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.
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.
The process in which precipitation particles grow by collision with, and by assimilation of, cloud particles or other precipitation particles.
The process in which solid precipitation particles combine in the atmosphere to produce larger particles, such as hailstones.
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.
[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.
See overshooting top.
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.
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.
[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.
[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.
[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.
(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.
Stationary orographic cloud on or above an isolated mountain peak forming a cap around the summit.
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.
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.
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.
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.
[colloq.]. In popular terminology, any sudden and heavy fall of rain, almost always of the shower type.
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.
Process of formation of a single liquid water drop by the union of two or more colliding drops.
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.
(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.
Level at which the air subjected to a lifting process becomes saturated.
Nucleus on which water vapour can condense.
Condensation trail (contrail)
See condensation trail.
Organized motions within a layer of air leading to the vertical transport of heat, momentum, etc.
Cumuliform cloud that forms in an atmospheric layer made unstable by heating at the base or cooling at the top.
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.
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).
The formation of ice on a surface directly from water vapour, without passing through a liquid phase.
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.
Violent and damaging downdraught reaching the surface, associated with a severe thunderstorm.
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.
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.
Popular public term for a thunderstorm.
Convection occurring within an elevated layer, i.e. a layer in which the lowest portion is based above the Earth’s surface.
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.
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).
The physical process by which a liquid or solid is transformed to the gaseous state; the opposite of condensation.
Supplementary feature virga.
Supplementary feature cavum.
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.
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.
Common term for a light snow shower, lasting for only a short period of time.
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.
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.
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.
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.
The temperature at which air is saturated with respect to water vapour over an ice surface.
See steam fog.
[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.
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.
A wave disturbance in which buoyancy acts as the restoring force on parcels displaced from hydrostatic equilibrium.
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.
Haboob ((habub) "blasting/drafting")
Strong wind, producing a dust storm or sandstorm, in arid or semi-arid regions of the world.
A substance that readily attracts water from its surroundings which, in meteorology, is applied principally to condensation nuclei such as salt, etc.
See diamond dust.
See accessory clouds flumen.
A beaver tail cloud with a stinger-like shape.
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.
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.
[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.
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.
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).
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.
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.
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.
Long, well-defined wisps of cirrus clouds, thicker at one end than the other. See Cirrus uncinus.
A storm-scale region of rotation, typically around 3-10 km in diameter within a thunderstorm. Often gives rise to a tornado.
Top of the mesosphere situated at about 80–85 km
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.
(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.
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.
An atmospheric gravity wave, formed when stable airflow passes over a mountain or mountain barrier.
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.
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.
A property that does not allow light to pass through so that objects on the other side are totally obscured. See variety opacus.
Cloud whose presence and shape are determined by the relief of the Earth’s surface.
Lifting of air caused by its passage up and over mountains or other sloping terrain.
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.
A characteristic severe depletion of stratospheric ozone that occurs each spring over the polar regions.
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).
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.
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.
A name for a broad class of mesoscale convective systems that have various linear configurations.
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.
Region, situated on the lee side of a mountain or mountain range, where the rainfall is much less than on the windward side.
The ratio (%) of the observed vapour pressure to the saturation vapour pressure with respect to water at the same temperature and pressure.
See species volutus.
Turbulent cloud formation in the lee of large mountain barriers. The air in the cloud rotates around an axis parallel to the range.
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.
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%.
(fractus). The accessory cloud pannus, ragged low clouds – usually Stratus fractus or Cumulus fractus – that occur below the main cloud base.
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.
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).
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.
See banner cloud.
Agglomeration of snow crystals.
Mass of snow heaped up by the wind and deposited along an obstruction or an irregularity of the terrain.
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.
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.
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.
Descriptive of clouds of extensive horizontal development, in contrast to the vertically developed cumuliform types.
Top of the inversion layer in the upper stratosphere at about 50 km to 55 km.
The region of the atmosphere, situated between the tropopause and the stratopause, in which the temperature generally increases with height.
The process of phase transition directly from solid to vapour in the absence of melting.
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.
Liquid water at a temperature below freezing point.
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.
The condition existing in a given portion of the atmosphere when the relative humidity is greater than 100%.
The cloud that forms over the top of Table Mountain, Cape Town, South Africa, due to orographic lift.
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.
Updraught produced locally above a surface warmer than its immediate surroundings.
Towering Cumulus (TCU)
Cumulus congestus of great vertical extent.
A property that allows light to pass through diffusely so that objects on the other side are not clearly visible. See variety translucidus.
A property that allows light to pass through so that objects on the other side are clearly visible.
Lower part of the Earth’s atmosphere, extending from the surface up to the tropopause, in which the temperature generally decreases with height.
Random and continuously changing air motions which are superposed on the mean motion of the air.
Alternate term for a rain-free base. Updraught produced locally above a surface warmer than its immediate surroundings.
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.
See supplementary feature murus.
The local variation of the wind vector or any of its components in a given direction.