3.2 Cloud Classification (Genera, Species, Varieties & Features)

3.2.1 Cloud genera

Cloud forms are based on ten main genera, conventionally grouped into three altitude bands — high, medium and low — plus a vertically developed group. About 90% of atmospheric moisture exists below 20 000 feet with 50% or more in the band below 6500 feet. The altitudes included in each band are dependent on the thickness of the troposphere at nominal locations — tropical, temperate or polar.

A two-letter code is used to identify cloud genera in meteorological reports, observations and aviation area forecasts.

Cirrus [CI] (Latin for 'curl'): white patches, banners or delicate filaments. They often have rather dense 'generating heads' with fall streaks of ice crystals dropping from them which are comma-shaped or hooked — ‘mares tails’. Cirrus are usually detached but may merge into CS or CB. They are formed by widespread ascent, but sometimes by turbulence.


Cirrostratus [CS]: a transparent, amorphous, whitish veil of fibrous or smooth appearance over much of the sky. They create the appearance of halos about the sun or moon. Cirrostratus may merge into CC or possibly AS, and are formed by widespread ascent.


Cirrocumulus [CC]: thin, white patches, sheets or layers with small, regularly arranged elements in the form of grains or ripples, which may be merged or separate — a true ‘mackerel sky’. The apparent width of elements is less than one degree. Cirrocumulus may merge into CS, and are produced by turbulence aloft — often associated with a front or upper-level disturbance.
Medium-level clouds

Altostratus [AS]: grey/bluish sheet, with coverage of possibly 8 oktas, of uniform appearance. They are often striated or fibrous, having parts thin enough to reveal a vague sun without any halo but possibly a corona. Altostratus often merges into NS. They are caused by widespread ascent and are usually associated with a front or upper-level disturbance.


Altocumulus [AC]: white/grey patches, bands or sheets of regularly arranged globular elements (sometimes called mackerel sky) — waves or rows with light shading, closely packed or merged. The element width is 2 to 5 degrees. (A finger width at arm's length is approximately 2 degrees; the spread between the tips of the little finger and thumb when a hand is splayed is about 22 degrees.) Altocumulus often shows coloured patches (irisation) around elements when illuminated by the sun or moon; a corona may be visible. They are usually caused by turbulence and are not associated with a change in the weather.


Nimbostratus [NS] (from the Latin 'nimbus' = cloud, aureole): thick, dense, dark grey layer, often with a ragged or diffused base, with continuous precipitation. Coverage is often 8 oktas. Scud (pannus) may form beneath it. Invariably they occur at medium level, but usually extend to high level and merge with AS; they may also extend to low levels and envelop hills. Nimbostratus are produced by widespread ascent.
Low-level clouds

Stratocumulus [SC]: grey/whitish patches, sheets or layers of separate or partly merged globular masses or rolls with dark shading and generally irregular appearance. If regularly arranged, the separate elements have apparent width exceeding 5 degrees. Coverage is often 8 oktas and may be penetrated by large CU or CB. Stratocumulus are probably the most frequently seen cloud in south-eastern Australia and are most frequent in winter anticyclones — ‘anticyclonic gloom’ — when moist air is trapped under an inversion. They are particularly noticeable around Melbourne.


Stratus [ST] (Latin = spread, laid down): grey, uniform layer with fairly even base from which drizzle may descend. The sun outline may be visible. Stratus envelops low hills. They sometimes appear in ragged patches, which are produced by frictional turbulence or possibly orographic ascent.


Cumulus [CU] (Latin = heap): white, heaped tops with generally grey, horizontal bases. Form is usually sharply outlined but may be ragged if evaporating. Vertical development varies greatly with atmospheric buoyancy, and bases can be at low or medium levels. Cumulus are formed by convection or possibly orographic ascent.
Vertically developed clouds

Cumulonimbus [CB]: heavy, dense cloud with massive vertical development, bases at low or medium levels, with tops possibly reaching (even overshooting) the tropopause. They may have a 'boiling' appearance during their vertical development stage. The base is usually very dark with lighter inflow areas. They are associated with heavy showers or virga — precipitation that evaporates before reaching the surface. Frequently low, ragged, turbulence cloud is mixed beneath it. Cumulonimbus are produced by vigorous convection. Refer to section 3.6.

For more information on the types and dangers of thunderstorms read sections 9.4 through 9.7.


Towering cumulus [TCU]: CU with cauliflower appearance, often of great vertical extent. Properly known as cumulus congestus [CU CON].
Cloud structure and composition
Cloud type Height of base Vertical extent Composition Associated precipitation
CI 20 000 + usually thin* ice crystals fall streaks
CS 20 000 + usually thin ice crystals nil
CC 20 000 + usually thin crystals/droplets nil
AS 6000 – 20 000+ up to 15 000 usually crystals, occasionally mixed rain/snow
AC 6000 – 20 000 usually thin usually droplets to –10 °C, some crystals to –30 °C occasionally mixed rain, drizzle

NS 0 – 8000+ merges into AS water droplets steady rain, snow, ice pellets
SC 1500 – 4000 500 – 3000 mainly droplets down to –15 °C rain, drizzle, virga
ST 0 – 2000 200 – 1000 usually water droplets drizzle
CU, TCU 1500 – 15 000 up to 15 000 water droplets rain showers
CB 1500 – 5000 15 000 – 35 000+ mainly droplets to –15 °C, mixed at lower temperatures rain/snow showers/virga, hail, ice pellets
*With fall streaks, the vertical extent of CI may exceed 5000 feet


Photographs and more information on cloud classes and identification techniques can be found at the Australian Severe Weather website.

3.2.2 Cloud species

Each of the cloud genera are subdivided into species by the addition of a common species descriptor (with a three-letter code), according to cloud shape and structure.

Fibratus [FIB]: CI and CS in the form of irregularly curved or nearly straight filaments, but without tufts or hooks. CI FIB, CS FIB


Spissatus [SPI]: dense or thickened CI plumes or CS, often originating from a CB anvil. Appears greyish when viewed towards the sun. CI SPI, CS SPI


Uncinus [UNC] (Latin = hook): CI that are hooked or comma-shaped. ‘Mares tail cirrus’. Ice crystals are forming at the high point of the fall streak where a small tuft of cloud may appear — the generating head. CI UNC


Nebulosus [NEB]: CS and AS as a nebulous veil with no detail. Also applied to low amorphous ST — lifted fog. CS NEB, AS NEB, ST NEB


Stratiformus [STR]: AC and SC, occasionally CC, spread out into an extensive sheet or layer. CC STR, AC STR, SC STR


Lenticularis [LEN]: AC of orographic wave origin, sometimes CC or SC; occurs as a lens or almond shape with a sharp margin, and often elongated if produced by a long ridge. They sometimes display iridescence. May form in long bands parallel to the Great Dividing Range and extend 50 to 100 nm downstream, towards the east; see mountain waves. CC LEN, AC LEN, SC LEN


Castellatus [CAS]: having a turreted or crenellated appearance and connected to a common cloud base line. They are generally AS (but forming AC), or sometimes SC, CI or CC, signifying increasing instability. AC CAS may precede the development of CB.


Floccus [FLO] (Latin = tuft of wool): CI, CC or particularly AC occurring in chaotic form, like a flock of sheep, each unit having a ragged base and a small cumuliform tuft above; 'thundery skies'. Often accompanied by virga. If developing CU reach this humid and unstable layer then energetic CB may develop.


Fractus [FRA]: ST or CU shreds with broken, ragged or wispy appearance, associated with formation or dispersion of low cloud. CU FRA often appears early in the morning, rising only slightly above the condensation level; they are also found in precipitation under CB. ST FRA is much darker than CU FRA when found under CB. ST FRA normally forms below NS or AS, and derives moisture from evaporating raindrops or surface water. Uplift from near-surface turbulence may produce ST FRA, particularly in areas of rising ground or low hills. If forming without overlying cloud, ST FRA forewarns of worsening low-level visibility and ST formation. Pannus or scud is a mix of CU FRA and ST FRA.


Humilis [HUM] (Latin = lowly): CU with small development and usually flattened at an inversion that is not far above the condensation level — 'fair weather CU'. Lifetime is 5 to 45 minutes. CU HUM


Mediocris [MED] (Latin = of middle degree): CU of intermediate vertical growth, occurring at no more than 3000 feet. They have tops showing small protuberances that are not actively growing. CU MED


Congestus [CON] (Latin = piled up): CU with cauliflower appearance, often of great vertical extent, perhaps 10 000 feet; generally known as towering CU [TCU]. Freezing does not occur. CU CON may produce heavy showers or microbursts, the latter particularly so in northern Australia.


Calvus [CAL] (Latin = bald): developing CB prior to anvil stage, but at least some of its upper part is losing its CU outline due to freezing. CB CAL


Capillatus [CAP] (Latin = hair): CB with distinct icy, upper fibrous or striated cirriform appearance. Frequently anvil-shaped, or untidy plumes, or disordered cirrus mass. CB CAP


3.2.3 Cloud varieties

Each of the cloud genera and species can be further classified into varieties by use of a common descriptor for element arrangement, transparency, etc.

Intortus [IN]: irregularly curved or tangled CI.


Vertebratus [VE]: CI looking like fish bone, ribs or vertebrae.


Lacunosus [LA]: thin CC or AC with regularly spaced, net-like holes.


Undulatus [UN]: parallel undulations in patches, sheets or layers of CC, CS, AC, AS, or SC.


Radiatus [RA]: broad, parallel bands of CI, AC, AS, CU or SC appearing to converge towards a radiation point on the horizon, or both horizons.


Duplicatus [DU]: more than one layer of CI, CS, AC, AS or SC at slightly different levels.


Translucidus [TR]: AC, AS, SC or ST in large sheets thin enough to show position of the sun or moon.


Perlucidus [PE]: AC or AS in broad layers or patches with small lanes that allow the sky to be seen.


Opacus [OP]: AS, AC, SC or ST that completely masks the sun or moon.
3.2.4 Accessory clouds

There are three cloud types that only exist in association with one of the main cloud genera:

Pileus (Latin = cap, hood, like mushroom cap): a short-lived, smooth lenticular cloud appearing in a humid stable layer above a CB or TCU when the rising thermal deflects the moving air in the layer up and over into the condensation level. Further CB or TCU development will push through the cap cloud, which may hang on as a temporary collar. There is a good photograph of such an event in the Sydney Storm Chasers website. In strong shear conditions, the cap cloud may form downwind.


Velum (Latin = veil): a thin, wide and persistent sheet of cloud accompanying a CB or TCU and forming in a humid, stable layer. Velum is dark in contrast to the convective cloud that generally rises through it.


Pannus (Latin = piece of cloth): a mix of CU FRA and ST FRA, or just a lump of ST. Scud rapidly forms or reforms generally at lower levels under precipitating CU, AS, CB or NS bases in turbulent lifting conditions, particularly when air rises rapidly at the edge of cool moist outflow, or a downburst or in upflow caused by the topography — and exacerbated by evaporation of moisture from forest canopies. Scud changes size and shape constantly, and may be drawn into the cloud base. Flight in a locality where pannus is forming — scud running — is a very dangerous activity for aviators.
3.2.5 Cloud features

Some notable cloud features are:

Incus (Latin = anvil): the anvil of a large CB, particularly a multicell or supercell storm, which has spread out, usually when upper-level winds are light. A severe storm attains maximum vertical development when the updraught reaches a stable layer which it is unable to break through — often the tropopause — and the cloud top spreads horizontally in all directions to form an overhanging anvil.

The photograph and text below appeared in the "NSW Lightning Bolt" of August 1997 — produced by the Severe Weather Section of the Bureau of Meteorology, NSW. That anvil had a spread of about 30 km. The rollover around the underside of the anvil indicates rapid expansion.

"Rose's magnificent photo (below) of a storm cloud near Millthorpe in NSW is familiar to many Bureau staff from the 1996 Weather Calendar, a 1995 Bureau Christmas card, and the new thunderstorm poster. The story of how the photo came to be taken may attract the writers at the Disney Studios. Rose relates the tale:

'... my son Ian phoned to tell me about the clouds and to ask if I had a spare film, as his camera was empty. I tied a film to our kelpie’s collar and sent him down the hill to Ian. Meanwhile, Ian’s daughter Melanie was cycling up to get the film ... by the time they both met Ian the cloud had started to break up. Fortunately by then I had climbed two fences and taken the two shots ...’ "



Arcus (Latin = arch, bow or curve): a shelf-like cloud indicating the inflow region at the leading edge of a thunderstorm or a squall line. If conditions are very humid the shelf cloud will be a low, thick, curved and well-formed cloud bank. If there is a sharp, severe gust front there may be a vortex indicated by twisting scud under, and leading up to, the shelf. A roll cloud, like a horizontal tube, may develop if the leading edge of the shelf speeds up and detaches. SC, AC roll clouds are also associated with mountain waves and solitary waves.

Granitus: a localised cloud (always forming below the lowest safe altitude [LSALT] marked on aeronautical charts) enclosing and obscuring a large chunk of land, usually in the form of a hill or peak. Granitus is sometimes known as 'stuffed CU', which refers to both the content and the consequences of entering such a cloud.

Wall cloud: a localised, possibly rotating, lowering from a CB cloud base. Situated at the main updraught with a diameter ranging from 0.5 km to 5 km. Refer to section 9.5.
The Sydney Storm Chasers website has many images of thunderstorm features. There are good photos of wall clouds, arcus, pannus and mammatus.



Mammatus: hard, downward protuberances, pouches or bulges from the underside of a CB anvil (frequently) or CI, CC, AS, AC or SC, indicating descending pockets of small droplets or ice crystals. The sinking, saturated air is cooler than the air around it. As it sinks it warms, but warming is retarded because some of the heat is used in evaporating cloud droplets in the saturated air. If more energy is required for evaporation than is generated by adiabatic warming, then the air and the cloud pouches will continue to sink and will elongate the protuberances. The mamma associated with CI and CC are very shallow, forming undulations in the cloud trails. Mamma associated with CB are an indication of a dissipating storm rather than severe turbulence.

Fall streaks: virga-like showers of ice crystals or snowflakes from CI generating heads, which sink at rates up to 0.5 m/sec but slowing as they sublimate. As they sink through several thousand feet they become deflected by falling into winds of lower velocity, or slow through sublimation, and thus appear to trail back from the parent head as hooks, mares’ tails, etc. Dense streaks combined with a strong drop in wind speed produce jet-stream banners — CI features that stream with the wind. AS and most stable cloud features lie across the wind.

Billow clouds: AC and AS found in a series of regular bands with clear areas between of similar width, occurring most frequently at 15 000 to 25 000 feet. At other times the upper surface of the cloud may have regular wave-like troughs and crests – undulatus.

When a higher-level inversion occurs, the upper and lower air layers are generally stable. If there is a significant difference in wind velocity between the layers then there is vertical wind shear at the interface, and a phenomenon known as 'Kelvin-Helmholtz shearing instability' causes the formation of long but short-lived waves across the interface — in much the same way as ocean waves — which grow in amplitude until they curl up and break. The waves produce an extensive but shallow area of clear air turbulence. If sufficient moisture exists, the waves become visible as Kelvin-Helmholtz billows. Billows always move with the wind so that in wave clouds they appear to move from the front to the rear of the formation, evaporating in the troughs and re-condensing in the crests.

Pyrocumulus: CU initiated by bushfire thermal activity. Ray Kennedy's photograph below shows a CU CON building above the brown smoke during the Gippsland bushfires on New Year's Day 1998.


3.2.6 Stratospheric clouds

Nacreous (mother-of-pearl) clouds are rare, high-latitude, stratospheric clouds resembling CC LEN or AC LEN. Small patches are occasionally formed in winter, usually in stationary standing waves, and often in the lee of mountain ranges, which provide abrupt uplift. They usually occur in the ozone layer at about 25 km with temperatures down to –80°C or –90°C. Nacreous clouds are probably composed of spherical ice crystals about one to two microns diameter. Brilliant iridescence is shown at angular distances up to 40 degrees from the sun, and green and pink colours predominate. These clouds are brightest at sunset but are rarely seen in daylight.


Noctilucent clouds [NLC] are rare, tenuous, mesospheric cloud formations only seen from higher-latitude locations, normally around 40° to 60° south, against a twilit (nautical and astronomical) sky in summer. Sufficient contrast for observation occurs when the sun is between 6° and 16° below the horizon with maximum contrast at 10° when solar illumination and light scatter is at the maximum. They are seen close to the sunward horizon and extend maybe 20° above, along the twilight arch, although the clouds can be seen at a much higher elevation. The clouds appear to be near the mesopause at about 80 km and are moving with the zonal easterlies. They resemble high CI with pronounced band or wave structures, commonly herring-bone, bluish-white to pure white with yellow beneath. They are probably composed of cosmic dust with thin ice deposition, saturation of traces of water vapour being reached through orographic waves resonated from the earth’s surface, or possibly oxidation of atmospheric methane.

The Australian Severe Weather website has many excellent images grouped into cloud classifications, cloud features and atmospheric phenomena.
3.2.7 ICAO / WMO Cloud continuity scale

SKC — sky clear, no cloud.
FEW — few clouds, one to two oktas cover.
SCT — scattered, 3 – 4 oktas cover. Clear intervals between clouds predominate.
BKN — broken, 5 – 7 oktas. Cloud masses predominate.
OVC — overcast, 8 oktas. Continuous, no clear intervals.

All information accredited to http://www.auf.asn.au/meteorology/index.html at page http://www.auf.asn.au/meteorology/section3.html