Principles of cloud classification

Clouds change their shape according to processes in the atmosphere and have many individual shapes but only a very few basic forms. The three principle families of clouds have the Latin names cirrus (meaning "fibre" or "hair"), cumulus ("heap or "pile"), and Stratus ("layer" or "sheet"). Luke Howard, an amateur meteorologist, first classified the various clouds based on their appearance in 1802.

The World Meteorological Organization (WMO) cloud classification scheme uses genera, species and varieties in its International Cloud Atlas. A cloud may be described in terms of genera, species and variety e.g. Altocumulus stratiformis duplicatus (a stratiform species of altocumulus that occurs in two or more layers). Clouds sometimes have supplementary features or accessory clouds.

Genera

The classification of clouds has 10 main groups, or genera. Each observed cloud is a member of one, and only one, genus.

High level cloud High level cloud refers to cloud with a base above 20,000 feet.

• (0) Cirrus (Ci) - thin fibrous cloud.
• (1) Cirrocumulus (Cc) - thin granular layer of small lumps of cloud .
• (2) Cirrostratus (Cs) - thin uniform cover of cloud.

Mid level cloud Mid level cloud is cloud with a base above about 6,500 feet. ('Alto' means medium.)

• (3) Altocumulus (Ac) - thin layer of lumps or heaps of cloud.
• (4) Altostratus (As) - thin uniform layer of cloud.
• (5) Nimbostratus (Ns) - thick layer of rain bearing cloud.

Low level cloud Low level cloud, described as having a base below about 6,500 feet, is of most interest to pilots and controllers because of the impact on the safe conduct of flight particularly with regard to poor visibility, turbulence and structural damage, icing, and runway contamination.

• (6) Stratocumulus (Sc) - layer of relatively small lumps and heaps (cumuliform) of cloud.
• (7) Stratus (St) - layer of uniform cloud.
• (8) Cumulus (Cu) - cumuliform clouds.
• (9) Cumulonimbus (Cb) (Cb) - Thunderstorm.

Species

Most genera are subdivided into species based on the shape of the clouds or their Internal structure. A cloud is therefore identified by a Latin name made up of the genus followed by a specific name alluding to a characteristic shape or structure. A cloud may bear the name of only one species.

• Fibratus: Detached clouds or a thin cloud veil, consisting of nearly straight or more or less irregularly curved filaments that do not terminate in hooks or tufts. This term applies mainly to Cirrus and Cirrostratus
• Uncinus: Cirrus without grey parts, 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.
• Spissatus: Cirrus in patches, sufficiently dense to appear greyish when viewed towards the sun; it may also veil the Sun, obscure its outline or even hide it. Cirrus spissatus often originates from the upper part of a Cumulonimbus.
• Castellanus: Clouds that present, in at least some portion of their upper part, cumuliform protuberances in the form of turrets or towers (crenellated), some of which are taller than they are wide, and 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. This term applies to Cirrus, Cirrocumulus, Attocumulus and Stratocumulus.
• Floccus: 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. This term applies to Cirrus, Cirrocumulus, Altocumulus and Stratocumulus.
• Stratiformis: Clouds spread out in an extensive horizontal sheet or layer. This term applies to Altocumulus, Stratocumulus and, occasionally, Cirrocumulus.
• Nebulosus: A cloud like a nebulous or ill-defined veil or layer of clouds showing no distinct details. This term applies mainly to Cirrostratus and Stratus.
• Lenticularis: Clouds having the shape of lenses or almonds, often very elongated and usually with well-defined outlines; they occasionally show irisations. Such clouds appear most often in cloud formations of orographic origin, but may also occur in regions without marked orography. This term applies mainly to Cirrocumulus, Altocumulus and Stratocumulus.
• Fractus: Clouds in the form of irregular shreds, which have a clearly ragged appearance. This term applies only to Stratus and Cumulus.
• Humilis: Cumulus characterized by only a small vertical extent and appearing generally as if flattened.
• Mediocris: Cumulus of moderate vertical extent, with small protuberances and sproutings at their tops.
• Congestus: Strongly sprouting Cumulus with generally sharp outlines and often great vertical extent. The bulging upper part of Cumulus congestus frequently resembles a cauliflower.
• Volutus: A long, typically low, horizontal, detached, tube-shaped cloud mass, often appearing to roll slowly about a horizontal axis. The roll cloud, volutus, is a soliton, not attached to other clouds and is an example of an undular bore. This species applies mostly to Stratocumulus and rarely Altocumulus.
• Calvus: 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 (grooves or channels in cloud formations, arranged parallel to the flow of air and therefore depicting the airflow).
• Capillatus: 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.

Varieties

Cloud varieties are used to identify certain additional characteristics, such as a cloud's relative transparency, or a particular arrangement of its elements.

• Intortus: Cirrus, the filaments of which are very irregularly curved and often seemingly entangled in an erratic and unpredictable (known as capricious) manner.
• Vertebratus: Clouds, the elements of which are arranged in a manner suggestive of vertebrae, ribs or a fish skeleton. This term applies mainly to Cirrus.
• Undulatus: 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. This term applies mainly to Cirrocumulus, Cirrostratus, Altocumulus, Altostratus, Stratocumulus and Stratus.
• Radiatus: 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)”. This term applies mainly to Cirrus, Altocumulus, Altostratus, Stratocumulus and Cumulus.
• Lacunosus: 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. This term applies mainly to Cirrocumulus and Altocumulus; it may also apply, though very rarely, to Stratocumulus.
• Duplicatus: Cloud patches, sheets or layers, at slightly different levels in two or more layers, sometimes partly merged. This term applies mainly to Cirrus, Cirrostratus, Altocumulus, Altostratus and Stratocumulus.
• Translucidus: 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. This term applies to Altocumulus, Altostratus, Stratocumulus and Stratus.
• Perlucidus: 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 overlying clouds to be seen. It may also be observed in combination with the varieties translucidus or opacus. This term applies to Altocumulus and Stratocumulus.
• Opacus: An extensive cloud patch, sheet or layer, the greater part of which is sufficiently opaque to mask completely the Sun or Moon. This term applies to Altocumulus, Altostratus, Stratocumulus and Stratus.

Supplementary cloud features

Clouds sometimes have supplementary features attached to or partly merged with them.

• Incus: The upper portion of a Cumulonimbus spread out in the shape of an anvil with a smooth, fibrous or striated appearance
• Mamma: Hanging protuberances, like udders, on the under surface of a cloud. Occurs mostly with Cirrus, Cirrocumulus, Altocumulus, Altostratus, Stratocumulus and Cumulonimbus.
• Virga: Vertical or inclined trails of precipitation (fallstreaks) attached to the under surface of a cloud that do not reach the Earth’s surface. Occurs mostly with Cirrocumulus, Altocumulus, Altostratus, Nimbostratus, Stratocumulus, Cumulus and Cumulonimbus
• Praecipitatio: Precipitation (rain, drizzle, snow, ice pellets, hail, etc.) falling from a cloud and reaching the Earth’s surface. Mostly encountered with Altostratus, Nimbostratus, Stratocumulus, Stratus, Cumulus and Cumulonimbus.
• Arcus: 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 arch. Occurs with Cumulonimbus and, less often, with Cumulus.
• Tuba: Cloud column or inverted cloud cone, protruding from a cloud base; it constitutes the cloudy manifestation of a more or less intense vortex. Occurs with Cumulonimbus and, less often, with Cumulus.
• Asperitas: Well-defined, wave-like structures in the underside of the cloud; more chaotic and with less horizontal organization than the variety undulatus. Asperitas is characterized by localized waves in the cloud base, either smooth or dappled with smaller features, sometimes descending into sharp points, as if viewing a roughened sea surface from below. Varying levels of illumination and thickness of the cloud can lead to dramatic visual effects. Occurs mostly with Stratocumulus and Altocumulus.
• Fluctus: A relatively short-lived wave formation, usually on the top surface of the cloud, in the form of curls or breaking waves (Kelvin-Helmholtz Waves). Occurs mostly with Cirrus, Altocumulus, Stratocumulus, Stratus and occasionally Cumulus.
• Cavum: A well-defined generally circular (sometimes linear) hole in a thin layer of supercooled water droplet cloud. Virga or wisps of Cirrus typically fall from the central part of the hole, which generally grows larger with time. Cavum is typically a circular feature when viewed from directly beneath, but may appear oval shaped when viewed from a distance. When resulting directly from the interaction of an aircraft with the cloud, it is generally linear (in the form of a dissipation trail). Virga typically falls from the progressively widening dissipation trail. Occurs in Altocumulus and Cirrocumulus and rarely Stratocumulus.
• Murus: A localized, persistent, and often abrupt lowering of cloud from the base of a Cumulonimbus from which tuba (spouts) sometimes form. Usually associated with a supercell or severe multicell storm; typically develop in the rain-free portion of a Cumulonimbus and indicate an area of strong updraft. Murus showing significant rotation and vertical motion may result in the formation of tuba (spouts). Commonly known as a 'wall cloud'.
• Cauda: A horizontal, tail-shaped cloud (not a funnel) at low levels extending from the main precipitation region of a supercell Cumulonimbus to the murus (wall cloud). It is typically attached to the wall cloud, and the bases of both are typically at the same height. Cloud motion is away from the precipitation area and towards the murus, with rapid upward motion often observed near the junction of the tail and wall clouds. Commonly known as a 'tail cloud'.

Accessory clouds

Clouds may sometimes be accompanied by other usually smaller clouds, known as accessory clouds, which are separate from the main cloud body or partly merged with it.

• Pileus: An accessory cloud of small horizontal extent, in the form of a cap or hood above the top or attached to the upper part of a cumuliform cloud that often penetrates it. Several pileus may fairly often be observed in superposition. Occurs principally with Cumulus and Cumulonimbus.
• Velum: An accessory cloud veil of great horizontal extent, close above or attached to the upper part of one or several cumuliform clouds that often pierce it. Occurs principally with Cumulus and Cumulonimbus.
• Pannus: Ragged shreds sometimes constituting a continuous layer, situated below another cloud and sometimes attached to it. Occurs mostly with Altostratus, Nimbostratus, Cumulus and Cumulonimbus
• Flumen: Bands of low clouds associated with a supercell severe convective storm (Cumulonimbus), arranged parallel to the low-level winds and moving into or towards the supercell. These accessory clouds form on an inflow band into a supercell storm along the pseudo-warm front. The cloud elements move towards the updraft into the supercell, the base being at about the same height as the updraft base. Note that flumen are not attached to the murus wall cloud and the cloud base is higher than the wall cloud. One particular type of inflow band cloud is the so-called 'Beaver's tail'. This is distinguished by a relatively broad, flat appearance suggestive of a beaver's tail.

Special clouds

In addition, there are special cases where clouds may form or grow as a consequence of certain, often localized, generating factors. These may be either natural, or the result of human activity. Several cases of “special clouds” can be distinguished:

• Flammagenitus: Clouds may develop as a consequence of convection initiated by heat from forest fires, wildfires or volcanic eruption activity. Clouds that are clearly observed to have originated as a consequence of localized natural heat sources, such as forest fires, wildfires or volcanic activity and which, at least in part, consist of water drops, will be given the name relevant to the genus followed, if appropriate, by the species, variety and any supplementary features, and finally by the special cloud name “flammagenitus”, (for example, Cumulus congestus flammagenitus or Cumulonimbus calvus flammagenitus). (Note: Cumulus flammagenitus is also known by the unofficial, common name, 'Pyrocumulus').
• Homogenitus: Clouds may also develop as a consequence of human activity. Examples are aircraft condensation trails (contrails), or clouds resulting from industrial processes, such as cumuliform clouds generated by rising thermals above power station cooling towers. Clouds that are clearly observed to have originated specifically as a consequence of human activity will be given the name of the appropriate genus, followed by the special cloud name “homogenitus”. For example, Cumulus cloud formed above industrial plants will be known as Cumulus (and, if appropriate, the species, variety and any supplementary features) followed by the special cloud name homogenitus; for example, Cumulus mediocris homogenitus.
• Cataractagenitus: Clouds may develop locally in the vicinity of large waterfalls as a consequence of water broken up into spray by the falls. The downdraft caused by the falling water is compensated for by the locally ascending motion of air. These special clouds will be given the name of the appropriate genus, followed by any appropriate species, variety and supplementary feature, and followed by the special cloud name “cataractagenitus” (for example, Cumulus cataractagenitus or Stratus cataractagenitus).
• Silvagenitus: Clouds may develop locally over forests as a result of increased humidity due to evaporation and evapotranspiration from the tree canopy. These special clouds will be given the name of the appropriate genus, followed by any appropriate species, variety and supplementary feature, and followed by the special cloud name “silvagenitus” (for example, Stratus silvagenitus).
• Supercillium: Short-lived broken thin whips of cloud, irregularly distributed, appearing to tumble and break in a turbulent way in the airflow over, and to the immediate lee of, steep mountain peaks during periods of strong mountain summit level winds. Applies mainly to Altocumulus and Stratocumulus, possibly Cirrocumulus, sometimes coincident with the Duplicatus variety. (Editor's note: Supercillium is a cloud feature proposed by the Cloud Appreciation Society and is not yet recognised by the WMO).

The cloud species and Varieties associated with the ten cloud genera

• Cirrus
  • uncinus
  • fibratus
  • floccus
  • castellanus
  • spissatus
  • intortus
  • radiatus
  • Vertebratus

• Cirrostratus
  • nebulosus
  • fibratus
  • duplicatus
  • undulatus

• Cirrocumulus
  • startiformis
  • lenticularis
  • castellanus
  • floccus
  • undulatus
  • lacunosus

• Altostratus
  • translucidus
  • opals

• Altocumulus
  • lenticularis
  • stratformis
  • cumulogenitus
  • castellanus
  • floccus
  • translucidus
  • duplicatus

• Nimbostratus

(no sub-species)

• Stratocumulus
  • cumulogenitus
  • stratformis
  • castellanus
  • lenticulari
  • translucidus
  • perlucidus
  • opacus
  • duplicatus
  • undulatus
  • radiatus
  • lacunosus

• Stratus
  • nebulosus
  • fractus
  • opacus
  • translucidus
  • undulatus

• Cumulus
  • humilis
  • fractus
  • congestus
  • mediocrity
  • radiatus

• Cumulonimbus
  • calvus
  • capillatus

Related Articles

• Cloud Classification: Cloud codes and chart symbols
• Upper Atmospheric Cloud

Further Reading

• International Cloud Atlas; World Meteorological Organization (WMO).