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  • Glossary

Glossary

(Section G.1)
 

Accretion

Growth of a cloud or precipitation particle by the collision and union of a frozen particle (ice crystal or snowflake) with a supercooled liquid droplet which freezes on impact.

(WMO-No. 182)

Advection

Transport of water or air along with its properties (e.g. temperature, chemical tracers) by the motion of the fluid. Regarding the general distinction between advection and convection, the former describes the predominantly horizontal, large-scale motions of the atmosphere or ocean, while convection describes the predominantly vertical, locally induced motions.

(https://www.ipcc.ch/pdf/glossary/ar4-wg1.pdf) Agglomeration

The process in which precipitation particles grow by collision with, and by assimilation of, cloud particles or other precipitation particles.

(WMO-No. 182) Aggregation

The process in which solid precipitation particles combine in the atmosphere to produce larger particles, such as hailstones.

(WMO-No. 182) Anvil

See supplementary feature incus.

A cirriform cloud with an anvil shape, which forms the upper part of a well-developed Cumulonimbus. Its glaciated top spreads out horizontally upon reaching the tropopause or by the action of the winds aloft.

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(WMO-No. 182) Anvil crawler

[colloq.]. A lightning discharge occurring within the anvil of a thunderstorm, characterized by one or more channels that appear to crawl along the underside of the anvil. They typically appear during the weakening or dissipating stage of the parent thunderstorm or during an active mesoscale convective system.

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(https://www.weather.gov/oun/spotterglossary) Anvil dome

See overshooting top.

Anvil rollover

[colloq.]. A circular or semicircular lip of clouds along the underside of the upwind part of a back-sheared anvil, indicating rapid expansion of the anvil. See cumuliform anvil, knuckles, mushroom.

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(https://www.weather.gov/oun/spotterglossary) Anvil zits

[colloq.]. Frequent (often continuous or nearly continuous), localized lightning discharges occurring from within a thunderstorm anvil.

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(https://www.weather.gov/oun/spotterglossary)

Back-building thunderstorm

A thunderstorm in which new development takes place on the upwind side, such that the storm seems to remain stationary or propagate in a backward direction.

(https://www.weather.gov/oun/spotterglossary) Back-sheared anvil

[colloq.]. A thunderstorm anvil which spreads upwind, against the flow aloft. A back-sheared anvil often implies a very strong updraft and a high potential for severe weather.

(https://www.weather.gov/oun/spotterglossary) Banner cloud

Stationary orographic cloud which forms in the neighbourhood of a mountain crest or peak and takes the shape of a banner streaming downwind therefrom. This type of cloud must not be confused with snow which is blown off a mountain summit and carried downwind. 

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(WMO-No. 182) Barber pole

[colloq.]. A thunderstorm updraft with cloud striations that are curved in a manner similar to the stripes of a barber pole. The structure is typically 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.

 

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(https://www.weather.gov/oun/spotterglossary) Bear's cage

[colloq.]. A region of storm-scale rotation, in a thunderstorm, which is wrapped in heavy precipitation. This area often coincides with a radar hook echo and/or a mesocyclone, especially one associated with a high-precipitation (HP) storm.

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(https://www.weather.gov/oun/spotterglossary) Beaver('s) tail

[colloq.]. A particular type of inflow band with a relatively broad, flat appearance suggestive of a beaver's tail. It is attached to a supercell's general updraft and is oriented roughly parallel to the pseudo-warm front. As with any inflow band, cloud elements move towards the updraft. Its size and shape change as the strength of the inflow changes. See also inflow stinger.

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(https://www.weather.gov/oun/spotterglossary) Bergeron-Findeisen process

A theoretical explanation of the process by which precipitation particles may form within a mixed cloud (composed of both ice crystals and liquid water drops).

(http://glossary.ametsoc.org/) Black ice

(1) Thin, new ice on freshwater or saltwater, appearing dark in colour because of its transparency, which is a result of its columnar grain structure. On lakes, black ice is commonly overlain by white ice formed from refrozen snow or slush. (2) A popular alternative for glaze. A thin sheet of ice, relatively dark in appearance, may form when light rain or drizzle falls on a road surface that is at a temperature below 0 °C or, alternatively, when water already on the road surface subsequently freezes when the temperature thereof falls below freezing point. It may also be formed when supercooled fog droplets are intercepted by buildings, fences and vegetation.

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(http://glossary.ametsoc.org/)

Cap cloud

Stationary orographic cloud on or above an isolated mountain peak forming a cap around the summit.

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(WMO-No. 182) Cell

Convection in the form of a single updraft, downdraft, or updraft/downdraft couplet, typically seen as a vertical dome or tower as in a Cumulus or towering Cumulus. A typical thunderstorm consists of several cells.

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(https://www.weather.gov/oun/spotterglossary) Cirriform

As Cirrus. More generally, descriptive of clouds composed of small particles, mostly ice crystals that are fairly widely dispersed, usually resulting in relative transparency and whiteness and often producing halo phenomena not observed with other cloud forms. They include all types of Cirrus, Cirrocumulus, and Cirrostratus clouds.

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(http://glossary.ametsoc.org/) Clear air turbulence (CAT)

A higher altitude (6–15 km) turbulence phenomenon occurring in cloud-free regions, associated with wind shear, particularly between the core of a jet stream and the surrounding air.

(http://glossary.ametsoc.org/) Cloud bank

A fairly well-defined mass of cloud observed at a distance. It covers an appreciable portion of the horizon sky but does not extend overhead.

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(WMO-No. 182) Cloudburst

[colloq.]. In popular terminology, any sudden and heavy fall of rain, almost always of the shower type.

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(http://glossary.ametsoc.org) Cloud streets

Clouds arranged in lines roughly parallel to the wind direction and appearing, because of perspective, to converge towards a point or two opposite points on the horizon called the radiation point(s). See variety radiatus. The cloud most frequently appearing in cloud streets is Cumulus mediocris.

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(WMO-No. 182) Coalescence

Process of formation of a single liquid water drop by the union of two or more colliding drops.

(WMO-No. 182) Cold pool

A region of relatively cold air, represented on a weather map analysis as a relative minimum in temperature surrounded by closed isotherms. Cold pools aloft represent regions of relatively low stability, while surface-based cold pools are regions of relatively stable air.

(https://www.weather.gov/oun/spotterglossary) Collar cloud

A ring of cloud that may be observed on rare occasions, surrounding the upper part of a wall cloud (murus). 

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(https://www.weather.gov/oun/spotterglossary) Condensation

(1) The transition from the gaseous to the liquid state. (2) The physical process by which water vapour is transformed into dew, fog or cloud droplets.

(WMO-No. 182) Condensation funnel

See tuba.

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Condensation level

Level at which the air subjected to a lifting process becomes saturated.

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(WMO-No. 182) Condensation nucleus

Nucleus on which water vapour can condense.

(WMO-No. 182) Condensation trail (contrail)

Cloud which forms in the wake of an aircraft when the atmosphere at flying level is sufficiently cold and moist. See contrails and dissipation trail.

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(WMO-No. 182) Contrail

See condensation trail.

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Convection

Organized motions within a layer of air leading to the vertical transport of heat, momentum, etc.

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(WMO-No. 182) Convective clouds

Cumuliform cloud that forms in an atmospheric layer made unstable by heating at the base or cooling at the top.

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(WMO-No. 182) Cumuliform anvil

A thunderstorm anvil (incus) with visual characteristics resembling Cumulus-type clouds (rather than the more typical fibrous appearance associated with Cirrus). A cumuliform anvil arises from rapid spreading of a thunderstorm updraft, and thus implies a very strong updraft. See anvil rollover, knuckles.  

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(https://www.weather.gov/oun/spotterglossary) Cumuliform

As Cumulus. Generally descriptive of all clouds, the principal characteristic of which is vertical development in the form of rising mounds, domes or towers. This is the contrasting form to the horizontally extended stratiform types. Cumulus clouds are driven by thermal convection and typically have vertical velocities in excess of one metre per second; cloud with the bulging appearance of a Cumulus. When such clouds, arranged in lines and joined by a common base, possess protuberances giving them a turreted appearance, they are classed in the species castellanus (of the genera Stratocumulus, Altocumulus, Cirrus and Cirrocumulus). When they constitute elements separated into tufts they are classed in the species floccus (of the genera Stratocumulus, Altocumulus, Cirrus and Cirrocumulus).

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(WMO-No. 182) 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.

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(https://www.weather.gov/oun/spotterglossary) Deposition

The formation of ice on a surface directly from water vapour, without passing through a liquid phase.

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(A Dictionary of Geology and Earth Sciences, Michael Allaby (Ed) Jan 2013) Dew-point temperature ([colloq.] dew point)

The temperature at which the air is saturated (the relative humidity is 100%) with respect to water vapour over a water surface.

Dissipation trail (distrail)

A clearly delineated lane forming behind an aircraft flying in a thin cloud layer; the opposite of a condensation trail.

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(WMO-No. 182)

Downburst

Violent and damaging downdraught reaching the surface, associated with a severe thunderstorm.

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(WMO-No. 182) Downslope windstorm

A very strong, usually gusty, and occasionally violent wind that blows down the lee slope of a mountain range, often reaching its peak strength near the foot of the mountains and weakening rapidly farther away from the mountains. 

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(http://glossary.ametsoc.org/)

Dry line

Narrow zone, other than a warm, cold, or occluded front, across which there is a distinct gradient in the moisture content of the air near the Earth’s surface.

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(WMO-No. 182)

Electrical storm

Popular public term for a thunderstorm.

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(http://glossary.ametsoc.org/) Elevated convection

Convection occurring within an elevated layer, i.e. a layer in which the lowest portion is based above the Earth’s surface. 

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(https://www.weather.gov/oun/spotterglossary) Entrainment

The mixing of environmental air into a pre-existing organized cloud or an air current so that the environmental air becomes part of the current or the cloud.

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(WMO-No. 182) Etage

A meteorological term used in the 1956 and 1975 editions of the International Cloud Atlas (ICA) to define the range of levels within which clouds of certain genera occur more frequently. Étage has been replaced with “level”, primarily because it can be easily translated into many different languages. The use of étage in this context was proposed by J.B. Lamarck (1802) in the first published classification of clouds; also, the grouping of cloud height in the WMO classification: high étage (3–8 km); middle étage (2–4 km); and low étage (surface to 2 km).

(ICA Vol I, 1956 edition) (http://glossary.ametsoc.org/) Evaporation

The physical process by which a liquid or solid is transformed to the gaseous state; the opposite of condensation.

(http://glossary.ametsoc.org/) Fallstreak

Supplementary feature virga.

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Fallstreak hole

Supplementary feature cavum.

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 Fast ice

Sea ice which forms and remains fast along the coast, where it is attached to the shore, to an ice wall, to an ice front, between shoals or grounded icebergs. Vertical fluctuations may be observed during changes of sea level. Fast ice may be formed in situ from seawater or by freezing of floating ice of any age to the shore. It may extend a few metres or several hundred kilometres from the coast.

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Flanking line

A line of Cumulus or towering Cumulus clouds connected to, and extending outward from, the most active part of a supercell. The line normally has a stair-step appearance, with the tallest clouds closest to the main storm.

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(https://www.weather.gov/oun/spotterglossary)

Flurry (snow)

Common term for a light snow shower, lasting for only a short period of time.

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(http://glossary.ametsoc.org/)

Foehn

A warm, dry wind on the lee side of a mountain range, the warmth and dryness of the air being due to adiabatic compression as the air descends the mountain slopes. In the USA, the term “chinook” is used for foehn winds in the Rocky and Sierra Mountains.

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(http://glossary.ametsoc.org/)

Foehn gap

A break in an extensive cloud deck or cloud shield, usually a band parallel to and downwind of the mountain ridge line. 

Especially visible in satellite pictures, this cloud-free zone results from the strong sinking motion on the lee side of a mountain barrier during a foehn.

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(http://glossary.ametsoc.org/)

Foehn wall (foehn bank)

Cloud formation which, during a foehn episode, lies over and along a mountain ridge and which presents, to an observer downwind from the ridge, the appearance of a vertical wall. 

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(WMO-No. 182)

Front

A boundary or transition zone between two airmasses of different density and thus (usually) of different temperature. A moving front is named according to the advancing airmass, e.g. a cold front, if colder air is advancing.

(https://www.weather.gov/oun/spotterglossary)

Frost point temperature

The temperature at which air is saturated with respect to water vapour over an ice surface.

Frost smoke

See steam fog

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Fumulus

[colloq.]. A contraction of the words fume and Cumulus, indicating water-droplet clouds that form within the top of rising plumes from smokestacks, cooling towers or open fires. This cloud is classified as Cumulus homogenitus.  

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(http://glossary.ametsoc.org/)

Graupel

Snow pellets. Precipitation, usually of brief duration, consisting of crisp, white, opaque ice particles, round or conical in shape and about 2–5 mm in diameter. Same as snow pellets or small hail. Soft hail was officially renamed “snow pellets” in 1956.

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Gravity wave

A wave disturbance in which buoyancy acts as the restoring force on parcels displaced from hydrostatic equilibrium.

(http://glossary.ametsoc.org/)

Gust front

The leading edge of a mesoscale pressure dome separating the outflow air in a convective storm from the environmental air. This boundary, which is marked by upward motion along it and downward motion behind it, is followed by a surge of gusty winds on or near the ground. A gust front is often associated with a pressure jump, wind shift, temperature drop, and sometimes with heavy precipitation. Gust fronts are often marked by arcus clouds.

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(http://glossary.ametsoc.org/)

Haboob ((habub) "blasting/drafting")

Strong wind, producing a dust storm or sandstorm, in arid or semi-arid regions of the world.

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(WMO-No. 182)

Hygroscopic substance

A substance that readily attracts water from its surroundings which, in meteorology, is applied principally to condensation nuclei such as salt, etc.

Ice prisms

See diamond dust.

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Ice storm

Intense formation of ice on objects by the freezing on impact, of rain or drizzle.

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(WMO-No. 182) Inflow bands

See accessory clouds flumen.

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Inflow stinger

A beaver tail cloud with a stinger-like shape.

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(https://www.weather.gov/oun/spotterglossary)

Inversion

An increase of temperature with height.

Inversions and stable layers inhibit vertical motion, often limiting the vertical extent of cumuliform clouds. If the air below an inversion is relatively moist, the inversion often caps a layer of stratiform cloud.

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Ionosphere

That part of the atmosphere, extending approximately from 70 km to 500 km, in which ions and free electrons exist in sufficient quantities to reflect electromagnetic waves.

(WMO-No. 182)

Knuckles

[colloq.] Lumpy protrusions on the edges, and sometimes the underside, of a thunderstorm anvil. They usually appear on the upwind side of a back-sheared anvil and indicate rapid expansion of the anvil due to the presence of a very strong updraft. They are not the supplementary feature mamma. See also cumuliform anvil, anvil rollover.

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(https://www.weather.gov/oun/spotterglossary) Lapse rate

The rate of change of an atmospheric variable, usually temperature, with height. For example, a steep temperature lapse rate is a rapid decrease of temperature with height.

(https://www.weather.gov/oun/spotterglossary)

Layer

Layer is used to describe a sheet of considerable horizontal extent; often covering the whole sky. A term of limited meaning used within the Technical Regulations of the International Cloud Atlas (1956 and 1975 and its predecessor of 1939).

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Lee Wave

Any wave disturbance that is caused by, and is therefore stationary with respect to, some barrier in the fluid flow. Whether the wave is a gravity wave, inertia wave, barotropic wave, etc., will depend on the structure of the fluid and the dimensions of the barrier. 

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(http://glossary.ametsoc.org/)

Levanter cloud

The Spanish and most widely used term for an east or northeast wind occurring along the coast and inland from southern France to the Straits of Gibraltar.

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(http://glossary.ametsoc.org/)

Macroburst

An intense, localized downdraft of air that spreads on the ground, usually below a Cumulonimbus, causing rapid changes in wind direction and speed. The diameter is greater than 4 km. Compare with microburst.

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Mares’ tails

Long, well-defined wisps of cirrus clouds, thicker at one end than the other. See Cirrus uncinus.

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(http://glossary.ametsoc.org/)

Mesocyclone

A storm-scale region of rotation, typically around 3-10 km in diameter within a thunderstorm. Often gives rise to a tornado.

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Mesopause

Top of the mesosphere situated at about 80–85 km

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(WMO-No. 182)

Mesosphere

The region of the atmosphere, situated between the stratopause and the mesopause, in which the temperature generally decreases with height. 

(WMO No. 182)

Microburst

An intense, localized downdraft of air that spreads over the ground, usually below a Cumulonimbus, causing rapid changes in wind direction and speed. The diameter is 4 km or less. Compare with macroburst.

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(WMO-No. 182)

Mixed cloud

(also known as a mixed phase cloud). A cloud in which ice particles are mixed with supercooled droplets of water. This can lead to mixed icing conditions.

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(WMO-No. 182)

Morning glory

An elongated cloud band, visually similar to a roll cloud, usually appearing in the morning hours, when the atmosphere is relatively stable. Morning glories result from perturbations related to gravitational waves in a stable boundary layer. They are similar to ripples on a water surface; several parallel morning glories can often be seen propagating in the same direction. See species volutus.

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(https://www.weather.gov/oun/spotterglossary)

Mountain waves

An atmospheric gravity wave, formed when stable airflow passes over a mountain or mountain barrier.

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(http://glossary.ametsoc.org/)

Multi-vortex tornado

A tornado in which two or more condensation funnels or debris clouds are present at the same time, often rotating about a common centre or about each other. Multiple-vortex tornadoes can be especially damaging.

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(https://www.weather.gov/oun/spotterglossary)

Needle ice

A phenomenon that appears similar to hoar frost but occurring when the temperature of the soil is above 0°C and the surface air temperature is below 0°C. The subsurface liquid water is brought by capillary action to the surface, where it freezes and contributes to a growing, needle-like ice column.

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Opaque

A property that does not allow light to pass through so that objects on the other side are totally obscured. See variety opacus.

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Orographic cloud

Cloud whose presence and shape are determined by the relief of the Earth’s surface. 

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(WMO No. 182)

Orographic lift

Lifting of air caused by its passage up and over mountains or other sloping terrain. 

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(https://www.weather.gov/oun/spotterglossary)

Overshooting top (penetrating top)

A dome-like protrusion above a thunderstorm anvil, representing a very strong updraft and hence a higher potential for severe weather with that storm. A persistent and/or large overshooting top is often present on a supercell. A short-lived overshooting top, or one that forms and dissipates in cycles, may indicate the presence of a pulse storm or a cyclic storm.

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(https://www.weather.gov/oun/spotterglossary)

Ozone hole

A characteristic severe depletion of stratospheric ozone that occurs each spring over the polar regions.

(http://glossary.ametsoc.org/)

Pile d’assiettes (pile of plates)

The usual term for a series of lenticular clouds stacked one above the other caused by wave motion in multiple humid layers of air.

Technical classification is Stratocumulus lenticularis duplicatus or AltocumuIus lenticularis duplicatus (depending on height of cloud base).

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(A Dictionary of Weather, second edition, Storm Dunlop)

Planetary boundary layer (PBL)

The bottom layer of the troposphere that is in contact with the surface of the Earth. It is often turbulent and capped by a statically stable layer of air or temperature inversion. PBL depth (the inversion height) is variable in time and space, ranging from tens of metres in strongly statically stable situations, to several kilometres in convective conditions over deserts. 

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(http://glossary.ametsoc.org/)

Polar vortex

The large-scale cyclonic circulation in the middle and upper troposphere and extending into the stratosphere, centred generally in the polar regions.

(WMO-No. 182)

Precipitation

A hydrometeor consisting of a fall of an ensemble of particles. The forms of precipitation are: rain, drizzle, snow, snow grains, snow pellets, diamond dust, hail and ice pellets.

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(WMO-No. 182)

Protuberance

A projection or a bulge used to describe swellings, turrets and towers seen in many clouds; e.g. the small towers seen in Cumulis mediocris, the sometimes complex mass of towers in Cumulus congestus or the very small turrets of Cirrus castellanus.

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Pyrocumulus

A Cumulus cloud (Cumulus flammagenitus) formed by a rising thermal from a fire, or enhanced by buoyant plume emissions from an industrial combustion process (Cumulus homogenitus). 

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(http://glossary.ametsoc.org/)

Quasi linear convective system (QLCS)

A name for a broad class of mesoscale convective systems that have various linear configurations.

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(http://wdtb.noaa.gov/courses/awoc/documentation/handouts-Severe/Threat-Assessment-of-QLCS.pdf)

Rain-free base

A dark, horizontal cloud base with no visible precipitation beneath it.  It typically marks the location of a thunderstorm updraft. Tornadoes may develop from wall cloud attached to the rain-free base, or from the rain-free base itself.

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(https://www.weather.gov/oun/spotterglossary)

Rain shadow

Region, situated on the lee side of a mountain or mountain range, where the rainfall is much less than on the windward side. 

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(WMO-No. 182)

Relative humidity

The ratio (%) of the observed vapour pressure to the saturation vapour pressure with respect to water at the same temperature and pressure.

(WMO-No. 8)

Roll cloud

See species volutus

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Rotor cloud

Turbulent cloud formation in the lee of large mountain barriers. The air in the cloud rotates around an axis parallel to the range.

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(WMO-No. 182)

Rotors

Circulation of flow about a horizontal or nearly horizontal axis that is usually associated with flow over the lee side of a barrier, such as a mountain range. The rotation may extend to the ground, cause hazards to aircraft, and carry large amounts of dust aloft.

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(http://glossary.ametsoc.org/)

Saturation

An atmospheric condition in which air holds the maximum amount of water vapour that it can hold at the observed temperature and pressure, whether over a surface of water or of ice. In this condition the relative humidity is 100%.

Scud

(fractus). The accessory cloud pannus, ragged low clouds – usually Stratus fractus or Cumulus fractus – that occur below the main cloud base.

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(http://glossary.ametsoc.org/)

Sea-of-cloud

The appearance of the upper surface of a layer of cloud which shows undulations of very different lengths. The whole aspect then suggests sea waves.

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(WMO-No. 182)

Sheet

Sheet is used to describe a relatively thin layer that covers less than the whole sky. See layer. A term of limited meaning used in the Technical Regulations of the International Cloud Atlas (1956 and 1975 and its predecessor of 1939).

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Shelf cloud

See supplementary feature arcus. A low, horizontal wedge-shaped arcus cloud, associated with a thunderstorm gust front (or occasionally with a cold front, even in the absence of a thunderstorms). Unlike the roll cloud, the shelf cloud is attached to the base of the parent cloud above it (usually a thunderstorm). Rising cloud motion can often be seen in the leading (outer) part of the shelf cloud, while the underside often appears turbulent, boiling, and wind-torn.

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(https://www.weather.gov/oun/spotterglossary)

Sleet

Depending on the region, precipitation of rain and snow mixed, rain and hail, rain and ice pellets, melting snow, or sudden and brief rainfall with wind and hail.

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(WMO-No. 182)

Smoking mountain

See banner cloud.

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Snowflake

Agglomeration of snow crystals.

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(WMO-No. 182)

Snowdrift

Mass of snow heaped up by the wind and deposited along an obstruction or an irregularity of the terrain.

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(WMO-No. 182)

Sprouting

Sprouting is used to describe the appearance of growing upper parts of clouds such as Cumulus mediocris and congestus and Cumulonimbus calvus.

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Squall

Atmospheric phenomenon characterized by an abrupt and large increase in wind speed with a duration of minutes which diminish rather suddenly. It is often accompanied by showers or thunderstorms. 

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(WMO-No. 182)

Stable layer

A layer in which the temperature lapse rate is less than the dry adiabatic lapse rate (or moist adiabatic lapse rate, if saturated). Vertical motion within the stable layer is inhibited. 

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Steam fog

Fog formed when water vapour is added to air that is much colder than the vapour's source; most commonly, when very cold air drifts across relatively warm water.

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(http://glossary.ametsoc.org/)

Stratiform

Descriptive of clouds of extensive horizontal development, in contrast to the vertically developed cumuliform types. 

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(http://glossary.ametsoc.org/)

Stratopause

Top of the inversion layer in the upper stratosphere at about 50 km to 55 km.

(WMO-No. 182)

Stratosphere

The region of the atmosphere, situated between the tropopause and the stratopause, in which the temperature generally increases with height.

(WMO-No. 182)

Sublimation

The process of phase transition directly from solid to vapour in the absence of melting.

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(http://glossary.ametsoc.org/) Supercell

A thunderstorm with a persistent rotating updraft. Supercells are rare, but are responsible for a remarkably high percentage of severe weather events. especially tornadoes, extremely large hailstones and damaging straight-line winds. 

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(https://www.weather.gov/oun/spotterglossary) Supercooled

Liquid water at a temperature below freezing point. 

(http://glossary.ametsoc.org/)

Superposed

Placed above or upon something else, or one upon another. Used in describing the cloud variety duplicatus where cloud patches, sheets or layers may be at slightly different levels or partly merged. 

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Supersaturation

The condition existing in a given portion of the atmosphere when the relative humidity is greater than 100%.

(http://glossary.ametsoc.org/)

Surface-based convection

Convection occurring within a surface-based layer, i.e. a layer in which the lowest portion is based at, or very near, the Earth’s surface. Compare with elevated convection.

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(https://www.weather.gov/oun/spotterglossary) Table cloth

The cloud that forms over the top of Table Mountain, Cape Town, South Africa, due to orographic lift.

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Tail cloud

A horizontal, tail-shaped cloud (not a funnel cloud) at low levels extending from the precipitation cascade region of a supercell towards the wall cloud. The base of the tail cloud is about the same as that of the wall cloud. Cloud motion in the tail cloud is away from the precipitation and towards the wall cloud, with rapid upward motion often observed near the junction of the tail and wall cloud.

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(https://www.weather.gov/oun/spotterglossary)

Thermal lifting

Updraught produced locally above a surface warmer than its immediate surroundings.

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(WMO No.182)

Towering Cumulus (TCU)

Cumulus congestus of great vertical extent.

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Translucent

A property that allows light to pass through diffusely so that objects on the other side are not clearly visible.  See variety translucidus.

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Transparent

A property that allows light to pass through so that objects on the other side are clearly visible.

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Tropopause

The boundary between the troposphere and the stratosphere, where an abrupt change in lapse rate usually occurs. 

(WMO-No. 182)

Troposphere

Lower part of the Earth’s atmosphere, extending from the surface up to the tropopause, in which the temperature generally decreases with height.

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Turbulence

Random and continuously changing air motions which are superposed on the mean motion of the air.

(WMO No. 182)

Twister

[colloq.]. Tornado.

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Updraft base

Alternate term for a rain-free base. Updraught produced locally above a surface warmer than its immediate surroundings.

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(https://www.weather.gov/oun/spotterglossary)

Vortex

A whirling mass of air in the form of a column or spiral. It need not be oriented vertically but could, for example, be rotating around a horizontal axis. 

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(http://w1.weather.gov/glossary/)

Wall cloud

See supplementary feature murus.

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Wave cloud

Orographic cloud, at the crest of a stationary wave, formed in an airflow crossing a range of hills or mountains. 

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(WMO-No. 182)

Wind shear

The local variation of the wind vector or any of its components in a given direction.

(http://glossary.ametsoc.org/)

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