor within condenses into water droplets.
Any form of visible electrical discharges produced by thunderstorms.
In probability of precipitation statements, the equivalent of a 60 or 70 percent chance.
An area of low pressure, usually accompanied by cyclonic and inward wind flow. Also known as a cyclone.
Low cloud
Stratus, stratocumulus, cumulus and cumulonimbus.
Low-level jet
A region of relatively strong winds in the lower part of the atmosphere.
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.
Mammatus (or mamma clouds)
These clouds appear as hanging, rounded protuberances or pouches on the under-surface of a cloud. With thunderstorms, mammatus are seen on the underside of the anvil. These clouds do not produce tornadoes, funnels, hail, or any other type of severe weather, although they often accompany severe thunderstorms.
Maritime air mass
An air mass that forms over water. It is usually humid, and may be cold or warm.
Maximum temperature
The highest temperature during a specified time period.
Mean temperature
The average of a series of temperatures taken over a period of time, such as a day or a month.
Medium cloud
Altostratus, altocumulus and nimbostratus.
The study of the physics, chemistry, and dynamics of the atmosphere and the direct effects of the atmosphere upon the Earth's surface, the oceans, and life in general.
In winter, mild refers to four to seven degrees Celsius above normal.
Minimum temperature
The lowest temperature during a specified time period.
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.
A persistent seasonal wind, often responsible for seasonal precipitation regime. It is most commonly used to describe meteorological changes in southern and eastern Asia.
Sunrise to noon or midnight to noon depending on context.
Mountain breeze
System of winds that blow downhill during the night.
Colloquially descriptive of warm and especially humid weather.
Multivortex tornado
A tornado in which two or more condensation funnels or debris clouds are present at the same time, often rotating about a common center or about each other. Multiple-vortex tornadoes can be especially damaging.
A thunderstorm with a well-defined anvil rollover, and thus having a visual appearance resembling a mushroom.
Night, overnight
Sunset to sunrise.
No weather data available
No weather information available for this time slot.
The long-term average value of a meteorological element for a certain area. For example, "temperatures are normal for this time of year" Usually averaged over 30 years.
Offshore breeze
A wind that blows from the land towards a body of water. Also known as a land breeze.
Offshore forecast
A marine weather forecast for the waters between 60 and 250 miles off the coast.
Onshore breeze
A wind that blows from a body of water towards the land. Also known as a sea breeze.
Air that flows outward from a thunderstorm.
Outflow winds
Winds that blow down fjords and inlets from the land to the sea.
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.
A condition that exists when a relatively warm air mass moves up and over a colder and denser air mass on the surface. The result is usually low clouds, fog and steady, light precipitation.
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.
A soil layer below the surface of tundra regions that remains frozen permanently.
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.
Polar Stratospheric Clouds (PSCs)
High altitude clouds that form in the stratosphere above Antarctica during the Southern Hemisphere winter. Their presence seems to initiate the ozone loss experienced during the ensuing southern hemisphere spring.
Polar vortex
A circumpolar wind circulation which isolates the Antarctic continent during the cold Southern Hemisphere winter, heightening ozone depletion.
Poor visibility
One to five kilometers’ visibility.
Probability of Precipitation. Probability forecasts are subjective estimates of the chances of encountering measurable precipitation at some time during the forecast period.
Popcorn convection
Clouds, showers and thundershowers that form on a scattered basis with little or no apparent organization, usually during the afternoon in response to diurnal heating.
Liquid or solid water that falls from the atmosphere and reaches the ground.
The force exerted by the interaction of the atmosphere and gravity. Also known as atmospheric pressure.
Pressure change
The net difference between pressure readings at the beginning and ending of a specified interval of time.
Pressure falling rapidly
A decrease in station pressure at a rate of 0.06 inch of mercury or more per hour which totals 0.02 inches or more.
Pressure rising rapidly
An increase in station pressure at a rate of 0.06 inch of mercury or more per hour which totals 0.02 inches or more.
Pressure tendency
The character and amount of atmospheric pressure change during a specified period of time, usually the three-hour period preceding an observation.
Prevailing westerlies
Winds in the middle latitudes (approximately 30 degrees to 60 degrees) that generally blow from west to east.
Prevailing wind
The direction from which the wind blows most frequently in any location.
Pulse storm
A thunderstorm within which a brief period (pulse) of strong updraft occurs, during and immediately after which the storm produces a short episode of severe weather. These storms generally are not tornado producers, but often produce large hail and/or damaging winds. See overshooting top, cyclic storm.
Qualitative forecasts
Forecasts that provide only a categorical value for the predicted variable. Examples of this include “rain/no rain” and “cloudy/partly cloudy”.
Quantitative forecasts
Forecasts in which the “amount” of the forecast variable is specified.
Quantitative precipitation forecast (QPF)
A forecast of rainfall, snowfall or liquid equivalent of snowfall.
Quasi-biennial oscillation (QBO)
Periodic variation in the direction, either easterly or westerly, of tropical lower stratospheric winds. The direction changes every 26 months.
Quasi-nonhydrostatic (QNH)
Pilots, air traffic control (ATC) and low frequency weather beacons use this pressure setting to refer to the barometric altimeter setting that causes the altimeter to read altitude above mean sea level within a certain defined region.
Quasi-stationary front
A front which is nearly stationary or moves very little since the last synoptic position. Also known as a stationary front.
An instrument used to detect precipitation by measuring the strength of the electromagnetic signal reflected back. (RADAR= Radio Detection and Ranging).
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.
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 gauge
An instrument used to measure rainfall amounts.
Rain showers
Rain showers stop and start suddenly and vary widely in intensity, and are gone in less than an hour.
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.
Return flow
South winds on the back (west) side of an eastward-moving surface high pressure system. Return flow over the central and eastern United States typically results in a return of moist air from the Gulf of Mexico (or the Atlantic Ocean).
An elongated area of high pressure in the atmosphere. Opposite of a trough.
River flood warning
Issued when main stem rivers are expected to reach a level above flood stage.
Roll cloud
A relatively rare, low-level horizontal, tube-shaped accessory cloud completely detached from the cumulonimbus base. When present, it is located along the gust front and most frequently observed on the leading edge of a line of thunderstorms. The roll cloud will appear to be slowly "rolling" about its horizontal axis. Roll clouds are not and do not produce tornadoes.
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.
A condition of the atmosphere in which a certain volume of air holds the maximum water vapor it can hold at a specific temperature.
A cloud layer that covers between 3/8ths and 1/2 of the sky.
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.
Sea-level pressure
The pressure value obtained by the theoretical reduction or increase of barometric pressure to sea-level.
Secondary cold front
A front that follows a primary cold front and ushers in even colder air.
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.
A low-level horizontal accessory cloud that appears to be wedge-shaped as it approaches. It is usually attached to the thunderstorm base and forms along the gust front.
Short-fuse warning
A warning issued by the National Weather Service for a local weather hazard of relatively short duration. Short-fuse warnings include tornado warnings, severe thunderstorm warnings, and flash flood warnings. Tornado and severe thunderstorm warnings typically are issued for periods of an hour or less, flash flood warnings typically for three hours or less.
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.
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.
Slight risk (of severe thunderstorms)
Severe thunderstorms are expected to affect between two and five percent of the area. A slight risk generally implies that severe weather events are expected to be isolated.
Pollution formed by the interaction of pollutants and sunlight (photochemical smog), usually restricting visibility, and occasionally hazardous to health.
A suspension in the air of small particles produced by combustion. A transition to haze may occur when smoke particles have traveled great distances (25 to 100 statute miles or more) and when the larger particles have settled out and the remaining particles have become widely scattered through the atmosphere.
Frozen precipitation composed of ice particles in complex hexagonal patterns. Snow forms in cold clouds by the direct transfer of water vapor to ice.
Snow burst
Very intense shower of snow, often of short duration, that greatly restricts visibility and produces periods of rapid snow accumulation.
Snow flurries
Light snow showers, usually of an intermittent nature and short duration with no measurable accumulation.
Snow grains
Light snow showers, usually of an intermittent nature and short duration with no measurable accumulation. In Canada, Snow grains are described as minute, white and opaque grains of ice. When they hit hard ground, they do not bounce or shatter. They usually fall in very small quantities, and never in the form of a shower.
Snow pellets
Precipitation of white, opaque grains of ice. The grains are round or sometimes conical. Diameters range from about 0.08 to 0.2 inch (2 to 5 mm). In Canada, snow pellets are brittle and easily crushed; when they fall on hard ground, they bounce and often break up. They always occur in showers and are often accompanied by snowflakes or rain drops, when the surface temperature is around zero degrees Celsius.
Snow shower
Snow falling at varying intensities for brief periods of time. Some accumulation is possible.
Snow squalls
Intense, but of limited duration, periods of moderate to heavy snowfall, accompanied by strong, gusty surface winds and possible lightning. In Canada, it is described that a snow squall brings strong winds, flurries and poor visibility.
The depth of new snow that has accumulated since the previous day or since the previous observation.
White ice crystals that have combined in a complex branched hexagonal form.
Special marine warning
Issued for brief or sudden occurrence of sustained wind or frequent gusts of 34 knots or more. This is usually associated with severe thunderstorms or waterspouts.
An ensemble of water droplets torn by the wind from an extensive body of water, generally from the crests of waves, and carried up into the air in such quantities that it reduces the horizontal visibility.
A strong wind characterized by a sudden onset in which the wind speed increases at least 16 knots and is sustained at 22 knots or more for at least one minute. In Canada, a squall is defined as a strong, sudden wind which generally lasts a few minutes then quickly decreases in speed. In Australia, it is described as a sudden increase of the mean wind speed which lasts for several minutes at least before the mean wind returns to near its previous value.
An indication of how easily a parcel of air is lifted. If the air is very stable it is difficult to make the parcel rise. If the air is very unstable the parcel may rise on its own once started.
Stable air
Air with little or no tendency to rise, usually accompanied by clear dry weather.
Steam fog
Fog that is formed when water vapor is added to air which is much colder than the vapor's source. This is most common when very cold air drifts across relatively warm water.
Steering winds (steering currents)
A prevailing synoptic scale flow which governs the movement of smaller features embedded within it.
In marine usage, winds 48 knots (55 miles per hour) or greater.
Storm surge
A rise of the sea level alone the shore that builds up as a storm (usually a hurricane) moves over water. It is a result of the winds of the storm and low atmospheric pressures.
Storm warning
A marine wind warning for sustained winds greater of 48 knots (55 miles per hour) or more from a non-tropical system.
Straight line winds
Thunderstorm winds most often found with the gust front.
Subtropical storm
A low pressure system that develops in subtropical waters (north of 20 north degrees latitude) and initially has non-tropical features (see table below for a list of tropical features) but does have some element of a tropical cyclone's cloud structure (located close to the center rather than away from the center of circulation).
Sunny or a few clouds means that less than half the sky has clouds.
A severe thunderstorm whose updrafts and downdrafts are in near balance allowing the storm to maintain itself for several hours. Supercells often produce large hail and tornadoes.
The condition which occurs in the atmosphere when the relative humidity is greater than 100 percent.
A measure of the warmth or coldness of an object or substance with reference to a standard value.
The sound caused by a lightning stroke as it heats the air and causes it to rapidly expand.
A storm with lightning and thunder produced by a Cumulonimbus cloud, usually producing gusty winds, heavy rain and sometimes hail. In Australia, they are described to be usually short-lived and hit on only a small area.
Tilted storm or tilted updraft
A thunderstorm or cloud tower which is not purely vertical but instead exhibits a slanted or tilted character. It is a sign of vertical wind shear, a favorable condition for severe storm development.
Until sunset.
Midnight to midnight.
Sunset to midnight.
A violent rotating column of air, in contact with the ground, pendant from a cumulonimbus cloud. A tornado does not require the visible presence of a funnel cloud. It has a typical width of tens to hundreds of meters and a lifespan of minutes to hours.
Trade winds
Persistent tropical winds that blow from the subtropical high pressure centers towards the equatorial low. They blow northeasterly in the northern hemisphere.
Tropical storm
An organized low pressure system in the tropics with wind speeds between 38 and 74 miles per hour. In Australia, it is a term used in the northern hemisphere for a tropical cyclone.
Tropical storm warning
A warning issued when sustained winds of 39 to 73 miles per hour (34 to 63 knots) are expected within 24 hours.
A warning issued when sustained winds of 39 to 73 mph (34 to 63 knots) are expected within 24 hours. In Canada, it is described as an elongated area of relatively low pressure extending from the centre of a region of low pressure.
A warning issued when sustained winds of 39 to 73 miles per hour (34 to 63 knots) are expected within 24 hours.
A colloquial term for a tornado.
A hurricane that forms in the Western Pacific Ocean. It is a term used in Australia for a tropical cyclone in the northwestern Pacific with maximum winds above 117 kilometers per hour (63 knots).
Unstable air
Air that rises easily and can form clouds and rain.
Very cool
Very cool, in the summer, refers to more than seven degrees Celsius below normal.
Very mild
In the winter, very mild means more than seven degrees Celsius above normal.
In summer, warm refers to four to seven degrees Celsius above normal.
Warm front
A narrow transitions zone separating advancing warmer air from retreating cooler air. The air behind a warm front is warmer and typically more humid than the air it is replacing. In Canada, it is defined as the trailing edge of a retreating cold air mass and moves in such a way that the warmer air replaces the colder air.
In meteorology any pattern identifiable on a weather map that has a cyclic pattern or a small cyclonic circulation in the early stages of development that moves along a cold front. In Canada, A wave, in meteorology, is the intersection of warm and cold fronts.
State of the atmosphere with respect to heat or cold, wetness or dryness, calm or storm, clearness or cloudiness. Also, weather is the meteorological day-to-day variations of the atmosphere and their effects on life and human activity. It includes temperature, pressure, humidity, clouds, wind, precipitation and fog.
Wind is the horizontal movement of air relative to the earth's surface and is caused by variations in temperature and pressure (for instance, air rises as it warms and a cool breeze moves in to take the place of the rising air.) Wind is also known as moving air.
Wind chill
The additional cooling effect resulting from wind blowing on bare skin. The wind chill is based on the rate of heat loss from exposed skin caused by the combined effects of wind and cold. The (equivalent) wind chill temperature is the temperature the body "feels" for a certain combination of wind and air temperature.
Winter storm
A heavy snow event. A snow accumulation of more than six inches in 12 hours or more than 12 inches in 24 hours.
Winter storm warning
Issued when seven or more inches of snow or sleet is expected in the next 24 hours, or 1/2 inch or more of accretion of freezing rain is expected. A warning is used for winter weather conditions posing a threat to life and property.
An electromagnetic wave of very short wavelength, able to pass through many materials opaque to light.
An inert gaseous chemical element, present in trace amounts in the air and used in some kinds of electric light.
Yellow wind
A strong, cold, dry west wind of eastern Asia that blows across the plains during winter and carries a yellow dust from the desert.
A hot wind during unsettled summer weather in the Mediterranean.
Zonal flow
A small amplitude pattern where winds blow mostly west-to-east.
Zonal wind
Wind blowing east-to-west or west-to-east.
Zulu time
The mean solar time for the meridian at Greenwich, England, used as a basis for calculating time in communications, military, aviation, maritime and other activities that cross time zones. Zulu time, which is also known as Coordinated Universal Time (UTC), is used internally among people working in the field of weather and meteorology, as well as among weather enthusiasts, for weather reports and forecasts.

Sources: The National Oceanic and Atmospheric Administration's (NOAA) National Weather Service, the Met Office in the United Kingdom, Canada’s Weather Office, and the Australian Government Bureau of Meteorology.