List of cloud types

The list of cloud types is a summarisation of the modern system of cloud classification according to their height, forming mechanism and other characteristics that have been adopted universally. In the troposphere, there are ten basic genus types with Latin names derived from five physical forms that are cross-classified into étages defined by altitude range. Most genera are divided into species and varieties, also with Latin names. The essentials of the modern nomenclature system for tropospheric clouds were proposed by Luke Howard, a British manufacturing chemist and an amateur meteorologist with broad interests in science, in an 1802 presentation to the Askesian Society. Since 1890, clouds have been classified and illustrated in cloud atlases. Mesospheric and stratospheric clouds have their own classifications of types and subtypes using mostly alpha-numeric nomenclature.

Cloud classification by altitude of occurrence. Towering vertical cumulus congestus not shown.

Formation of clouds

Clouds form in the Earth's atmosphere when water evaporates into vapor from oceans, lakes, ponds, and even streams and rivers; and by evaporation or transpiration over moist areas of Earth's land surface. The vapor rises up into colder areas of the atmosphere due to convective, orographic, or frontal lifting. This subjects the rising air to a process called adiabatic cooling.

The water vapor attaches itself to condensation nuclei which can be anything from dust to microscopic particles of salt and debris. Once the vapor has been cooled to saturation, the cloud becomes visible. All weather-producing clouds form in the troposphere, the lowest major layer of the atmosphere. However very small amounts of water vapor can be found higher up in the stratosphere and mesosphere and may condense into very thin clouds if the air temperatures are sufficiently cold. The nephology branch of meteorology is focused on the study of cloud physics.

Cloud classification: Order of listed types

Terrestrial mesospheric, stratospheric, and tropospheric classes are listed on this page in descending order of altitude range. Within the troposphere, étages of non-vertical clouds are also listed in descending order of altitude. The genus types within each étage are arranged in descending order of average cloud base height. Their constituent species, varieties, supplementary features and mother clouds are arranged in approximate order of frequency of occurrence. Vertical or multi-étage cloud groups and their constituent genera and species are listed in ascending order of average altitude of cloud tops. Their varieties, supplementary features, and mother clouds are arranged in order of approximate frequency of occurrence. A count of basic tropospheric variants is shown as a number in parentheses after each variety, after nimbostratus that has no sub-types, and after certain species that are not always dividable into varieties. Other planets in our solar system that have clouds are listed in order of their distance from the sun, and the clouds on each planet are in approximate descending order of altitude.

Polar mesospheric classification

Clouds that form above the mesosphere have a generally cirriform structure, but are not given Latin names based on that characteristic. Polar mesospheric clouds are the highest in the atmosphere and are given the Latin name noctilucent which refers to their illumination during deep twilight. They are sub-classified alpha-numerically according to specific details of their cirriform physical structure.

Extremely high cirriform

Noctilucent cloud over Estonia

Noctilucent clouds are thin, mostly cirriform-looking clouds, based from about 264,000 to 280,000 feet (80–85 km) and occasionally seen in deep twilight after sunset and before sunrise.

Type 1 

Very tenuous; resembles cirrus.

Type 2 

Bands. Long streaks, often in parallel groups or interwoven at small angles.

Subtypes

2A 

Streaks with diffuse, blurred edges.

2B 

Streaks with sharply defined edges.

Type 3

Billows. Clearly spaced, roughly parallel short streaks.

Subtypes

3A 

Short, straight, narrow streaks.

3B 

Wave-like structures with undulations.

Type 4 

Whirls. Partial (or, more rarely, complete) rings with dark centers.

Subtypes

4A 

Whirls possessing a small angular radius of curvature, sometimes resembling light ripples on a water surface.

4B 

Simple curve of medium angular radius with one or more bands.

4C 

Whirls with large-scale ring structures.

Polar stratospheric classification

Polar stratospheric clouds form at very high altitudes in polar regions of the stratosphere. Those that show mother-of-pearl colors are given the name nacreous. Both these and non-nacreous types are classified alpha-numerically according to their physical state and chemical makeup.

Very high cirriform

Nacreous (mother of pearl) and non-nacreous

Stratospheric nacreous clouds over Antarctica

A thin usually cirriform-looking cloud based from about 60,000 to 100,000 feet (18–30 km) and seen most often between sunset and sunrise.

Type 1 (non-nacreous) 

Contains supercooled nitric acid and water droplets.

Subtypes

1A 

Crystals of nitric acid and water.

1B 

Additionally contains supercooled sulfuric acid in ternary solution.

Type 2 (nacreous) 

Consists of ice crystals only.

Columnar clouds – rare, column-shaped.

Tropospheric classification

Tropospheric clouds are divided into physical forms defined by structure, and étages defined by altitude range. They are cross-classified to produce ten basic genus-types. They have Latin names as authorized by the World Meteorological Organization (WMO) that indicate physical structure, altitude or étage, and process of formation

High cirriform, stratocumuliform, and stratiform

High clouds form in the highest and coldest region of the troposphere from about 16,500 to 40,000 ft (5 to 12 km) in temperate latitudes. At this altitude water almost always freezes so high clouds are generally composed of ice crystals or supercooled water droplets.

Genus cirrus

Cirrus spissatus undulatus clouds

Cirrus uncinus clouds

Abbreviation: Ci

Cirrus tends to be wispy and are mostly transparent or translucent. Isolated cirrus clouds do not bring rain; however, large amounts of cirrus clouds can indicate an approaching storm system eventually followed by fair weather.

There are several variations of clouds of the cirrus genus based on species and varieties:

Species

• Cirrus fibratus (1)
  High clouds having the traditional "mare's tail" appearance. These clouds are long, fibrous, and curved, with no tufts or curls at the ends.
• Cirrus uncinus (2)
  Filaments with up-turned hooks or curls.
• Cirrus spissatus (3)
  Dense and opaque or mostly opaque patches.
• Cirrus castellanus (4)
  A series of dense lumps, or "towers", connected by a thinner base.
• Cirrus floccus (5)
  Elements which take on a rounded appearance on the top, with the lower part appearing ragged.

Opacity-based varieties

• None; always translucent except species spissatus which is inherently opaque.

Fibratus pattern-based varieties

• Cirrus fibratus intortus (6)
  Irregularly curved or tangled filaments.
• Cirrus fibratus vertebratus (7)
  Elements arranged in the manner of a vertebrate or fish skeleton.

Pattern-based variety radiatus
Large horizontal bands that appear to converge at the horizon; normally associated with fibratus and uncinus species.

• Cirrus fibratus radiatus (8)
• Cirrus uncinus radiatus (9)

Pattern-based variety duplicatus
Sheets at different layers of the upper troposphere, which may be connected at one or more points; normally associated with fibratus and uncinus species.

• Cirrus fibratus duplicatus (10)
• Cirrus uncinus duplicatus (11)

• Varieties are not commonly associated with Ci species spissatus, castellanus, or floccus.

Precipitation-based supplementary features

• Not associated with cirrus.

Cloud-based supplementary feature

• Mamma
  Bubble-like downward protuberances; mostly seen with species castellanus.

Genitus mother clouds

• Cirrus cirrocumulogenitus
• Cirrus altocumulogenitus
• Cirrus cumulonimbogenitus

Mutatus mother cloud

• Cirrus cirrostratomutatus

Genus cirrocumulus

A large field of cirrocumulus stratiformis

Cirrus fibratus radiatus

Abbreviation: Cc.

Clouds of the genus cirrocumulus form when moist air at high tropospheric altitude reaches saturation, creating ice crystals or supercooled water droplets. Limited convective instability at the cloud level gives the cloud a rolled or rippled appearance. Despite the lack of a strato- prefix, cirrocumulus is physically more closely related to stratocumulus than the more freely convective cumulus genus.

Species

• Cirrocumulus stratiformis (12)
  Sheets or relatively flat patches of cirrocumulus.
• Cirrocumulus lenticularis (13)
  Lenticular, or lens-shaped high cloud.
• Cirrocumulus castellanus (14)
  Cirrocumulus with "towers", or turrets.
• Cirrocumulus floccus (15)
  Tufts with ragged bases.

Opacity-based varieties

• None (always translucent)

Pattern-based variety undulatus
Cirrocumulus with an undulating base; normally associated with stratiformis and lenticularis species.

• Cirrocumulus stratiformis undulatus (16)
• Cirrocumulus lenticularis undulatus (17)

Pattern-based variety lacunosus
Cirrocumulus with large clear holes; normally associated with stratiformis, castellanus, and floccus species.

• Cirrocumulus stratiformis lacunosus (18)
• Cirrocumulus castellanus lacunosus (19)
• Cirrocumulus floccus lacunosus (20)

Precipitation-based supplementary feature

• Virga
  Light precipitation that evaporates well above ground level; mostly seen with species stratiformis, castellanus, and floccus.

Cloud-based supplementary feature

• Mamma
  Bubble-like downward protuberances; mostly seen with species castellanus.

Genitus mother clouds

• None associated with cirrocumulus.

Mutatus mother clouds

• Cirrocumulus cirromutatus
• Cirrocumulus cirrostratomutatus
• Cirrocumulus altocumulomutatus

Genus cirrostratus

Cirrostratus nebulosus merging into darker altostratus translucidus

Abbreviation: Cs

Clouds of the genus cirrostratus consist of mostly continuous, wide sheets of cloud that covers a large area of the sky. It is formed when convectively stable moist air cools to saturation at high altitude, forming ice crystals. Frontal cirrostratus is a precursor to rain or snow if it thickens into mid-level altostratus and eventually nimbostratus as the weather front moves closer to the observer.

Species

• Cirrostratus fibratus (21)
  Cirrostratus sheet with a fibrous appearance, but not as detached as cirrus.
• Cirrostratus nebulosus (22)
  Featureless, uniform sheet.

Opacity-based varieties

• None (always translucent)

Fibratus pattern-based varieties

• Cirrostratus fibratus duplicatus (23)
  Separate or semi-merged sheets with one layer slightly above the other.
• Cirrostratus fibratus undulatus (24)
  Undulating waves.

Varieties are not commonly associated with Cs species nebulosus.

Supplementary features/accessory clouds

• Not associated with cirrostratus.

Genitus mother clouds

• Cirrostratus cirrocumulogenitus
• Cirrostratus cumulonimbogenitus

Mutatus mother clouds

• Cirrostratus cirromutatus
• Cirrostratus cirrocumulomutatus
• Cirrostratus altostratomutatus.

Middle stratocumuliform and stratiform

Middle cloud forms from 6,500 to about 23,000 ft (2 to 7 km) in temperate latitudes, and may be composed of water droplets or ice crystals depending on the temperature profile at that altitude range.

Genus altocumulus

Altocumulus floccus

Altocumulus lenticularis duplicatus

Altocumulus castellanus

Altocumulus stratiformis translucidus undulatus

Fallstreak hole -altocumulus stratiformis translucidus lacunosus

Abbreviation: Ac

Clouds of the genus altocumulus are not always associated with a weather front but can still bring precipitation, usually in the form of virga which does not reach the ground. This genus is generally an indicator of limited convective instability, and is therefore structurally more closely related to stratocumulus than to the more freely convective cumulus genus.

Species

• Altocumulus stratiformis (Always dividable into opacity-based varieties)
  Sheets or relatively flat patches of altocumulus.
• Altocumulus lenticularis (25)
  Lens-shaped middle cloud. Includes informal variant altocumulus Kelvin–Helmholtz cloud, lenticular spiral indicative of severe turbulence.
• Altocumulus castellanus (26)
  Turreted middle cloud.
• Altocumulus floccus (27)
  Tufted middle clouds with ragged bases.

Stratiformis opacity-based varieties

• Altocumulus stratiformis translucidus (28)
  Translucent altocumulus through which the sun or moon can be seen.
• Altocumulus stratiformis perlucidus (29)
  Opaque middle clouds with translucent breaks.
• Altocumulus stratiformis opacus (30)
  Opaque altocumulus that obscures the sun or moon.

Pattern-based variety radiatus
Rows of altocumulus that appear to converge at the horizon; normally associated with stratiformis species.

• Altocumulus stratiformis translucidus radiatus (31)
• Altocumulus stratiformis perlucidus radiatus (32)
• Altocumulus stratiformis opacus radiatus (33)

Pattern-based variety duplicatus
Altocumulus in closely spaced layers, one above the other; normally associated with stratiformis and lenticularis species.

• Altocumulus stratiformis translucidus duplicatus (34)
• Altocumulus stratiformis perlucidus duplicatus (35)
• Altocumulus stratiformis opacus duplicatus (36)
• Altocumulus lenticularis duplicatus (37)

Pattern-based variety undulatus
Altocumulus with wavy undulating base; normally associated with stratiformis and lenticularis species.

• Altocumulus stratiformis translucidus undulatus (38)
• Altocumulus stratiformis perlucidus undulatus (39)
• Altocumulus stratiformis opacus undulatus (40)
• Altocumulus lenticularis undulatus (41)

Pattern-based variety lacunosus
Ac with circular holes caused by localized downdrafts; normally associated with stratiformis, castellanus, and floccus species.

• Altocumulus stratiformis translucidus lacunosus (42)
• Altocumulus stratiformis perlucidus lacunosus (43)
• Altocumulus stratiformis opacus lacunosis (44)
• Altocumulus castellanus lacunosus (45)
• Altocumulus floccus lacunosus (46)

Precipitation-based supplementary feature

• Virga
  Altocumulus producing precipitation that evaporates before reaching the ground; usually associated with species stratiformis, castellanus, and floccus.

Cloud-based supplementary feature

• Mamma
  Altocumulus (usually species castellanus) with downward facing bubble-like protuberances caused by localized downdrafts within the cloud.

Genitus mother clouds

• Altocumulus cumulogenitus
• Altocumulus cumulonimbogenitus

Mutatus mother clouds

• Altocumulus cirrocumulomutatus
• Altocumulus alsostratomutatus
• Altocumulus nimbostratomutatus
• Altocumulus stratocumulomutatus

Genus altostratus

Altostratus translucidus near top of photo merging into altostratus opacus near bottom

Altocumulus stratiformis perlucidus undulatus clouds merging into altostratus opacus, with higher layer of cirrus fibratus

Abbreviation: As

Clouds of the genus altostratus form when a large convectively stable airmass is lifted to condensation in the middle étage of the troposphere, usually along a frontal system. Altostratus can bring light rain or snow. If the precipitation becomes continuous, it may thicken into nimbostratus which can bring precipitation of moderate to heavy intensity.

Species

• No differentiated species (always nebulous).

Opacity-based varieties

• Altostratus translucidus (47)
  Altostratus through which the sun can be seen.
• Altostratus opacus (48)
  Altostratus that completely blocks out the sun.

Pattern-based variety radiatus
Bands that appear to converge at the horizon.

• Altostratus translucidus radiatus (49)
• Altostratus opacus radiatus (50)

Pattern-based variety duplicatus
Altostratus in closely spaced layers, one above the other.

• Altostratus translucidus duplicatus (51)
• Altostratus opacus duplicatus (52)

Pattern-based variety undulatus
Altostratus with wavy undulating base.

• Altostratus translucidus undulatus (53)
• Altostratus opacus undulatus (54)

Precipitation-based supplementary features

• Virga
  Accompanied by precipitation that evaporates before reaching the ground. Seen mostly with opacus varieties.
• Praecipitatio
  Produces precipitation that reaches the ground; associated with opacus varieties.

Cloud-based supplementary feature

• Mamma
  Altostratus with downward facing bubble-like protuberances caused by localized downdrafts within the cloud.

Accessory cloud
Seen mostly with opacus varieties

• Pannus
  Accompanied by ragged lower layer of fractus species clouds forming in precipitation.

Genitus mother clouds

• Altostratus altocumulogenitus
• Altostratus cumulonimbogenitus

Mutatus mother clouds

• Altostratus cirrostratomutatus
• Altostratus nimbostratomutatus

Low stratocumuliform, stratiform, and cumuliform

Low cloud forms from near surface to ca. 6,500 feet (2.0 km) and are generally composed of water droplets.

Genus stratocumulus

Stratocumulus cumulogenitus with higher layer of altocumulus stratiformis

Stratocumulus castellanus

Abbreviation: Sc

Clouds of the genus stratocumulus are lumpy, often forming in slightly unstable air, and they can produce very light rain or drizzle.

Species

• Stratocumulus stratiformis (always dividable into opacity-based varieties)
  Sheets or relatively flat patches of stratocumulus
• Stratocumulus lenticularis (55)
  Lens-shaped low cloud.
• Stratocumulus castellanus (56)
  Layer of turreted stratocumulus cloud with tower-like formations protruding upwards.

Stratiformis opacity-based varieties

• Stratocumulus stratiformis translucidus (57)
  Thin translucent stratocumulus through which the sun or moon can be seen.
• Stratocumulus stratiformis perlucidus (58)
  Opaque low clouds with translucent breaks.
• Stratocumulus stratiformis opacus (59)
  Opaque stratocumulus clouds.

Pattern-based variety radiatus
Stratocumulus arranged in parallel bands that appear to converge on the horizon; normally associated with stratiformis species..

• Stratocumulus stratiformis translucidus radiatus (60)
• Stratocumulus stratiformis perlucidus radiatus (61)
• Stratocumulus stratiformis opacus radiatus (62)

Pattern-based variety duplicatus
Closely spaced layers of stratocumulus, one above the other; normally associated with stratiformis and lenticularis species.

• Stratocumulus stratiformis translucidus duplicatus (63)
• Stratocumulus stratiformis perlucidus duplicatus (64)
• Stratocumulus stratiformis opacus duplicatus (65)
• Stratocumulus lenticularis duplicatus (66)

Pattern-based variety undulatus
Stratocumulus with wavy undulating base; normally associated with stratiformis and lenticularis species.

• Stratocumulus stratiformis translucidus undulatus (67)
• Stratocumulus stratiformis perlucidus undulatus (68)
• Stratocumulus stratiformis opacus undulatus (69)
• Stratocumulus lenticularis undulatus (70)

Pattern-based variety lacunosus
Sc with circular holes caused by localized downdrafts; normally associated with stratiformis and castellanus species.

• Stratocumulus stratiformis translucidus lacunosus (71)
• Stratocumulus stratiformis perlucidus lacunosus (72)
• Stratocumulus stratiformis opacus lacunosus (73)
• Stratocumulus castellanus lacunosus (74)

Precipitation-based supplementary features
Usually associated with species stratiformis and castellanus:

• Virga
  Low cloud producing precipitation that evaporates before reaching the ground.
• Praecipitatio
  Stratocumulus clouds producing precipitation that reaches the ground.

Cloud-based supplementary feature

• Mamma
  Stratocumulus with bubble-like protrusions on the underside; usually associated with species castellanus.

Genitus mother clouds

• Stratocumulus altostratogenitus
• Stratocumulus nimbostratogenitus
• Stratocumulus cumulogenitus
• Stratocumulus cumulonimbogenitus

Mutatus mother clouds

• Stratocumulus altocumulomutatus
• Stratocumulus nimbostratomutatus
• Stratocumulus stratomutatus

Genus stratus

At level with stratus nebulosus clouds

Stratus fractus cloud

Abbreviation: St

Clouds of the genus stratus form in low horizontal layers having a ragged or uniform base. Ragged stratus often forms in precipitation while more uniform stratus forms in maritime or other moist stable air mass conditions. The latter often produces drizzle.

Species

• Stratus nebulosus (75)
  Uniform fog-like low cloud.
• Stratus fractus (76)
  Ragged shreds of stratus clouds usually under base of precipitation clouds.

Nebulosus opacity-based varieties

• Stratus nebulosus translucidus (77)
  Thin translucent stratus.
• Stratus nebulosus opacus (78)
  Opaque stratus that obscures the sun or moon.

Pattern-based variety undulatus
Wavy undulating base.

• Stratus nebulosus translucidus undulatus (79)
• Stratus nebulosus opacus undulatus (80)

• Varieties are not commonly associated with St species fractus.

Precipitation-based supplementary feature

• Praecipitatio
  Stratus (usually species nebulosus) producing precipitation.

Accessory clouds

• Not usually seen with stratus.

Genitus mother clouds

• Stratus nimbostratogenitus
• Stratus cumulogenitus
• Stratus cumulonimbogenitus

Mutatus mother cloud

• Stratus stratocumulomutatus

Genus cumulus (little vertical extent)

Cumulus humilis

Abbreviation: Cu

These are fair weather cumuliform clouds of limited convection that do not grow vertically. The vertical height from base to top is generally less than the width of the cloud base. They appear similar to stratocumulus but the elements are generally more detached and less wide at the base.

Species

• Cumulus fractus (81)
  Ragged shreds of cumulus clouds.
• Cumulus humilis (82)
  "Fair weather clouds" with flat light grey bases and small white domed tops.

Opacity-based varieties

• None (always opaque except species fractus which is always translucent).

Humilis pattern-based variety

• Cumulus humilis radiatus (83)
  Small cumulus clouds arranged in parallel lines that appear to converge at the horizon.

Supplementary features/accessory clouds

• Not commonly seen with cumulus fractus or humilis.

Genitus mother clouds

• Cumulus altocumulogenitus
• Cumulus stratocumulogenitus

Mutatus mother clouds

• Cumulus stratocumulomutatus
• Cumulus stratomutatus

Vertical or multi-étage stratiform, cumuliform, and cumulonimbiform (low to middle cloud base)

Clouds with upward-growing vertical development usually form below 6,500 feet (2.0 km), but can be based as high as 8,000 feet (2.4 km) in temperate climates, and often much higher in arid regions. Downward-growing cloud forms mostly above 6,500 feet (2.0 km) and achieves vertical extent as the base subsides into the low altitude range during precipitation.

Genus nimbostratus: Moderate or deep vertical

Nimbostratus virga

Abbreviation: Ns (86)

Clouds of the genus nimbostratus tend to bring constant precipitation and low visibility. This cloud type normally forms above 6,500 feet (2.0 km) from altostratus cloud but tends to thicken into the lower levels during the occurrence of precipitation. The top of a nimbostratus deck is usually in the middle level of the troposphere.

Species

• No differentiated species (always nebulous).

Varieties

• No varieties (always opaque and never forms in patterns).

Precipitation-based supplementary features

• Virga
  Accompanied by precipitation that evaporates before reaching the ground.
• Praecipitatio
  Produces precipitation that reaches the ground.

Accessory cloud

• Pannus
  Nimbostratus with lower layer of fractus species cloud forming in precipitation.

Genitus mother clouds

• Nimbostratus cumulogenitus
• Nimbostratus cumulonimbogenitus

Mutatus mother clouds

• Nimbostratus altocumulomutatus
• Nimbostratus altostratomutatus
• Nimbostratus stratocumulomutatus

Genus cumulus: Moderate vertical

Cumulus mediocris (from above)

Cumulus mediocris pileus

Cumulus mediocris arcus

Abbreviation: Cu

Moderate vertical cumulus is the product of free convective airmass instability. Continued upward growth suggests showers later in the day.

Species

• Cumulus mediocris (84)

Moderate vertical clouds with flat medium grey bases and higher tops than cumulus humilis.

Opacity-based varieties

• None (always opaque)

Pattern-based variety

• Cumulus mediocris radiatus (85)
  Moderate cumulus clouds arranged in parallel lines that appear to converge at the horizon.

Precipitation-based supplementary features

• Virga
  Accompanied by precipitation that evaporates before reaching the ground.
• Praecipitatio
  Produces precipitation that reaches the ground.

Cloud-based supplementary feature

• Mamma
  Downward facing bubble-like protuberances caused by localized downdrafts within the cloud.

Accessory clouds

• Pileus
  Small cap-like cloud over parent cumulus cloud.
• Velum
  A thin horizontal sheet that forms around the middle of a cumulus cloud.

Mother clouds

• Genitus and mutatus types are the same as for cumulus of little vertical extent.

Genus cumulus: Towering vertical

Cumulus congestus

Abbreviation: Tcu (towering cumulus)

Species

• Cumulus congestus (87)

These large cumulus clouds have flat dark grey bases and very tall tower-like formations with tops mostly in the high level of the troposphere. The International Civil Aviation Organization (ICAO) designates this species as towering cumulus (Tcu).

Opacity-based varieties

• None (always opaque).

Pattern-based variety

• None (not generally discerned with highly unstable cumulus congestus).

Precipitation-based supplementary features

• Virga
  Accompanied by precipitation that evaporates before reaching the ground.
• Praecipitatio
  Produces precipitation that reaches the ground.

Cloud-based supplementary features

• Mamma
  Downward facing bubble-like protuberances caused by localized downdrafts within the cloud.
• Arcus (including roll and shelf clouds)
  Low horizontal cloud formation associated with the leading edge of a thunderstorm outflow.
• Tuba
  Column hanging from the cloud base which can develop into a small funnel cloud.

Accessory clouds

• Pannus
  Accompanied by a lower layer of fractus species cloud forming in precipitation.
• Pileus
  Small cap-like cloud over parent cumulus cloud.
• Velum
  A thin horizontal sheet that forms around the middle of a cumulus cloud.

Mother clouds

• Genitus and mutatus types are the same as for small and moderate cumulus.

Genus cumulonimbus: Towering vertical

Cumulonimbus calvus

Single-cell Cumulonimbus capillatus incus

Abbreviation: Cb

Clouds of the genus cumulonimbus have very dark gray to nearly black flat bases and very high tops that can penetrate the tropopause. They develop from cumulus when the airmass is convectively highly unstable. They generally produce thunderstorms, rain or showers, and sometimes hail, strong outflow winds, and/or tornadoes at ground level.

Species

• Cumulonimbus calvus (88)
  Cumulonimbus with high domed top.
• Cumulonimbus capillatus (89)
  Towering vertical cloud with high cirriform top.

Varieties

• No varieties (always opaque and does not form in patterns visible from surface level).

Precipitation-based supplementary features
Associated with calvus and capillatus species.

• Virga
  Precipitation that evaporates before reaching the ground.
• Praecipitatio
  Precipitation that reaches the ground.

Cloud-based supplementary features

• Incus (species capillatus only)
  Cumulonimbus with flat anvil-like cirriform top caused by wind shear where the rising air currents hit the inversion layer at the tropopause.
• Mamma
  Also sometimes called Mammatus, consisting of bubble-like protrusions on the underside caused by localized downdrafts.
• Arcus (including roll and shelf clouds)
  Low, horizontal cloud formation associated with the leading edge of thunderstorm outflow.
• Tuba
  Column hanging from the cloud base which can develop into a funnel cloud or tornado.

Accessory clouds
Seen with species and capillatus except where noted.

• Pannus
  Accompanied by a lower layer of fractus species cloud forming in precipitation.
• Pileus (species calvus only)
  Small cap-like cloud over parent cumulonimbus.
• Velum
  A thin horizontal sheet that forms around the middle of a cumulonimbus.

Genitus mother clouds

• Cumulonimbus altocumulogenitus
• Cumulonimbus altostratogenitus
• Cumulonimbus nimbostratogenitus
• Cumulonimbus stratocumulogenitus
• Cumulonimbus cumulogenitus

Mutatus mother cloud

• Cumulonimbus cumulomutatus

Alphabetical lists of tropospheric cloud types with Latin etymologies where applicable

WMO genera

• Altocumulus – altus and cumulus – high heap; now applied to middle-étage stratocumuliform.
• Altostratus – altus and stratus – high sheet; now applied to middle-étage stratiform.
• Cirrocumulus – cirrus and cumulus – thin, wispy heap; applied to high-étage stratocumuliform.
• Cirrostratus – cirrus and stratus – thin, wispy sheet; applied to high-étage stratiform.
• Cirrus – thin and wispy; applied to high-étage cirriform.
• Cumulonimbus – cumulus and nimbus (Latin for "raincloud") – precipitation-bearing heap; applied to vertical/multi-étage cumulonimbiform.
• Cumulus – Latin for "heap"; applied to low-étage and vertical/multi-étage cumuliform.
• Nimbostratus – nimbus and stratus – precipitation-bearing sheet; applied to multi-étage stratiform with vertical extent.
• Stratocumulus – stratus and cumulus – heap partly spread into a sheet; applied to low-étage stratocumuliform.
• Stratus – Latin for "sheet"; applied to low mostly shallow stratiform.

WMO species

• Castellanus – castle-like stratocumuliform (Sc cas, Ac cas, Cc cas) and dense cirriform (Ci cas) with a series of turret shapes – indicates air mass instability.
• Congestus – cumuliform (Cu con/Tcu) with great vertical development and heaped into cauliflower shapes – indicates considerable airmass instability and strong upcurrents.
• Fibratus – cirriform (Ci fib) or high stratiform (Cs fib) in the form of filaments, can be straight or slightly curved.
• Floccus – tufted middle and high stratocumuliform (Ac flo, Cc flo) and high cirriform (Ci flo) – indicates some mid and/or high level instability.
• Fractus – low stratiform (St fra) or cumuliform (Cu fra) with an irregular shredded appearance – forms in precipitation and/or gusty winds.
• Humilis – small, low, flattened cumuliform (Cu hum) – indicates relatively slight airmass instability.
• Lenticularis – stratocumuliform (Sc len, Ac len, Cc len) having a lens-like appearance – formed by standing waves of wind passing over mountains or hills.
• Mediocris – medium size cumuliform (Cu med) with bulges at the top – indicates moderate instability and upcurrents.
• Nebulosus – indistinct low and high stratiform (St neb, Cs neb) without features – indicates light wind if any and stable air mass.
• Spissatus – thick cirriform (Ci spi) with a grey appearance – indicates some upward movement of air in the upper troposphere.
• Stratiformis – horizontal cloud sheet of flattened stratocumuliform (Sc str, Ac str, Cc str) – indicates very slight airmass instability.
• Uncinus – cirriform (Ci unc) with a hook shape at the top – indicates a nearby backside of a weather system.

WMO varieties

• Duplicatus – double – closely spaced often partly merged layers of cloud in one of several possible forms.
• Intortus – twisted – curved and tangled cirriform.
• Lacunosus – full of holes – thin stratocumuliform cloud distinguished by holes (sometimes known as fallstreak holes) and ragged edges.
• Opacus – thick and shadowy – an opaque sheet of stratiform or stratocumuliform cloud.
• Perlucidus – translucent – sheet of stratocumuliform cloud with small spaces between elements.
• Radiatus – radial – clouds in one of several possible forms arranged in parallel lines that appear to converge at a central point near the horizon.
• Translucidus – transparent – thin translucent patch or sheet of stratiform or stratocumuliform.
• Undulatus – wavy – stratiform or stratocumuliform cloud displaying an undulating pattern.
• Vertebratus – skeletal and bone-like – cirriform arranged to look like bones, a skeleton or calcium.

WMO supplementary features and free-convective mother clouds

A translucent wave cloud -altocumulus lenticularis

• Arcus – arch or a bow – feature mostly attached to cumulus, thick with ragged edges.
• Cumulogenitus – formed by the spreading out of cumulus mother clouds.
• Cumulonimbogenitus – formed by the spreading out of cumulonimbus mother clouds.
• Incus – anvil – top part of Cb cloud, anvil-shaped feature.

Mammatus over Squaw Valley

• Mammatus (WMO term mamma) – breast – feature in the form of round pouches on under-surface of a cloud.
• Pannus – shredded cloth – shredded accessory cloud that forms in precipitation below main cloud.
• Pileus – capped – hood-shaped accessory cloud.
• Praecipitatio – falling – cloud whose precipitation reaches the ground.
• Tuba – shaped like a tube – feature in the form of a column hanging from the bottom of cumulus or cumulonimbus.
• Velum – a ship's sail – an accessory cloud in the form of a sail.

Informal terms officially proposed for WMO classification

• Aviaticus cloud - persistent condensation trails (contrails) formed by ice crystals originating from water vapor emitted by aircraft engines. May resemble cirrus, cirrocumulus, or cirrostratus depending on atmospheric stability and wind shear. Proposed as a WMO genitus cloud homogenitus (man-made).
• Fallstreak hole – supercooled altocumulus or cirrocumulus distinguished by a hole with ragged edges and virga or wisps of cirrus. Proposed as a WMO supplementary feature, possibly to be named cavus (hole).
• Kelvin-Helmholtz - Crested wave-like clouds formed by wind-shear instability that may occur at any altitude in the troposphere. Proposed as a WMO supplementary feature, possibly with the Latin name fluctus.
• Pyrocumulus and pyrocumulonimbus - cumulus and cumulonimbus clouds formed by quickly generated ground heat; including forest fires, volcanic eruptions and low level nuclear detonation. Proposed as a WMO genitus cloud, possibly with the Latin name flammagenitus, or homogenitus in the case of small cumulus formed by contained human activity.

Roll cloud over Wisconsin

• Roll cloud – elongated, low-level, tube shaped, horizontal formation not associated with a parent cloud. Proposed as a WMO stratocumulus species, possibly to have the Latin name volutus.

WMO and informal terms related to free-convective cloud types and storms

• Accessory cloud (WMO supplementary feature) – secondary cloud that is associated with but separate from a main cloud.
• Anvil (WMO supplementary feature incus) – the anvil top of a cumulonimbus cloud.
• Anvil dome (WMO supplementary feature incus) – the overshooting top on a Cb that is often present on a supercell.
• Anvil rollover – (slang) circular protrusion attached to underside of anvil.
• Arcus cloud (WMO supplementary feature) – arch or a bow shape, attached to cumulus, thick with ragged edges.
• Backsheared anvil – (slang) anvil that spreads upwind, indicative of extreme weather.
• Clear slot or dry slot (informal term) – an evaporation of clouds as a rear flank downdraft descends and dries out cloud and occludes around a mesocyclone.
• Cloud tags (WMO species fractus) – ragged detached portions of cloud.
• Collar cloud (WMO velum accessory cloud) – ring shape surrounding upper part of wall cloud.
• Condensation funnel (WMO supplementary feature tuba) – the cloud of a funnel cloud aloft or a tornado.
• Altocumulus castellanus (WMO genus and species) – castle crenellation-shaped altocumulus clouds.
• Cumulus (WMO genus) – heaped clouds.
• Cumulus castellanus – (informal variation of WMO genus and species cumulus congestus) cumulus with tops shaped like castle crenellations.
• Cumulus congestus (WMO genus and species) – considerable vertical development and heaped into cauliflower shapes.
• Cumulus fractus (WMO genus and species) – ragged detached portions of cumulus cloud.
• Cumulus humilis (WMO genus and species) – small, low, flattened cumulus, early development.
• Cumulus mediocris (WMO genus and species) – medium-sized cumulus with bulges at the top.
• Cumulus pileus (WMO genus and accessory cloud) – capped, hood-shaped cumulus cloud.
• Cumulus praecipitatio (WMO genus and supplementary feature) – cumulus whose precipitation reaches the ground.
• Cumulus radiatus (WMO genus and variety) – cumulus arranged in parallel lines that appear to converge near the horizon.
• Cumulus tuba (WMO genus and supplementary feature) – column hanging from the bottom of cumulus.
• Cumulonimbus (WMO genus) – heaped towering rain-bearing clouds that stretch to the upper levels of the troposphere.
• Cumulonimbus calvus (WMO genus and species) – cumulonimbus with round tops like cumulus congestus.
• Cumulonimbus capillatus (WMO genus and species) – Cb with cirriform top.
• Cumulonimbus incus (WMO genus and supplementary feature) – Cb capillatus with anvil top.
• Cumulonimbus mamma (WMO genus and supplementary feature) – Cb with pouch-like protrusions that hang from under anvil or cloud base.
• Cumulonimbus pannus (WMO genus and accessory cloud) – shredded sections attached to main Cb cloud.
• Cumulonimbus pileus (WMO genus and accessory cloud) – capped, hood-shaped cloud above a cumulonimbus cloud.
• Cumulonimbus praecipitatio (WMO genus and supplementary feature) – Cb whose precipitation reaches the ground.
• Cumulonimbus tuba (WMO genus and supplementary feature) – column hanging from the bottom of cumulonimbus.
• Debris cloud (informal term) – rotating "cloud" of debris found at base of tornado.
• Funnel cloud (WMO supplementary feature tuba) – rotating funnel of cloud hanging from under Cb, not making contact with ground.
• Hail fog (informal term) – a shallow surface layer of fog that sometimes forms in vicinity of deep hail accumulation, can be very dense.
• Hot tower (informal term) - a tropical cumulonimbus cloud that penetrates the tropopause.
• Inflow band (informal term) – a laminar band marking inflow to a Cb, can occur at lower or mid levels of the cloud.
• Inverted cumulus (informal variation of WMO supplementary feature mamma) – cumulus which has transferred momentum from an exceptionally intense Cb tower and is convectively growing on the underside of an anvil.
• Knuckles (informal variation of WMO supplementary feature mamma) – lumpy protrusion that hangs from edge or underside of anvil.
• Pyrocumulus and Pyrocumulonimbus– intense ground-heat cloud proposed for WMO classification (see above).
• Rope – (slang) narrow, sometimes twisted funnel type cloud seen after a tornado dissipates.
• Rope cloud (informal term) – A narrow, long, elongated lines of cumulus cloud formation that develop at the leading edge of an advancing cold front or weather fronts that is often visible in satellite imagery.
• Scud cloud (informal term for WMO species fractus) – ragged detached portions of cloud that usually form in precipitation.
• Shelf cloud (informal term for WMO supplementary feature arcus) – wedge-shaped cloud often attached to the underside of Cb.
• Stratus fractus (WMO genus and species) – ragged detached portions of stratus cloud that usually form in precipitation (see also scud cloud).
• Striations (informal term for WMO accessory cloud velum) – a groove or band of clouds encircling an updraft tower, indicative of rotation.
• Tail cloud (informal term) – an area of condensation consisting of laminar band and cloud tags extending from a wall cloud towards a precipitation core.
• Towering cumulus (TCu) (aviation term for WMO genus and species cumulus congestus) – a large cumulus cloud with great vertical development, usually with a cauliflower-like appearance, but lacking the characteristic anvil of a Cb.
• Wall cloud (informal term) – distinctive fairly large lowering of the rain-free base of a Cb, often rotating.

Other planets

Venus

Thick overcast clouds of sulfur dioxide in three main layers at altitudes of 45 to 65 km that obscure the planet's surface and can produce virga.

Stratiform

Overcast opaque clouds sheets.

Stratocumuliform

Wave clouds with clear gaps through which lower stratiform layers may be seen.

Cumuliform and cumulonimbiform

Embedded convective cells that can produce lightning.

Mars

Clouds resembling several terresrial genus-types can be seen over Mars and are believed to be composed of water-ice.

High cirriform

Thin scattered wispy cloud resembling cirrus through which the planet's surface can be seen.

High stratocumuliform

Thin scattered wave-cloud resembling cirrocumulus.

Low stratocumuliform

Wave-cloud resembling stratocumulus, especially as a polar cap cloud over the winter pole which is mostly composed of suspended frozen carbon dioxide.

Surface-based

Morning fog of water and/or carbon dioxide commonly forms in low areas of the planet.

Jupiter and Saturn

Cloud decks in parallel latitudinal bands at and below the tropopause alternatingly composed of ammonia crystals and ammonium hydrosulfate.

Cirriform

Bands of cloud resembling cirrus located mainly in the highest of three main layers that cover Jupiter.

Stratiform and Stratocumuliform

Wave and haze clouds that are seen mostly in the middle layer.

Cumuliform and cumulonimbiform

Convective clouds in the lowest layer that are capable of producing thunderstorms and may be composed at least partly of water droplets. an intermediate deck of ammonium hydrosulfide, and an inner deck of cumulus water clouds.

Uranus and Neptune

Clouds layers made mostly of methane gas.

Cirriform

High wispy formations resembling cirrus.

Stratiform

Layers of haze-cloud that lack any distinct features.

Cumuliform and cumulonimbiform

Lower-based convective clouds that can produce thunderstorms.

See also

Cloud species

External links

• http://nephology.eu International Cloud Atlas online
• Cloud Classification (National Weather Service)
• Ten Basic Cloud Types (National Weather Service)
• Cloud Appreciation Society
• Texas A&M Cloud Glossary
• Cloud-identification site
• UK Met Office cloud classification page
• Cloud Atlas (Atlas Chmur) (in Polish)
• NOAA