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Glossary of terms for clouds


Definition of a cloud: a hydrometeor consisting of minute particles of liquid water or ice, or of both, suspended in the free air and usually not touching the ground. It may also include larger particles of liquid water or ice as well as non-aqueous or solid particles such as those present in fumes, smoke or dust. 

Principles of cloud classification: clouds are continuously in a process of evolution and appear, therefore, in a infinite variety of forms. It is possible, however, to define a limited number of characteristic forms, frequently observed all over the world, into which clouds can be broadly grouped. A classification of the characteristic forms of clouds, in terms of "genera", "species", and "varieties" has been established. Definitions and descriptions of each of the characteristic
forms corresponding to this classification are given in the list below. Intermediate or transitional forms, although observed fairly frequently, are not described; they are of little intrest, as they are less stable and as their appearance is not very different from that indicated in the definitions of the characteristic forms. Finally, there exists a group of clouds, rarely or occassionally observed, not included in this classification. Some of these so called "special clouds" consists for the greater part or in their entirety of non-aqueous liquid or solid particles. The above mentioned definition of a cloud is therefore not applicable to all special clouds.
Special clouds are: nacreous clouds, noctilucent clouds, condensation trails, clouds from waterfalls, clouds from fires, clouds from volcanic eruptions, clouds resulting from industry and clouds resulting from explosions.


Genera of clouds

Main characteristics mutually exclusive forms of clouds, constituting the basis of the cloud classification included in the WMO International Cloud Atlas Volume I & II of 1956, and Volume I of 1975 & Volume II of 1987. The cloud genera are ten in number:

Cirrus (Ci): Detached clouds in the form of white, dedicate filaments or white or mostly white patches or narrow bands. These clouds have fibrous (hair-like) appearance, or a silky sheen, or both.

Cirrocumulus (Cc)
: Thin, white patch, sheet or layer of cloud without shading, composed of very small elements in the form of grains, ripples, etc., merged or separate, and more or less regularly arranged; most of the elements have an apparent width of less than one degree.

Cirrostratus (Cs)
: Transparent, whitish cloud veil of fibrous (hair-like) or smooth appearance, totally or partly covering the sky, and generally producing halo phenomena.
Altostratus (As)
: Greyish or bluish cloud sheet or layer of striated, fibrous or uniform appearance, totally or partly covering the sky, and having parts thin enough to reveal the Sun at least vaguely, as through ground glass. Altostratus cloud does not show halo phenomena.

Altocumulus (Ac): White or grey, or both white and grey, patch, sheet or layer of cloud, generally with shading, composed of laminae, rounded masses, rolls, etc., which are sometimes partly fibrous or diffuse and which may or may not be merged; most of the regularly arranged small elements usually have an apparent width of between one and five degrees.

Nimbostratus (Ns)
: Grey cloud layer, often dark, the appearance of which is rendered diffuse by more or less continuously falling rain or snow, which in most cases reaches the ground. It is thick enough throughout to blot out the Sun.

Stratocumulus (Sc)
: Grey or whitish, or both grey and whitish, patch, sheet or layer of cloud which almost always has dark parts, composed of tessellations, rounded masses, rolls, etc., which are non-fibrous (except for virga) and which may or may not be merged; most of the regularly arranged small elements have an apparent width of more than five degrees.

Stratus (St)
: Generally grey cloud layer with a fairly uniform base, which may give drizzle, ice prisms or snow grains. When the sun is visible through the cloud, its outline is clearly discernible. Stratus does not produce halo phenomena except, possible, at very low temperatures.

Cumulus (Cu)
: Detached clouds, generally dense and with sharp outlines, developing vertically in the form of rising mounds, domes or towers, of which the bulging upper parts often resembles a cauliflower. The sunlit parts of these clouds are mostly brilliant white; their base is relatively dark and nearly horizontal. 

Cumulonimbus (Cb)
: Heavy and dense cloud, with a considerable vertical extent, in the form of a mountain or huge towers. At least part of its upper portion is usually smooth, or fibrous or striated, and nearly always flattened; this part often spreads out in the shape of an anvil or vast plume. Under the base of this cloud which is often very dark, there are frequently low ragged clouds ether merged with it or not, and precipitation sometimes in the form of virga.


Alto: means height or upper air, in meteorological context = medium level.

Cirro, from which cirrus is obtained means "wisp of hair".

Cumulo, from which cumulus is obtained means "heap". Clouds with this designator appear to be piled up. These type clouds form in unstable layers of air. The initial lifting may be due to convective lifting or forced (mechanical) lifting, such as; orographic, frontal or convergence lifting. If the layer is unstable, the air parcel will continue to rise producing a cumulo-form cloud.

Nimbo: means "rain". Thus, nimbostratus and cumulonimbus are clouds from which precipitation occurs.

Strato, from which stratus is obtained means "layer" or "layered". Clouds with this designator form in stable layers of air; except the stratocumulus type which forms in a thin, unstable layer of air.

Species of clouds

Subdivision of the cloud genera taking into account one or more of the following features:

  • Their form (cloud in banks, veils, sheets, layers, etc.);
  • Their dimensions (areas of their constituent elements, vertical extension, etc.);
  • Their internal structure (clouds made up of ice crystals, water droplets, etc.);
  • Known or assumed physical processes which may enter into cloud formation (clouds due to orographic phenomena, etc.).

Species of a given type are 14 in number and are self-excluding.

Calvus (cal) (bald): Cumulonimbus in which at least some protuberances of the upper part are beginning to lose their cumuliform outlines but in which no cirriform parts can be distinguished. Protuberances and sproutings tend to form a whitish mass, with more or less vertical striations.

Capillatus (cap) (having hair): Cumulonimbus characterized by the presence, mostly in its upper portion, of distinct cirriform parts of clearly fibrous or striated structure, frequently having the form of an anvil, a plume or a vast, more or less disorderly mass of hair. Cumulonimbus capillatus is usually accompanied by a shower or by a thunderstorm, often with squalls and sometimes with hail; it frequently produces very well-defined virga.

Castellanus (cas) (castle): Clouds which present, in at least some portion of their upper part, cumuliform protuberances in the form of turrets which generally give the clouds a crenelated appearance. The turrets, some of which are taller than they are wide, are connected by a common base and seem to be arranged in lines. The castellanus character is especially evident when the clouds are seen from the side.

Congestus (con) (to pile up): Cumulus clouds which are markedly sprouting and are often of great vertical extent. Their bulging upper part frequently resembles a cauliflower.

Fibratus (fib) (fibrous): Detached clouds or a thin cloud veil, consisting of nearly straight or more or less irregularly curved filaments which do not terminate in hooks or tufts.

Floccus (flo) (tuft of wool): A species in which each cloud unit is a small tuft with a cumuliform appearance, the lower part of which is more or less ragged and often accompanied by virga.

Fractus (fra) (to fracture) : Clouds in the form of irregular shreds, which have a clearly ragged appearance.

Humilis (hum) (small size-low): Cumulus clouds of only a slight vertical extent. They generally appear flattened.

Lenticularis (len) (lentil): Clouds having the shape of lenses or almonds, often very elongated and usually with well-defined outlines; they occasionally show irisation. Such clouds appear most often in cloud formations of orographic origin, but may also occur in regions without marked orography.

Mediocris (med) (medium size): Cumulus clouds of moderate vertical extent, the tops of which show fairly small protuberances.

Nebulosus (neb) (full of mist): A cloud like a nebulous veil or layer, showing no distinct details.

Spissatus (spi) (to make thick): Cirrus of sufficient optical thickness to appear grey when viewed towards the sun.

Stratiformis (str) (to spread out): Clouds spread out in an extensive horizontal sheet or layer.

Uncinus (unc) (hooked): Cirrus often shaped like a comma, terminating at the top in a hook, or in a tuft the upper part of which is not in the form of a rounded protuberance.


Varieties of clouds

Subdivision of cloud genera and their species, determined by taking into account one or other of the two following characteristics:

  • Their transparency (clouds allowing the Sun or Moon to appear or masking them altogether);
  • The arrangement of their macroscopic elements (clouds whose constituent elements are associated in a particular way).

The varieties of a given genus or species are 9 in number and are not self-exclusive except for opacus and translucidus. In the list below the first six describes the arrangement, the last three the degree of transparency.

Duplicatus (du) (doubled): Superposed cloud patches, sheets or layers, at slightly different levels, sometimes partly merged.

Intortus (in) (to twist): Cirrus, the filaments of which are very irregularly curved and often seemingly entangled in a capricious manner.

Lacunosus (la) (having holes): Cloud patches, sheets or layers, usually rather thin, marked by more or less regularly distributed round holes, many of them with fringed edges. Cloud elements and clear spaces are often arranged in a manner suggesting a net or a honeycomb.

Undulatus (un) (wave): Clouds in patches, sheets or layers, showing undulations. These undulations may be observed in fairly uniform cloud layers or in clouds composed of elements, separate or merged. Sometimes a double system of undulations is in evidence.

Vertebratus (ve) (having vertebrae): Clouds, the elements of which are arranged in a manner suggestive of vertebrae, ribs, or a fish skeleton.

Radiatus (ra) (having rays): Clouds showing broad parallel bands or arranged in parallel bands, which, owing to the effect of perspective, seem to converge towards a point on the horizon or, when the bands cross the whole sky, towards two opposite points on the horizon, called "radiation point(s).".

Opacus (op) (thick-shady): An extensive cloud patch, sheet or layer, the greater part of which is sufficiently opaque to mask completely the sun or moon.

Perlucidus (pe) (allowing light to pass trough it): An extensive cloud patch, sheet or layer, with distinct but sometimes very small spaces between the elements. The spaces allow the sun, the moon, the blue of the sky or over-lying clouds to be seen. Can be observed together with opacus and translucidus.

Translucidus (tr) (transparent): Clouds in an extensive patch, sheet or layer, the greater part of which is sufficiently translucent to reveal the position of the sun or moon.

Supplementary features and accessory clouds

Clouds attached to the main part of a cloud, generally smaller than the latter. A given cloud may present simultaneously one or more supplementary features and are 9 in number. The first six listed below are supplementary features, the last three accessory clouds

Arcus (arc) (bow) often called roll or shelf cloud: A dense, horizontal roll with more or less tattered edges, situated on the lower front part of certain clouds and having, when extensive, the appearance of a dark, menacing arc.

Incus (inc) (anvil): The upper portion of a Cumulonimbus spread out in the shape of an anvil with smooth, fibrous or striated appearance.

Mamma (mam) (udder-breast) also called mammatus: Hanging protuberances, like pouches, on the undersurface of a cloud.

Praecipitatio (pra) (fall down): Precipitation (rain, drizzle, snow, ice pellets, hail, etc.) falling from a cloud and reaching the earth's surface.

Tuba (tub) (trumpet) commonly called funnel cloud; also called pendant cloud, tornado cloud: Cloud column or inverted cloud cone, protruding from a cloud base; it constitutes the cloudy manifestation of more or less intense vortex, namely, a tornado or waterspout.

Virga (vir) (stick-branch) also called fallstreaks, precipitation trails: Vertical or inclined trails of precipitation attached to the under surface of a cloud, which do not reach the earth's surface.

Pannus (pan) (rag): Ragged shreds, sometimes constituting a continuous layer, situated below another cloud and sometimes attached to it.

Pileus (pil) (cap) also called cap cloud, scarf cloud: An accessory cloud of small horizontal extent, in the form of a cap or hood above or attached to the top of a cumuliform cloud which often penetrates it. Several pileus clouds may fairly often be observed in superposition.

Velum (vel) (sail of a ship): An accessory cloud veil of great horizontal extent, close above or attached to the upper part of one or several cumuliform clouds which often pierce it.


Clouds may form in clear air. They may also form or grow from other clouds, called "mother-clouds"; two cases can be distinguished.

1) A part of a cloud may develop and more or less pronounced extensions may form. These extensions, whether attached to the mother-cloud or not, may become clouds of a genus different from that of the mother-cloud. They are given the name of the appropriate genus, followed by the name of the genus of the mother-cloud with the addition of the suffix "genitus" (e.g. Cirrus altocumulogenitus, Stratocumulus cumulogenitus)

2) The whole or large part of a cloud may undergo complete internal transformations, thus changing from one genus into another. The new cloud is then given the name of the appropriate genus, followed by the name of the genus of the mother-cloud with the addition of the suffix "mutatus" (e.g. Cirrus cirrostratomutatus, Stratus stratocumulomutatus). The internal transformation of clouds should not be confused with changes in the appearance of the sky resulting from the relative movement of clouds and observer.

( ) = abbreviations of the genera, species, varieties  & supplementary features and accessory clouds

( ) = meaning of Latin names of species, varieties  & supplementary features and accessory clouds


Source: WMO publication No. 266. TP.150 & WMO publication No. 407

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