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.
The list of cloud types groups all cloud 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 is 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 amateurmeteorologist 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[edit]
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:
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.
A count of basic tropospheric variants that result from the division and subdivision of genus types into species and varieties is shown as a number in parentheses from V-1 (variant 1) through V-92 after each variety, after nimbostratus that has no sub-types, and after certain species that are not always dividable into varieties.
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 our 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[edit]
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,[1] 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.
For another tabular overview that also covers the entire homosphere, see Template:Cloud types.
Mesospheric cloud identification and classification[edit]
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[edit]
Mesospheric noctilucent clouds over Estonia
Noctilucent clouds are thin clouds that come in a variety of forms based from about 80 to 85 kilometres (262,000–279,000 ft) and occasionally seen in deep twilight after sunset and before sunrise.[6][7]
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[edit]
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[edit]
Nacreous polar stratospheric cloud (mother of pearl)
Sometimes known as type 2, a thin usually cirriform or lenticular (stratocumuliform) looking cloud based from about 18 to 30 kilometres (59,000–98,000 ft) and seen most often between sunset and sunrise.[8] Consists of ice crystals only.
Troposphere cloud identification and classification[edit]
Tropospheric clouds are divided into physical forms defined by structure, and levels defined by altitude range. These divisions 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-level cirriform, stratocumuliform, and stratiform[edit]
High clouds form in the highest and coldest region of the troposphere from about 5 to 12 km (16,500 to 40,000 ft) in temperate latitudes.[9][10] At this altitude water almost always freezes so high clouds are generally composed of ice crystals or supercooled water droplets.
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:
Sheets at different layers of the upper troposphere, which may be connected at one or more points; normally associated with fibratus and uncinus species.
High-level stratocumuliform 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, layered cirrocumulus is physically a high stratocumuliform genus.[14]
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.[16] 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.
Middle cloud forms from 2 to 7 km (6,500–23,000 ft) in temperate latitudes, and may be composed of water droplets or ice crystals depending on the temperature profile at that altitude range.[10]
Mid-level stratocumuliform 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. Layered forms of altocumulus are generally an indicator of limited convective instability, and are therefore mainly stratocumuliform in structure.
Altocumulus with circular holes caused by localized downdrafts; normally associated with stratiformis and castellanus species (also with cumuliform floccus species).[12]
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.[13]
Stratiform clouds of the genus altostratus form when a large convectively stable air mass is lifted to condensation in the middle level 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.
Accompanied by ragged lower layer of fractus species clouds forming in precipitation.[13]
Genitus mother clouds
Altostratus altocumulogenitus
Altostratus cumulonimbogenitus
Mutatus mother clouds
Altostratus cirrostratomutatus
Altostratus nimbostratomutatus
Towering vertical cumulonimbiform and cumuliform (low to mid-level cloud base)[edit]
Clouds with upward-growing vertical development usually form below 2 kilometres (6,600 ft),[10] but can be based as high as 2.5 kilometres (8,200 ft) in temperate climates, and often much higher in arid regions.
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 air mass is convectively highly unstable. They generally produce thunderstorms, rain or showers, and sometimes hail, strong outflowwinds, and/or tornadoes at ground level.
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).
Clouds of the genus nimbostratus tend to bring constant precipitation and low visibility. This cloud type normally forms above 2 kilometres (6,600 ft)[10] 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.
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.
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. Stratus that touches the Earth's surface is given the common name, fog, rather than a Latin name that applies only to clouds that form and remain aloft in the troposphere.
Latin for "castle-like": Applies to stratocumuliform (Sc cas, Ac cas, Cc cas) and dense cirriform (Ci cas) with a series of turret shapes – indicates air mass instability.
Congestus (Con)
Latin for "congested": Applies to cumuliform (Cu con/Tcu) with great vertical development and heaped into cauliflower shapes – indicates considerable air mass instability and strong upcurrents.
Fibratus (Fib)
"Fibrous": Cirriform (Ci fib) or high stratiform (Cs fib) in the form of filaments, can be straight or slightly curved; indicates strong, continuous upper winds.
Floccus (Flo)
"Tufted": Applies to stratocumuliform (Sc flo, Ac flo, Cc flo) and high cirriform (Ci flo); indicates some mid or high-level instability.
"Lens–like": Stratocumuliform (Sc len, Ac len, Cc len) having a lens-like appearance – formed by standing waves of wind passing over mountains or hills.
A translucent wave cloud - altocumulus lenticularis
Mediocris (Med)
"Medium-size": Cumuliform (Cu med) with moderate vertical extent; indicates moderate instability and upcurrents.
Nebulosus (Neb)
"Nebulous": Indistinct low and high stratiform (St neb, Cs neb) without features; indicates light wind if any and stable air mass.
Spissatus (Spi)
"Dense": Thick cirriform (Ci spi) with a grey appearance; indicates some upward movement of air in the upper troposphere.
Stratiformis (Str)
"Sheet-like": Horizontal cloud sheet of flattened stratocumuliform (Sc str, Ac str, Cc str); indicates very slight air mass instability.
Uncinus (Unc)
"Hook-like": Cirriform (Ci unc) with a hook shape at the top; indicates a nearby backside of a weather system.
Volutus (Vol)
"Rolled": Elongated, low or mid-level, tube shaped, stratocumuliform (Sc vol, Ac vol).
The division of genus types into species is as shown in the following table. The genus types (including some cumulus sub-types) are arranged from top to bottom in the left column in approximate descending order of average overall altitude range. The species are sorted from left to right in approximate ascending order of instability or vertical extent of the forms to which each belongs:
Stratiform species,
Cirriform species,
Stratocumuliform species,
Cumuliform species,
Cumulonimbiform species.
These ordinal instability numbers appear in each box where a particular genus has a particular species.
Latin for "Opaque". A thick sheet of stratiform or stratocumuliform cloud.
Perlucidus
"Semi-transparent". Sheet of stratocumuliform cloud with small spaces between elements.
Translucidus
"Translucent". Thin translucent patch or sheet of stratiform or stratocumuliform.
Pattern-based
Duplicatus
Latin for "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 and ragged edges.
Radiatus
"Radial". Clouds in one of several possible forms arranged in parallel lines that appear to converge at a central point near the horizon.
Undulatus
"Wavy". Stratiform or stratocumuliform cloud displaying an undulating pattern.
Vertebratus
"In the form of a back-bone". Cirriform arranged to look like the back-bone of a vertebrate.
The following table shows the cloud varieties arranged across the top of the chart from left to right in approximate descending order of frequency of appearance. The genus types and some sub-types associated with each variety are sorted in the left column from top to bottom in approximate descending order of average overall altitude range. Where applicable, the genera and varieties are cross-classified to show the species normally associated with each combination of genus and variety. The exceptions comprise the following: Altostratus that have varieties but no species so the applicable boxes are marked without specific species names; cumulus congestus, a species that has its own altitude characteristic but no varieties; cumulonimbus that have species but no varieties, and nimbostratus that has no species or varieties. The boxes for genus and species combinations that have no varieties are left blank.
"A ship's sail": An accessory cloud in the form of a sail.
Cumulonimbus velum
The supplementary features are associated with particular genera as follows. They are sorted from left to right in approximate decreasing order of frequency of occurrence for each of three categories. 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 supplementary feature.
Cataractagenitus (cataracta-/pertaining to a river cataract)
Formed from the mist at a waterfall, the downdraft caused from the cloud is counteracted by the ascending air displacement from the waterfall and may go on to form other types of clouds such as cumulus cataractagenitus.[20]
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.
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.
Level
Name
Class
Genitus mother
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
High
Cirrus
Ci
+
+
+
+
Cirrocumulus
Cc
+
+
+
Cirrostratus
Cs
+
+
+
+
+
Middle
Altocumulus
Ac
+
+
+
+
+
+
Altostratus
As
+
+
+
+
Towering vertical
Cumulonimbus
Cb
+
+
+
+
+
+
Cumulus congestus
Cu con or Tcu
Moderate vertical
Nimbostratus
Ns
+
+
+
+
+
Cumulus mediocris
Cu med
+
+
+
+
Low
Stratocumulus
Sc
+
+
+
+
+
+
+
Cumulus humilis
Cu hum
+
+
+
+
Stratus
St
+
+
+
+
Informal terms recently accepted for WMO classification with Latin nomenclature[edit]
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.
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).
Fluctus clouds over Mount Duval in AustraliaCrested 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.
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 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.[21]
WMO and informal terms related to free-convective cloud types and storms[edit]
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.
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.
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 line of cumulus clouds that sometimes develop at the leading edge of an advancing cold front that is often visible in satellite imagery.[22]
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.
Thick overcast clouds of sulfur dioxide and carbon dioxide in three main layers at altitudes of 45 to 65 km that obscure the planet's surface and can produce virga.[23]
Stratiform
Overcast opaque clouds sheets.
Stratocumuliform
Wave clouds with clear gaps through which lower stratiform layers may be seen.[24]
Cumuliform and cumulonimbiform
Embedded convective cells that can produce lightning.
Wave-cloud resembling stratocumulus, especially as a polar cap cloud over the winter pole which is mostly composed of suspended frozen carbon dioxide.[25][26]
Surface-based
Morning fog of water and/or carbon dioxide commonly forms in low areas of the planet.
Cloud decks in parallel latitudinal bands at and below the tropopause alternately 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.[28]
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.[29] an intermediate deck of ammonium hydrosulfide, and an inner deck of cumulus water clouds.[30][31]
^Michael Gadsden & Pekka Parviainen (September 2006). Observing Noctilucent Clouds(PDF). International Association of Geomagnetism & Aeronomy. p. 9. Archived from the original(PDF) on 31 October 2008. Retrieved 31 January 2011.
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