thumb|300px|Tropospheric cloud classification by altitude of occurrence. Multi-level and vertical genus-types not limited to a single altitude level include nimbostratus, cumulonimbus, and some of the larger cumulus species.|alt=g

The list of cloud types groups all genera as high (cirro-, cirrus), middle (alto-), multi-level (nimbo-, cumulo-, cumulus), and low (strato-, stratus). These groupings are determined by the altitude level or levels in the troposphere at which each of the various cloud types are normally found. Small cumulus are commonly grouped with the low clouds because they do not show significant vertical extent. Of the multi-level genus-types, those with the greatest convective activity are often grouped separately as towering vertical. The genus types all have Latin names.

The genera are also grouped into five physical forms. These are, in approximate ascending order of instability or convective activity: stratiform sheets; cirriform wisps and patches; stratocumuliform patches, rolls, and ripples; cumuliform heaps, and cumulonimbiform towers that often have complex structures. Most genera are divided into species with Latin names, some of which are common to more than one genus. Most genera and species can be subdivided into varieties, also with Latin names, some of which are common to more than one genus or species. 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. Very low stratiform clouds that touch the Earth's surface are given the common names fog and mist, which are not included with the Latin nomenclature of clouds that form aloft in the troposphere.

Above the troposphere, stratospheric and mesospheric clouds have their own classifications with common names for the major types and alpha-numeric nomenclature for the subtypes. They are characterized by altitude as very high level (polar stratospheric) and extreme level (polar mesospheric). Three of the five physical forms in the troposphere are also seen at these higher levels, stratiform, cirriform, and stratocumuliform, although the tops of very large cumulonimbiform clouds can penetrate the lower stratosphere.

Cloud identification and classification: Order of listed types

In section two of this page (Classification of major types), height ranges are sorted in approximate descending order of altitude expressed in general terms. On the cross-classification table, forms and genus types (including some genus sub-types) are shown from left to right in approximate ascending order of instability.

In sections three to five, terrestrial clouds are listed in descending order of the altitude range of each atmospheric layer in which clouds can form:

  • mesospheric layer;
  • stratospheric layer;
  • tropospheric layer.
  • Within the troposphere, the cloud levels are listed in descending order of altitude range.
  • Non-vertical genus types (including some genus sub-types) are sorted into approximate descending order of altitude of the cloud bases.
  • Vertical or multi-level genera and genus sub-types can be based in the low or middle levels and are therefore placed between the non-vertical low and mid-level genus types and sub-types. These thick clouds are listed in approximate descending order of altitude of the cloud tops.
  • The species associated with each genus type are listed in approximate ascending order of instability where applicable.
  • The constituent varieties and associated supplementary features and mother clouds for each genus or species are arranged in approximate order of frequency of occurrence.

In section six, the cloud types in the general lists and the mother clouds in the applicable classification table are sorted in alphabetical order except where noted. The species table shows these types sorted from left to right in approximate ascending order of the convective instability of each species. The table for supplementary features has them arranged in approximate descending order of frequency of occurrence.

In section seven, extraterrestrial clouds can be found in the atmospheres of other planets in the Solar System and beyond. The planets with clouds are listed (not numbered) in order of their distance from the Sun, and the clouds on each planet are in approximate descending order of altitude.

Cloud cross-classification throughout the homosphere

right|thumb|200px|Cloud chart showing major tropospheric cloud types identified by standard two-letter abbreviations and grouped by altitude and form. See table below for full names and classification.

The table that follows is very broad in scope much like the cloud genera template near the bottom of the article and upon which this table is partly based. There are some variations in styles of nomenclature between the classification scheme used for the troposphere (strict Latin except for surface based aerosols) and the higher levels of the homosphere (common terms, some informally derived from Latin). However, the schemes presented here share a cross-classification of physical forms and altitude levels to derive the 10 tropospheric genera, the fog and mist that forms at surface level, and several additional major types above the troposphere. The cumulus genus includes four species that indicate vertical size which can affect the altitude levels.

{| class="wikitable"

!

!Stratiform <br> non-convective

!Cirriform <br> mostly non-convective

!Stratocumuliform <br> limited-convective

!Cumuliform <br> free-convective

!Cumulonimbiform <br> strong-convective

|-

!Extreme-level

|Noctilucent (NLC) veils ||Noctilucent billows or whirls ||Noctilucent bands || ||

|-

!Very high-level

|Nitric acid & water polar stratospheric (PSC)||Cirriform nacreous PSC||Lenticular nacreous PSC|| ||

|-

!High-level

|Cirrostratus (Cs)||Cirrus (Ci)||Cirrocumulus (Cc)|| ||

|-

!Mid-level

|Altostratus (As)|| ||Altocumulus (Ac)|| ||

|-

!Towering vertical

| || || ||Cumulus congestus (Cu con)||Cumulonimbus (Cb)

|-

!Multi-level or moderate vertical

|Nimbostratus(Ns)|| || || Cumulus mediocris (Cu med)||

|-

!Low-level

|Stratus (St)|| ||Stratocumulus (Sc)||Cumulus humilis (Cu hum) or fractus (Cu fr)||

|-

!Surface-level

|Fog or mist|| || || ||

|}

Mesospheric cloud identification and classification

Clouds that form in the mesosphere come in a variety of forms such as veils, bands, and billows, but are not given Latin names based on these characteristics. These clouds are the highest in the atmosphere and are given the Latin-derived name noctilucent which refers to their illumination during deep twilight rather than their physical forms. They are sub-classified alpha-numerically and with common terms according to specific details of their physical structures.

Extreme-level stratiform, stratocumuliform, and cirriform

thumb|right|Mesospheric noctilucent clouds over Estonia

Noctilucent clouds are thin clouds that come in a variety of forms based from about and occasionally seen in deep twilight after sunset and before sunrise.

;Type 1 : Veils, very tenuous stratiform; resembles cirrostratus or poorly defined cirrus.

;Type 2 : Long stratocumuliform bands, often in parallel groups or interwoven at small angles. More widely spaced than cirrocumulus bands.

:;2A : Bands with diffuse, blurred edges.

:;2B : Bands with sharply defined edges.

;Type 3: Billows. Clearly spaced, fibrous cirriform, roughly parallel short streaks.

:;3A : Short, straight, narrow streaks.

:;3B : Wave-like structures with undulations.

;Type 4 : Whirls. Partial (or, more rarely, complete) cirriform rings with dark centers.

:;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 streaks.

:;4C : Whirls with large-scale ring structures.

Stratospheric cloud identification and classification

thumb|right|Stratospheric nacreous clouds over Antarctica

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.

Very high-level stratiform

;Nitric acid and water polar stratospheric : Sometimes known as type 1, a thin sheet-like cloud resembling cirrostratus or haze. Contains supercooled nitric acid and water droplets; sometimes also contains supercooled sulfuric acid in ternary solution.

Very high-level cirriform and stratocumuliform

;Nacreous polar stratospheric cloud (mother of pearl): Sometimes known as type 2, a thin usually cirriform or lenticular (stratocumuliform) looking cloud based from about and seen most often between sunset and sunrise. At this altitude water almost always freezes so high clouds are generally composed of ice crystals or supercooled water droplets.

Genus cirrus

Abbreviation: Ci

Cirriform clouds tend to be wispy and are mostly transparent or translucent. Isolated cirrus do not bring rain; however, large amounts of cirrus 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

thumb|Cirrus spissatus

thumb|Cirrus fibratus radiatus

thumb|Cirrus uncinus

; Cirrus fibratus: 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: Filaments with up-turned hooks or curls.

; Cirrus spissatus: Dense and opaque or mostly opaque patches.

; Cirrus castellanus: A series of dense lumps, or "towers", connected by a thinner base.

; Cirrus floccus: Elements which take on a rounded appearance on the top, with the lower part appearing ragged.

Varieties

; Opacity-based: None; always translucent except species spissatus which is inherently opaque.

; Fibratus pattern-based:

:; Cirrus fibratus intortus: Irregularly curved or tangled filaments.

:; Cirrus fibratus vertebratus: 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:

:; Cirrus uncinus radiatus:

; 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:

:; Cirrus uncinus duplicatus:

; Spissatus, castellanus, or floccus

; Genitus mother clouds:

:; Cirrus cirrocumulogenitus:

:; Cirrus altocumulogenitus:

:; Cirrus cumulonimbogenitus:

:; Cirrus homogenitus: Cirrus formed by spreading of aircraft contrails.

; Mutatus mother cloud:

:; Cirrus cirrostratomutatus:

:; Cirrus homomutatus: Cirrus formed by the complete transformation of cirrus homogenitus.

Genus cirrocumulus

thumb|right|250px|alt=A A large field of cirrocumulus clouds in a blue sky, beginning to merge near the upper left. | A large field of cirrocumulus stratiformis

Abbreviation: Cc.

High stratocumuliform species

; Cirrocumulus stratiformis: Sheets or relatively flat patches of cirrocumulus.

; Cirrocumulus lenticularis 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

:; 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; a funnel cloud or tornado.

Species

; Cumulus congestus; Moderate cumulus clouds arranged in parallel lines that appear to converge at the horizon.

; Flammagenitus (flamma-/pertaining to fire): Formed by convection associated with large wildfires.

; Homogenitus (homo-/pertaining to humans): Formed as a result of human activities, particularly aircraft at high altitudes and heat-generating industrial activities at surface level. If a homogenitus cloud of one genus changes to another genus type, it is then termed a homomutatus cloud.

; Silvagenitus (silva-/pertaining to trees or forests): Formed by low-level condensation of water vapor released by vegetation, especially forest canopies.

Mutatus mother clouds

Nomenclature works the same way as for genitus mother clouds except for the mutatus suffix to indicate the complete rather than the partial transformation of the original cloud type. e.g. Altocumulomutatus – formed by the complete transformation of altocumulus mother cloud.

The possible combinations of genera and mother clouds can be seen in this table. The genitus and mutatus clouds are each sorted from left to right in alphabetical order. The genus types and some sub-types are arranged from top to bottom in approximate descending order of average overall altitude range. Each box is marked where a particular genus or sub-type has a particular genitus or mutatus mother cloud.

{| class="wikitable" style="font-size: 85%;"

|-

| rowspan=3 | Level

| rowspan=3 | Name

| Class || colspan="10"|Genitus mother || colspan="10"|Mutatus mother

|-

| Abbrev. || Ac || As || Ci || Cc || Cs || Cu || Cb || Ns || St || Sc || Ac || As || Ci || Cc || Cs || Cu || Cb || Ns || St || Sc

|-

| Abbrev. || altocumulo || altostrato || cirro || cirrocumulo || cirrostrato || cumulo || cumulonimbo || nimbostrato || strato || stratocumulo || altocumulo || altostrato || cirro || cirrocumulo || cirrostrato || cumulo || cumulonimbo || nimbostrato || strato || stratocumulo

|-

|rowspan="3"|High || Cirrus || Ci || + || || || + || || || + || || || || || || || || + || || || || ||

|-

|Cirrocumulus || Cc || || || || || || || || || || || + || || + || || + || || || || ||

|-

|Cirrostratus || Cs || || || || + || || || + || || || || + || || + || + || || || || || ||

|-

|rowspan="2"|Middle || Altocumulus || Ac || || || || || || + || + || || || || || + || || + || || || || + || + ||

|-

|Altostratus || As || + || || || || || || + || || || || || || || || + || || || + || ||

|-

|rowspan="2"|Towering<br>vertical || Cumulonimbus || Cb || + || + || || || || + || || + || || + || || || || || || + || || || ||

|-

|Cumulus<br>congestus || Cu con<br> or Tcu || || || || || || || || || || || || || || || || || || || ||

|-

|rowspan="2"|Moderate<br>vertical || Nimbostratus || Ns || || || || || || + || + || || || || + || + || || || || || || || || +

|-

|Cumulus<br>mediocris || Cu med || + || || || || || || || || || + || || || || || || || || || + || +

|-

|rowspan="3"|Low || Stratocumulus || Sc || || + || || || || + || + || + || || || + || || || || || || || + || + ||

|-

|Cumulus<br>humilis || Cu hum || || || || || || + || + || + || || || || || || || || || || || || +

|-

|Stratus || St || + || || || || || || || || || + || || || || || || || || || + || +

|}

Informal terms recently accepted for WMO classification with Latin nomenclature

; Aviaticus cloud: Persistent condensation trails (contrails) formed by ice crystals originating from water vapor emitted by aircraft engines. Usually resembles cirrus; recognized as a WMO genitus cloud cirrus homogenitus (man-made). Further transformation into cirrus, cirrocumulus, or cirrostratus homomutatus is possible depending on atmospheric stability and wind shear.

; Fallstreak hole: Supercooled altocumulus or cirrocumulus distinguished by a hole with ragged edges and virga or wisps of cirrus. Accepted as a WMO supplementary feature to be named cavum (hole).

; Kelvin–Helmholtz cloud: thumb|Fluctus clouds over [[Mount Duval (New South Wales)|Mount Duval in Australia]] thumb|Fluctus clouds over [[San Diego Bay | San Diego Bay in California]] Crested wave-like clouds formed by wind-shear instability that may occur at any altitude in the troposphere. Accepted as a WMO supplementary feature 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. Accepted as a WMO genitus cloud with the Latin name flammagenitus, or homogenitus in the case of small cumulus formed by contained human activity.

; Roll cloud: thumb|Roll cloud over WisconsinElongated, low-level, tube shaped, horizontal formation not associated with a parent cloud. Accepted as a WMO stratocumulus or altocumulus species with the Latin name volutus.

  • 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.

thumb|Anvil rollover

  • 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.

thumb|Cumulus radiatus clouds

  • 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 or dust, often surrounding the base of a tornado, beneath a condensation funnel.
  • 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.
  • Marine layer
  • Overcast
  • 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 line of cumulus clouds that sometimes develop at the leading edge of an advancing cold front 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.
  • Sea of clouds
  • 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 and carbon dioxide in three main layers at altitudes of 45 to 65&nbsp;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 terrestrial types can be seen over Mars and are believed to be composed of water-ice.

; Extremely high cirriform: Noctilucent clouds are known to form near the poles at altitudes similar to or higher than the same type of clouds over Earth.

; 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.

; 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

Cloud layers composed mainly 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.