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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:
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.
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:
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
Subdivision of cloud genera and their species, determined by taking into account one or other of the two following characteristics:
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.
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,
Virga (vir) (stick-branch) also called fallstreaks, precipitation
or inclined trails of precipitation attached to the under surface of a cloud,
which do not reach the earth's surface.
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.
( ) = 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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