Clouds continuously evolve and appear in an infinite variety of forms. However, there is a limited number of characteristic forms frequently observed all over the world,so these are into which clouds can be broadly grouped in a classification scheme. The scheme uses genera(defined according to their appearance and position in the sky), species(describing shape and structure) and varieties(describing transparency and arrangement).This is similar to the systems used in the classification of plants or animals, and similarly uses Latin names.
There also two additional cloud classifications: Special clouds and Upper atmospheric clouds. These tend to be only rarely or occasionally observed and, in some cases, only in certain parts of the world.
The classification of clouds has ten main groups, called genera. Each observed cloud is a member of one, and only one, genus.
Most of the genera are thus subdivided into species, based on the shape of the clouds or their internal structure. A cloud, observed in the sky and identified as a specific genus, may bear the name of only one species.
Varieties are different arrangements of the visible elements of clouds and varying degrees of transparency. A variety may be common to several genera, and a cloud may show characteristics of more than one variety. When this is the case, all the observed varieties are included in the name of the cloud.
Most clouds are confined within their level with the following few notable exceptions:
(a) Altostratus is usually found in the middle level, but it often extends higher;
(b) Nimbostratus is almost always found in the middle level, but it usually extends into the other two levels;
(c) Cumulus and Cumulonimbus usually have their bases in the low level, but their vertical extent is often so great that their tops may reach into the middle and high levels.
When the height of a particular cloud is known, the concept of levels may be of some help to the observer in identifying the cloud. The genus can be determined by making a choice from among the genera normally encountered in the level corresponding to its height.
New cloud types
As might be expected, no additions have been made to the 10 genera or primary cloud types. However, a new cloud species has been added:
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
Five new supplementary features have also been added: asperitas, cavum, cauda, fluctus and murus.
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.
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.
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’.
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.
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’.
The updated Cloud Atlas recognizes a new accessory cloud – the name for features that merge with another cloud –
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.
To round things off, there are also five new “special clouds”, which describe localised conditions where clouds are generated due to either natural causes or human activity.The new special clouds recognised are: cataractagenitus, flammagenitus, homogenitus, silvagenitus and homomutatus.
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).
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.
Important examples 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).
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.
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).
Persistent contrails (Cirrus homogenitus) may be observed, over a period of time and under the influence of strong upper winds, to grow and spread out over a larger portion of sky, and undergo internal transformation such that the cloud eventually takes on the appearance of more natural cirri-form clouds.
In this instance, the resulting clouds will be given the name of the appropriate genus (for example, Cirrus, Cirrocumulus, or Cirrostratus) followed by any appropriate species, variety and supplementary features, followed by the special cloud name “homomutatus”, (for example, Cirrus floccus homomutatus or Cirrus fibratus homomutatus).