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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 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 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
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-93 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 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
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.
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
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
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.
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
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
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.
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
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
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.[22]
Anvil dome (WMO supplementary feature incus) – the overshooting top on a Cb that is often present on a supercell.[23]
Anvil rollover – (slang) circular protrusion attached to underside of anvil.[24]
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.[25]
Clear slot[26] or dry slot[27] (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.[28]
Collar cloud (WMO velum accessory cloud) – ring shape surrounding upper part of wall cloud.[29]
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 or dust, often surrounding the base of a tornado, beneath a condensation funnel.[30]
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.[31]
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.[32]
Stratiform
Overcast opaque clouds sheets.
Stratocumuliform
Wave clouds with clear gaps through which lower stratiform layers may be seen.[33]
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.[34][35]
Surface-based
Morning fog of water and/or carbon dioxide commonly forms in low areas of the planet.
Cloud decks in parallel bands of latitude 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.[37]
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.[38] an intermediate deck of ammonium hydrosulfide, and an inner deck of cumulus water clouds.[39][40]
^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.
^ abCarr, M. H.; Baum, W. A.; Blasius, K. R.; Briggs, G. A.; Cutts, J. A.; Duxbury, T. C.; Greeley, R.; Guest, J.; Masursky, H.; Smith, B. A. (January 1980). "NASA SP-441: Viking Orbiter Views of Mars". National Aeronautics and Space Administration. Retrieved 26 January 2013.
Clouds in Earth's atmosphere are classified into various types based on their altitude, shape, structure, and other physical characteristics, providing a standardized system for meteorologists and scientists to identify and study them. This classification, primarily developed and maintained by the World Meteorological Organization (WMO), recognizes ten main genera of clouds, grouped into high-level (above 6 km), middle-level (2–6 km), low-level (below 2 km), and vertically developed types that span multiple altitudes.[1]The foundational framework for this list traces back to the early 19th century, when British pharmacist Luke Howard proposed the initial Latin-based nomenclature for cloud genera, which evolved into the modern system codified in the WMO's International Cloud Atlas. High-level clouds, such as cirrus, cirrocumulus, and cirrostratus, are typically composed of ice crystals and appear thin or wispy due to their cold, upper-atmosphere formation. Middle-level clouds, including altocumulus and altostratus, often consist of water droplets or a mix of ice and water, forming layered or patchy formations that may signal approaching weather changes. Low-level clouds like stratus, stratocumulus, and nimbostratus are usually water-based and closer to the surface, frequently associated with overcast skies or precipitation. Vertically developed clouds, notably cumulus and cumulonimbus, exhibit significant upward growth from convection, with cumulonimbus often producing thunderstorms and heavy rain.[1][2]Beyond the genera, the classification system incorporates finer details through species (e.g., fibratus for fibrous shapes or stratiformis for layered forms), varieties (e.g., opacus for opaque or radiatus for radiating patterns), and supplementary features such as virga (trailing precipitation that evaporates before reaching the ground) or mamma (pouch-like structures). Accessory clouds (e.g., pileus caps) and other phenomena like contrails or noctilucent clouds are also noted, though the core list focuses on naturally occurring tropospheric types. This hierarchical structure enables precise observation and forecasting, with the WMO updating the atlas periodically to incorporate new observations from satellites and ground-based tools.[1][3]
Cloud Classification Fundamentals
Identification and Naming Conventions
The World Meteorological Organization (WMO) International Cloud Atlas serves as the authoritative reference for cloud classification, first published in 1896 as a collaborative effort among international meteorologists to standardize observations and nomenclature.[4] This atlas has evolved through multiple editions, with the 2017 version marking a significant update by transitioning to a fully digital, web-based format that incorporates high-resolution imagery, time-lapse videos, and global contributions to enhance accessibility and precision in identification. Since 2017, the digital atlas has undergone minor revisions, including 12 updates accepted in 2022 that added new supplementary features to reflect emerging observations from advanced imaging and satellite technology.[5] The system emphasizes objective criteria to ensure consistency across observers worldwide, drawing on photographic and descriptive evidence rather than subjective interpretations.Clouds are primarily identified based on three core criteria: their form and structure, altitude above the surface, and potential for precipitation. Form refers to the basic appearance, categorized into cirriform (detached, wispy elements like ice crystals), stratiform (layered or sheet-like), cumuliform (piled or heaped with vertical development), and combinations thereof, which reflect the underlying atmospheric processes such as convection or stability.[1] Altitude divides clouds into levels in temperate zones—high (5–13 km for cirrus, cirrocumulus, and cirrostratus), mid-level (2–7 km for altocumulus, altostratus, and nimbostratus), and low (0–2 km for stratus, stratocumulus, cumulus, and cumulonimbus)—though these ranges vary with latitude and season due to differences in tropopause height.[6] Precipitation potential is assessed by features indicating moisture release, such as virga (trailing precipitation that evaporates before reaching the ground) or praecipitatio (reaching the surface), which distinguish rain-bearing types like nimbostratus from non-precipitating ones.[1]The naming conventions follow a hierarchical Latin-based system established by the WMO, comprising 10 main tropospheric genera (e.g., cirrus for high-level wispy forms), 15 species that refine shape and texture (e.g., fibratus for fibrous appearance), 9 varieties describing opacity or arrangement (e.g., translucidus for translucent layers), and 14 supplementary features highlighting transient elements (e.g., arcus for roll clouds).[7] Accessory clouds (4 types, such as pileus caps) and other clouds (e.g., homogenitus from human activity) provide additional descriptors.[1] The 2017 revisions expanded this framework by officially recognizing new special clouds, including flammagenitus (formed by intense heat from fires or volcanoes), as well as new supplementary features and an additional accessory cloud.[8]Certain clouds defy strict categorization, spanning multiple levels or exhibiting hybrid characteristics; for instance, cumulonimbus often extends from low to high altitudes with anvil tops, while altocumulus castellanus combines mid-level layers with cumuliform turrets indicating instability.[9] These cross-classifications are noted in the atlas to reflect real-world variability, prioritizing the dominant level and form for primary identification while allowing supplementary notations for vertical extent or mixed features.[6]
Hierarchy and Order of Types
The cloud classification system adopted by the World Meteorological Organization (WMO) organizes clouds into a multi-level hierarchy to facilitate systematic identification and analysis. At the highest level are genera, which represent broad categories based primarily on height, form, and structure, encompassing ten principal types for tropospheric clouds plus distinct types for upper atmospheric layers.[1] Within each genus, species provide subdivisions according to specific shapes, stability, or developmental stages, such as fibratus for thread-like forms or castellanus for turreted elements.[1] Further refinement occurs through varieties, which describe subtle differences in transparency, layering, or internal structure, like intortus for twisted filaments or opacus for opaque patches.[1] Finally, features denote accessory elements that modify the overall appearance, including supplementary aspects such as virga (trailing precipitation) or mamma (pouch-like appendages), as well as accessory clouds like pileus (cap-like) and mother-clouds indicating origins from other types.[1]The logical order for listing cloud types begins with the highest altitudes to reflect the atmospheric layering, starting with mesospheric and stratospheric clouds before descending to tropospheric ones. Mesospheric clouds, such as noctilucent types, are addressed first due to their extreme elevation around 80-85 km, followed by stratospheric clouds like nacreous or polar stratospheric clouds at 15-30 km.[10] Within the troposphere, the sequence proceeds by altitude levels: high-level (above 6 km, e.g., cirrus genus), mid-level (2-7 km, e.g., altocumulus), low-level (below 2 km, e.g., stratus), and vertical development clouds (spanning multiple levels, e.g., cumulonimbus).[6] Supplementary and special types, including hybrid or unusual forms, are placed last to complete the coverage without disrupting the altitude-based progression.[1]Prioritization principles emphasize non-precipitating high-level clouds first, as they often signal approaching weather changes without immediate effects, followed by mid- and low-level types, and concluding with precipitating and vertical clouds that indicate active or severe conditions.[11] This order also groups by stability, listing stratiform (layered, horizontally developed) clouds before cumuliform (heaped, vertically developed) ones within levels, to align with their formation in stable versus unstable air masses and aid in forecasting sequences./06%3A_Clouds/6.04%3A_Cloud_Classification) For instance, in high-level tropospheric clouds, cirrus (diffuse and fibrous) precedes cirrostratus (sheet-like) and cirrocumulus (patchy heaps), mirroring this stability gradient.[1]In the homosphere—encompassing the troposphere, stratosphere, and mesosphere—cloud classifications interrelate through shared atmospheric dynamics but maintain distinct categories without overlap, as each layer's temperature, humidity, and stability dictate unique formation processes. Tropospheric clouds dominate weather phenomena, while stratospheric and mesospheric types arise under extreme polar or seasonal conditions, ensuring the hierarchy avoids redundancy by segregating them by altitude thresholds.[10]
Clouds by Atmospheric Layer
Mesospheric Clouds
Mesospheric clouds form in the upper atmosphere above approximately 50 km altitude, in a layer characterized by extremely low temperatures and minimal water vapor. The primary type is noctilucent clouds, also known as polar mesospheric clouds (PMCs), which occur at altitudes of 80-85 km, typically between 82 and 86 km.[12][13] These clouds consist of tiny water ice crystals, often less than 0.1 micrometers in diameter, nucleated on minute dust particles of meteoric origin.[14][15] Unlike lower-altitude clouds, they produce no precipitation and remain illuminated by direct sunlight even when the troposphere below is in darkness, creating their distinctive glow during twilight.[14]Noctilucent clouds are visible primarily from latitudes between 50° and 70° north or south during summer months, when the mesopause reaches its coldest temperatures of around -130°C, allowing supersaturation of water vapor.[16][17] They appear as faint, bluish-white formations in the deep twilight sky, shortly after sunset or before sunrise, when the Sun is 6° to 16° below the horizon.[18] Formation is driven by this extreme cold, combined with trace water vapor introduced via meteor ablation and oxidation in the mesosphere.[17] Satellite observations, such as those from NASA's Aeronomy of Ice in the Mesosphere (AIM) mission, have documented an increase in their frequency, with a notable rise in occurrence rates since the early 2000s, correlating to mesospheric cooling trends potentially linked to anthropogenic climate change. Additionally, the 2022 Hunga Tonga eruption's stratospheric water vapor has been observed to potentially enhance noctilucent cloud occurrence and brightness in subsequent seasons, as analyzed in 2025 studies.[19][20][21]In classification systems, noctilucent clouds hold supplementary status outside the standard World Meteorological Organization (WMO) genera for tropospheric clouds, described as extreme-level cirriform or stratiform formations with subtypes including wave-like billows and delicate veils.[17] Studies, including analyses from 2023, indicate a long-term descent in these clouds' altitudes by a few kilometers due to mesospheric cooling, as evidenced by long-term density trends and mesopause temperature analyses.[22][23] This phenomenon distinguishes them from stratospheric clouds, which form at lower altitudes (15-50 km) and are more associated with winter polar conditions or volcanic influences.[13]
Stratospheric Clouds
Stratospheric clouds form in the stratosphere, the atmospheric layer extending from approximately 12 to 50 km altitude, where temperatures are extremely low and conditions differ markedly from the troposphere below. Unlike tropospheric clouds, which primarily consist of water droplets or ice crystals involved in weather systems, stratospheric clouds often involve nitric acid, sulfuric acid, or pure ice particles and play a significant role in atmospheric chemistry rather than precipitation. These clouds are rare and typically occur over polar regions during winter, influenced by the cold polar vortex, and can also arise from volcanic injections of material into the stratosphere.[24][25]The primary types of stratospheric clouds are polar stratospheric clouds (PSCs), classified into Type I and Type II based on composition and formation conditions. Type I PSCs, forming at altitudes of 15-25 km, consist of nitric acid trihydrate (NAT) solids or supercooled liquid droplets of nitric and sulfuric acids mixed with water, appearing when temperatures drop below -78°C in the polar vortex. Type II PSCs, also known as nacreous or mother-of-pearl clouds, are composed of water ice crystals and form at even colder temperatures below -85°C, the frost point for ice; they are most prevalent in Antarctica but occur in the Arctic as well. Nacreous clouds exhibit a wispy, cirriform or stratiform structure and are classified by the World Meteorological Organization (WMO) as supplementary features due to their rarity and high altitude above typical cirrus clouds. Their thin layers, often just 10 μm in particle size, produce unique optical effects such as iridescence—brilliant, pastel colors like pink and green from light diffraction and interference—and coronas, sequences of colored rings around the sun or moon, which are more vivid than in lower clouds.[26][24][25][27]Formation of PSCs is driven by the isolated, descending air in the winter polar vortex, where radiative cooling creates supersaturated conditions for particle nucleation on background sulfuric acid aerosols. These clouds contribute critically to ozone depletion by providing surfaces for heterogeneous chemical reactions that activate chlorine and bromine compounds from inert reservoirs (e.g., ClONO₂ and HCl) into reactive forms (e.g., ClO), leading to catalytic ozone destruction upon sunlight exposure in spring; denitrification from nitric acid removal further prolongs this process, exacerbating the Antarctic ozone hole. Observations indicate increasing PSC frequency and seasonal duration due to stratospheric cooling from rising greenhouse gases, which lowers temperatures and expands the volume of air susceptible to PSC formation—trends projected to persist or intensify despite declining halogens. For instance, analysis of Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) data from 2007-2021 reveals a significantly longer PSC season (by 3-5 days per decade) between 30 and 100 hPa altitudes, with expanded coverage linked to this cooling.[28][24][29][28]Volcanic eruptions can also generate stratospheric clouds through sulfate aerosol veils, distinct from PSCs but similarly influential on climate. The 2022 Hunga Tonga-Hunga Ha'apai eruption injected about 150 million tons of water vapor into the stratosphere, accelerating sulfate aerosol formation and creating persistent thin layers that enhanced ozone depletion by up to 5% globally in the following months; these effects, including elevated water vapor and aerosol persistence, continued to influence stratospheric chemistry into 2025. As higher-altitude analogs, mesospheric noctilucent clouds share icy compositions but form in summer at 80-85 km.[30][31]
Tropospheric Clouds
Tropospheric clouds form in the lowest layer of Earth's atmosphere, the troposphere, which extends from the surface up to an average height of about 12 km, though this varies with latitude and season. These clouds comprise the vast majority of observable atmospheric clouds, with nearly all weather-related phenomena occurring within this layer due to its high concentration of water vapor and dynamic mixing. They are primarily composed of water droplets at lower altitudes and ice crystals at higher altitudes, with mixed-phase clouds possible in the middle levels where supercooled water can coexist with ice.[32][33]Tropospheric clouds are subdivided into broad categories based on altitude and form: high-level clouds (typically above 5-6 km), which consist mainly of ice crystals and rarely produce precipitation; mid-level clouds (around 2-7 km), featuring a mix of water droplets and ice crystals that may yield light precipitation; low-level clouds (below 2 km), dominated by water droplets and often associated with drizzle or steady rain; and vertically developed clouds, which form through convection and can extend from low levels to the upper troposphere, leading to thunderstorms and heavy precipitation. For example, high-level cirrus clouds illustrate the ice-crystal dominance in this category. These classifications align with the World Meteorological Organization's standards, emphasizing height-based étages while accounting for vertical extent in convective types.[34][33]Globally, tropospheric clouds cover approximately 67% of Earth's surface, playing a crucial role in the hydrological cycle by facilitating precipitation and in climate regulation through albedo effects that reflect solar radiation back to space, thereby cooling the planet. However, observations as of 2025 indicate a slight decline in global cloud cover, with contractions of 1.5-3% per decade in storm-forming regions over oceans, potentially enhancing warming effects.[35][36][37] They influence Earth's energy balance by both reflecting incoming sunlight and trapping outgoing infraredradiation, with net effects varying by cloud type and location. In the hydrological cycle, these clouds drive the majority of rainfall and snow, sustaining ecosystems and water resources.[35][37]Cloud base heights in the troposphere vary regionally due to differences in temperature and moisture profiles: high-level bases range from 3-8 km in polar regions, 5-13 km in mid-latitudes, and 6-18 km in the tropics, with corresponding adjustments for mid- and low-level clouds. These variations reflect warmer tropical air supporting higher cloud formation and colder polar air leading to lower bases. Updates to cloud classification, including the 2017 WMO International Cloud Atlas, incorporate anthropogenically influenced tropospheric clouds, such as ship tracks designated as ship-genitus, into observational categories to better account for human impacts on cloud formation and properties.[33][38]
High-Level Tropospheric Clouds
Cirrus Genus
Cirrus clouds are high-level tropospheric clouds characterized by their detached, fibrous structure, composed entirely of ice crystals, and typically occurring at altitudes between 5 and 13 km in temperate regions.[39] These clouds often appear as delicate white threads, hooks, or patches, with a silky or fibrous texture that results from wind shear dispersing the ice crystals.[2] Their thin nature is reflected in a low optical depth, generally less than 0.3, allowing them to minimally obscure the sun or moon while contributing to subtle atmospheric veiling.[40]Cirrus form through the sublimation of water vapor directly into ice crystals when moist air in gentle updrafts rises to cold upper-tropospheric levels, where temperatures drop below -40°C, leading to icesupersaturation and nucleation on aerosol particles such as dust or soot.[41] This process often occurs ahead of advancing warm fronts, where rising warm air overrides cooler air masses, providing an early indicator of approaching weather systems.[42] Unlike lower clouds, cirrus do not produce precipitation due to their high altitude and sparse ice content, though they can gradually thicken and merge into cirrostratus sheets as moisture increases.[2]The genus includes several distinct species based on shape and arrangement, as defined by the World Meteorological Organization:
Fibratus: Fine, hair-like filaments, straight or slightly curved, without hooks.
Uncinus: Hooked or comma-shaped filaments, resembling mare's tails.
Spissatus: Dense and opaque tufts, patches, or layers that may veil the sun.
Duplicatus: Superposed layers at slightly different levels, sometimes merging.
Intortus: Twisted or tangled filaments, appearing irregular.
Radiatus: Parallel bands or streaks converging toward a vanishing point.
Vertebratus: Fishbone-like patterns with perpendicular side branches from a main axis.
Castellatus: Swelling protuberances on the upper surface, like small towers.
These variations arise from local wind patterns and shear at formation heights.[1][43]Cirrus clouds serve as indicators of upper-air moisture availability, signaling potential changes in large-scale atmospheric circulation.[2] Additionally, aviation-induced contrail cirrus, formed from aircraft exhaust in ice-supersaturated regions, has been increasing in prevalence and coverage, contributing to radiative forcing estimated at 0.057 W m⁻² in 2018, with trends tied to rising air traffic.[44] Observers note halo phenomena around the sun or moon due to refraction by the hexagonal ice crystals, often appearing as 22° rings, which highlight the clouds' optical properties without implying active precipitation.[2]
Cirrocumulus Genus
Cirrocumulus clouds consist of thin, white patches, sheets, or layers composed primarily of ice crystals, appearing as small, rounded elements arranged in grains, ripples, or waves without shading the ground below. These high-level clouds form at altitudes between 5 and 13 kilometers (16,000 to 43,000 feet) in the upper troposphere, often resembling the scales of a fish—commonly known as a "mackerel sky"—due to their dappled, rippled texture. They typically cover small portions of the sky in isolated patches, and are rarer than other high clouds like cirrus or cirrostratus.[2][45]Cirrocumulus clouds develop through wave-like instabilities in moist upper-level air, where rising parcels of humid air cool to the point of ice crystal formation, often in association with mid-latitude weather systems or embedded within broader cirrus layers. This process frequently occurs alongside cirrus or cirrostratus clouds, sometimes evolving from their degraded forms as atmospheric instability fragments smoother sheets into rippled patches. The clouds are generally short-lived, dissipating quickly as conditions stabilize.[2][45][46]The species of cirrocumulus, as defined by the World Meteorological Organization, include:
Stratiformis: Sheet-like with ripples or waves.
Floccus: Small, tufted clouds with ragged lower edges.
Castellanus: With turreted or cumuliform protuberances.
Lenticularis: Lens-shaped, often in wave formations.
Varieties include lacunosus, characterized by regularly distributed round holes with fringed edges resembling a net or honeycomb, and undulatus, featuring wave-like undulations across the cloud layer. These forms signal upper-atmospheric instability, often appearing briefly before transitions to other high-cloud types.[47][48][45]As relatively rare high clouds, cirrocumulus indicate fragmentation of cirrus decks and are frequently linked to perturbations in the jet stream, where strong wind shears promote their rippled structures near the jet core. Their presence can precede fair weather but may hint at approaching instability or system changes.
Cirrostratus Genus
Cirrostratus clouds are high-level, thin, sheet-like formations composed primarily of ice crystals, occurring at altitudes between 5 and 13 km in temperate and tropical regions.[6] They present as a uniform, whitish or pale grey veil that can cover the entire sky or large portions of it, often with a fibrous or smooth texture that allows the sun or moon to shine through faintly.[49] These clouds are well-known for producing optical phenomena, such as the 22° halo around the sun or moon, caused by the refraction of light through their ice crystals.[2]These clouds typically form through the spreading and merging of cirrus clouds under the influence of upper-level wind shear, or by the gradual thickening of existing cirrus in stable air currents.[50] The species of cirrostratus include nebulosus, featuring a hazy, structureless appearance without distinct filaments or undulations, and fibratus, with a fibrous veil showing thin striations; nebulosus often expands to blanket the sky as a precursor to weather changes.[51][52]Cirrostratus serves as an early indicator of approaching precipitation, commonly signaling the advance of a warm front 12 to 24 hours prior to rain or snow.[50] By reflecting and scattering incoming solar radiation, these thin ice clouds can reduce surface insolation by 10-20%, contributing to a modest cooling effect during the day while trapping some outgoing longwave radiation at night.[53] Observational records show the 22° halo as a frequent and reliable identifier, and recent climate modeling efforts as of 2025 project increased persistence and coverage of cirrostratus-like high ice clouds in a warming atmosphere due to enhanced upper-tropospheric humidity.[54] In contrast to the thicker, water-droplet-bearing altostratus at mid-levels, cirrostratus remains exclusively ice-based and more translucent.
Mid-Level Tropospheric Clouds
Altocumulus Genus
Altocumulus clouds form as white or gray patches, sheets, or layers in the mid-levels of the troposphere, typically between 2 and 7 kilometers (6,500 to 23,000 feet) above the surface in temperate latitudes, with variations by region: 2–4 km in polar regions and 2–8 km in tropical regions.[2][55][6] They consist of regularly arranged small elements, such as rounded masses, laminae, or rolls, with individual elements showing shading and apparent widths of 1° to 5°.[56] These clouds are primarily composed of water droplets, though they may include a mixture of supercooled droplets and ice crystals, especially in colder conditions.[57][55]Altocumulus develop from instability within moist mid-level air masses, where rising air parcels cool adiabatically and condense into cloud elements.[58] This process is often triggered by post-frontal clearing, when cooler air settles after a cold front passage, promoting convection in the middle troposphere.[2]The genus includes several species that describe variations in structure and arrangement. Altocumulus translucidus appears semi-transparent, allowing partial visibility of the sun or higher clouds through thin areas.[59] Altocumulus opacus forms opaque layers that obscure the sun completely.[59] Altocumulus duplicatus features multi-layered sheets with distinct upper and lower parts.[59] Altocumulus undulatus exhibits wave-like patterns across the cloud sheet.[59] Altocumulus lenticularis takes a lens-shaped form, often aligned in rows due to orographic wave motion over mountains.[60] Altocumulus floccus consists of small, tufted elements with ragged lower edges.[60] Altocumulus castellanus displays vertically developed, turret-like protuberances indicating localized instability.[60]These clouds signify mid-level atmospheric convection and instability, often preceding changes in weather patterns.[61] They rarely produce significant precipitation but can generate virga, where light drizzle or snow evaporates before reaching the ground.[55] Unlike the higher, ice-only cirrocumulus, altocumulus incorporates water droplets and occurs at lower mid-level altitudes.[2]
Altostratus Genus
Altostratus clouds form a uniform, greyish or bluish sheet or layer that typically covers the entire sky, exhibiting a fibrous, striated, or smooth appearance with parts thin enough to reveal the position of the Sun or Moon through a diffused glow, while thicker sections obscure it entirely without producing halo phenomena. These mid-level clouds occupy altitudes between 2 and 7 kilometers in temperate regions, with variations by latitude: 2–4 km in polar regions and 2–8 km in tropical regions, and their thicknesses ranging from 1 to 3 kilometers, with bases appear diffuse and hazy due to falling precipitation that often evaporates before reaching the ground. Composed primarily of water droplets in the lower portions, with ice crystals dominating the upper levels and a mixed phase in between, altostratus lacks distinct edges and merges seamlessly with adjacent cloud layers.[62][63][64][6]The formation of altostratus arises from the gradual, uniform lifting of extensive moist air masses, commonly linked to frontal systems where warmer air ascends over cooler air, leading to widespread condensation. It frequently develops through the thickening of higher cirrostratus clouds as a precursor or by the thinning aloft of denser nimbostratus layers, often signaling the approach of a warm front. Unlike more turbulent cloud types, this process occurs in stable atmospheric conditions, resulting in a persistent layered structure that can span hundreds of kilometers horizontally. Altostratus often serves as a thinner precursor to lower, more precipitous clouds, evolving as moisture accumulates and the layer descends.[65][34]Owing to its consistent uniformity, altostratus is not classified into distinct species, though it commonly displays the supplementary feature of praecipitatio, where continuous light to moderate rain or snow falls from the cloud base, sometimes reaching the surface as drizzle or flurries but often appearing as virga. These clouds hold meteorological significance as indicators of steady, widespread precipitation associated with frontal weather systems, contributing to overcast conditions that reduce visibility. In aviation contexts, altostratus poses risks through supercooled water droplets, which can accrete as rime or clear ice on aircraft surfaces, particularly in temperatures between 0°C and -15°C, potentially degrading lift and control.[66][2][67]Observations confirm that water droplets prevail in the lower reaches of altostratus, blurring solar or lunar outlines and preventing optical effects like halos, while ice crystals aloft influence radiative properties.[63]
Low-Level and Vertical Development Tropospheric Clouds
Stratocumulus Genus
Stratocumulus clouds form a low-level genus characterized by grey or whitish layers composed of large, rounded masses, rolls, or tessellations, often exhibiting a honeycomb-like pattern with darker undersides. These clouds typically occur at altitudes between 500 and 2,000 meters (0-2 km), where they appear as extensive sheets or patches covering 50-90% of the sky in many cases, particularly over oceans and land in stable atmospheric conditions.[68][69]They develop through boundary layer mixing, where cool, moist air near the surface rises gently and interacts with a temperature inversion above, or via the breakup of higher stratiform clouds like stratus. This process is prevalent in maritime and continental environments with high humidity and moderate cooling, leading to shallow convection that organizes the clouds into rolls or patches without significant vertical development.[69][2]Key species include undulatus, featuring wave-like undulations across the cloud layer; translucidus, with semi-transparent gaps allowing sunlight to penetrate; opacus, appearing dark and opaque due to dense composition; and duplicatus, where multiple layers stack vertically, creating a tiered appearance. These variations highlight the cloud's response to subtle atmospheric instabilities or shear.[70]Stratocumulus often produce light precipitation such as drizzle or snow, especially in thicker formations, but generally indicate fair weather with limited turbulence. Marine stratocumulus decks play a crucial role in Earth's climate by reflecting incoming solar radiation, thereby exerting a net cooling effect that helps regulate global temperatures. Altocumulus serves as an elevated counterpart at mid-levels, sharing similar layered structures but forming higher in the troposphere. In 2024, satellite observations revealed that reduced sulfur emissions from shipping regulations have diminished ship tracks within stratocumulus layers over major ocean routes, leading to less persistent cloud cover and enhanced warming in those regions.[71][72][73]
Cumulus Genus
Cumulus clouds belong to the low-level cloud genus, featuring discrete, heaped formations with a characteristic flat base and rounded, dome-shaped tops that give them a puffy, cotton-like appearance. These clouds typically have bases at altitudes between 0 and 2 kilometers above the surface, where rising moist air reaches its condensation level, and are composed primarily of liquid water droplets, though supercooled droplets may occur in cooler conditions.[6][74][75]The formation of cumulus clouds is driven by surface heating that initiates thermal convection, where parcels of warm air rise, cool adiabatically, and condense into visible clouds upon saturation. This process is most prominent over land during clear skies, following a diurnal cycle that begins in the morning, peaks in vertical development during the afternoon due to maximum solar heating, and often dissipates by evening as the surface cools.[2][76][74]Cumulus clouds are classified into subtypes based on their vertical extent and development stage. Cumulus humilis represents the flattest form, with minimal vertical growth where the cloud's width exceeds its height, often spreading horizontally and indicating stable, fair-weather conditions without significant precipitation. Cumulus mediocris exhibits moderate development, with vertical extent roughly equal to its width, reaching tops around 1 to 2 kilometers above the base, and featuring more pronounced domes but still limited to light turbulence. Cumulus congestus shows greater vigor, with towering structures 2 to 6 kilometers tall—often taller than wide—resembling cauliflowers and capable of producing brief, heavy showers through localized precipitation.[75][77][78][79]As indicators of atmospheric convection, cumulus clouds signal fair weather when limited to humilis or mediocris forms, but congestus subtypes can herald increasing instability and short-lived heavy rain, serving as precursors to more intense development. In extreme cases, sustained growth may lead to cumulonimbus clouds with severe weather potential. Recent meteorological models, such as those incorporating large eddy simulations, increasingly use observations of cumulus humilis as a baseline for parameterizing shallow convection, enhancing predictions of boundary layer stability as of 2025 updates.[74][78][80]
Stratus Genus
Stratus clouds form a uniform, grey layer at low altitudes, typically with bases between the surface and 2 kilometers (6,500 feet), though often as low as 300 meters or less. These featureless clouds resemble fog elevated aloft and consist primarily of small water droplets, occasionally mixed with ice crystals, drizzle droplets, or snow grains. They present a smooth, unbroken deck that obscures the sun's disk without producing halos, except under extremely low temperatures, and their thickness varies from tens to several hundred meters.[81][82][83]The formation of stratus primarily results from the cooling of moist air in the lower troposphere combined with wind-induced turbulence. Over land, this occurs through nocturnal radiative cooling under clear skies with light winds or via the advection of warm, moist air over cooler surfaces, leading to condensation near the ground. In marine environments, stratus develops when warm air advects over colder water, promoting widespread stratiform layering. Unlike more convective clouds, stratus lacks vertical development and remains horizontally extensive.[84]Stratus is classified into two main species based on appearance. The nebulosus species (St neb) appears as a nebulous, grey, and fairly uniform layer without distinct structure, representing the most common form. In contrast, the fractus species (St fra) manifests as irregular, ragged shreds with continuously changing outlines, often signaling a transitional phase during formation or dissipation. These species highlight stratus's adaptability to local atmospheric conditions.[85][86]Stratus clouds significantly reduce visibility, particularly in valleys and coastal regions where they are prevalent, and they commonly produce light precipitation such as drizzle or mist rather than heavy rain. Observations indicate that increasing atmospheric instability can cause stratus to evolve into stratocumulus, introducing breaks and rolls in the layer. Recent studies, including a 2023 analysis of urban influences, suggest that pollution from aerosols enhances the persistence and cooling effects of low-level stratus by modifying cloud microphysics in polluted environments. Nimbostratus serves as a thicker, precipitating variant extending to mid-levels.[81][87][88]
Nimbostratus Genus
Nimbostratus clouds form a thick, dark gray to black layer that extends through low and mid-levels of the troposphere, typically with bases between 0 and 2 kilometers above the surface and tops reaching 4 to 8 kilometers, often appearing amorphous and featureless due to their uniform composition of water droplets and ice crystals.[2][89] Their ragged, diffuse undersides result from falling precipitation, which obscures visibility and can merge with lower ragged clouds, creating a multi-level structure that blocks sunlight completely.[2] These clouds produce continuous, moderate to heavy precipitation in the form of rain or snow, with rates commonly ranging from 1 to 5 mm per hour but capable of reaching up to 10 mm per hour in intense cases, distinguishing them from lighter drizzles associated with thinner layers.[90]Nimbostratus develop through large-scale synoptic processes, primarily via the gradual uplift of moist warm air over cooler air masses in warm or occluded fronts, leading to widespread cooling and condensation that thickens preexisting altostratus into a deeper, precipitating layer.[91][89] This frontal uplift often merges altostratus as a non-precipitating precursor with lower stratus formations, resulting in a stable, thermodynamically persistent cloud deck that covers extensive areas.[2] Rarely, they can evolve from thickening stratocumulus layers under similar lifting conditions.[91]As a genus, nimbostratus lacks distinct species or varieties, defined instead by its uniform, edge-less form and the supplementary feature of praecipitatio, which denotes the continuous moderate to heavy precipitation falling from its base without significant virga or evaporation before reaching the ground.[1][92] Other occasional features include pannus, ragged shreds below the main layer, but these do not alter the core classification.[1]Nimbostratus serve as primary producers of steady, widespread precipitation in mid-latitude weather systems, particularly ahead of advancing warm fronts where they signal prolonged rainy periods lasting hours to days.[89] Their significance lies in contributing substantially to regional water cycles, with studies showing they account for much of the continuous rain in frontal systems, often exceeding 50% of total precipitation in affected areas.[93] In terms of impacts, these clouds pose notable flooding risks due to their persistent output, especially when intensified by atmospheric rivers—narrow corridors of enhanced moisture transport—where 2025 observations indicate heightened nimbostratus activity leading to extreme events, such as the Pacific Northwest's November storms that delivered up to 100 mm of rain and prompted flood warnings.[93][94] Such intensification, linked to warmer sea surface temperatures, has amplified flood damages, with atmospheric rivers, often featuring nimbostratus clouds, contributing to about 80% of related economic losses in coastal regions.[95][96]
Cumulonimbus Genus
Cumulonimbus clouds represent the most intense form of convective clouds, characterized by their massive vertical development and association with severe weather phenomena. These heavy, dense clouds form towering structures resembling mountains or huge pillars, with bases typically at low altitudes around 0.5 to 2 kilometers above the surface and tops extending well into the upper troposphere, often exceeding 12 kilometers in height. The upper portions frequently flatten into a smooth, anvil-like shape due to the spreading of ice crystals at the stabletropopause level, while the lower parts remain turbulent and cauliflower-like. Cumulonimbus are renowned for producing heavy showers, large hailstones, frequent lightning, and strong wind gusts, distinguishing them from less vigorous cloud types.[97][98][99]The formation of cumulonimbus relies on powerful upward convection triggered by atmospheric instability, where warm, moist air parcels rise rapidly, cooling adiabatically and condensing into towering updrafts. This process is fueled by high convective available potential energy (CAPE), often exceeding 2,000 J/kg, leading to rapid vertical growth that can reach maturity within an hour. In environments with significant wind shear, these clouds may evolve into supercell variants, featuring persistent rotating updrafts that sustain their intensity for several hours. Cumulonimbus typically develop from the mature stage of cumulus clouds when instability intensifies.[100][101][102]Within the cumulonimbus genus, distinct species are identified based on the appearance of their upper portions. Cumulonimbus calvus features rounded, puffy summits without sharp outlines or cirriform elements, indicating a transitional stage before full glaciation. Cumulonimbus capillatus exhibits fibrous, striated, or hair-like upper parts, often spreading into an anvil plume, signaling the cloud's penetration into colder, ice-dominated regions. The incus subtype of capillatus displays a classic anvil top with a smooth, hammer-shaped fibrous structure, formed by wind shear at the tropopause. These species reflect the cloud's evolutionary progression from vigorous convection to dissipation.[103][104][105]Cumulonimbus clouds play a critical role in severe weather, generating phenomena such as tornadoes through mesocyclone rotation and gust fronts via downdraft outflows that propagate cool air ahead of the storm. Globally, they account for approximately 16 million thunderstorm events each year, contributing significantly to precipitation patterns and atmospheric electrification. Supplementary features like mammatus—pouch-like protuberances on the cloud's underside—often appear in mature cumulonimbus, formed by sinking pockets of cold, moist air and serving as indicators of post-storm stability in severe systems. Recent research highlights that climate change is intensifying cumulonimbus activity, with warmer temperatures enabling greater moisture uptake and thus more extreme rainfall and storm vigor in convective systems, as observed in WMO updates through 2025.[101][99][106][107][108]
Supplementary Cloud Classifications
Species and Varieties
Cloud species represent finer subdivisions within cloud genera, primarily based on the clouds' shape, internal structure, or degree of development. The World Meteorological Organization (WMO) officially recognizes 15 species in its International Cloud Atlas, enabling meteorologists to identify subtle variations that reflect specific atmospheric processes such as convection, stability, or wind shear. These species are applied selectively across genera; for instance, fibratus appears in high-level cirrus clouds, while castellanus is common in mid-level altocumulus formations indicating potential instability. By specifying the species, observers can better assess local weather dynamics, as certain types like cumulus congestus often precede thunderstorm development.[109][1]The following table summarizes the WMO-defined cloud species, their key characteristics, and primary genera of application:
Species
Description
Primary Genera
Calvus
Cumulonimbus with a whitish, very fibrous upper portion lacking a distinct anvil, showing partial dissipation of the dome-like top.
Cumulonimbus
Capillatus
Cumulonimbus featuring a fibrous or striated cirriform upper part, often with a spreading anvil.
Cumulonimbus
Castellanus
Clouds with distinct turreted or cumuliform protuberances rising from a common horizontal base, suggesting local instability.
Cirrus, Cirrocumulus, Altocumulus, Stratocumulus
Congestus
Cumulus with considerable vertical development and bulging upper parts, but without anvil or fibrous features.
Cumulus
Fibratus
Thin, fine hair-like or silky filaments, straight or slightly curved, without tufts or hooks.
Cirrus, Cirrostratus, Cirrocumulus
Floccus
Small, tufted clouds with ragged lower parts, often accompanied by virga (trailing precipitation).
Cirrus, Cirrocumulus, Altocumulus
Fractus
Broken, irregular shreds with uneven edges, appearing as fragments of the more characteristic form.
Cumulus, Stratus
Humilis
Small, low cumulus with flattened tops and horizontal bases, showing little vertical growth.
Cumulus
Lenticularis
Lens-shaped or saucer-like clouds, often in a stack, associated with mountain waves and stationary relative to terrain.
Cirrocumulus, Altocumulus, Stratocumulus
Mediocris
Cumulus with moderate vertical extent, distinct outlines, and flat bases, but lacking significant bulging.
Cumulus
Nebulosus
A uniform layer resembling a diffuse veil without individual elements or structure.
Cirrostratus, Stratus
Spissatus
Dense, thick patches of cirrus with very low transparency, often obscuring the sun or moon.
Cirrus
Stratiformis
Large horizontal sheets or layers subdivided into smaller elements, uniform in appearance.
Cirrocumulus, Altocumulus, Stratocumulus
Uncinus
Cirrus with hook- or comma-shaped filaments, tufted at the top and curving downward.
Cirrus
Volutus
Complete horizontal roll or tube-like cloud, low and elongated, often associated with wind shear.
Altocumulus, Stratocumulus
This classification, updated in the 2017 edition of the International Cloud Atlas to include volutus, enhances observational precision; for example, stratocumulus volutus may indicate gust fronts ahead of storms.[7][11][109]Cloud varieties further refine classification by describing the arrangement of visible elements within a genus or the degree of transparency. The WMO identifies nine principal varieties, which can overlap and are denoted when multiple are present. These help distinguish between uniform sheets and patterned formations, aiding in the interpretation of synoptic-scale patterns like converging air masses in radiatus varieties. Varieties such as undulatus often signal stable layers with wave disturbances, while lacunosus indicates clearing or evaporative processes. In forecasting, combining species and varieties—such as altocumulus lenticularis undulatus—provides cues for orographic influences and potential turbulence.[1][110]The table below outlines the WMO cloud varieties with their characteristics and typical applications:
Variety
Description
Primary Genera
Duplicatus
Superimposed layers of clouds partially joined or separated by clear layers.
Filaments or bands arranged in a herringbone or fishbone pattern.
Cirrus
These species and varieties collectively support aviation safety, climate modeling, and short-term weather predictions by revealing instability (e.g., castellanus in altocumulus forecasting convection) or uniformity (e.g., nebulosus indicating overcast conditions). Recent WMO discussions in 2025 explore aerosol-influenced patterns, potentially leading to new varieties, though none have been formally added beyond the 2017 updates.[1][11][111]
Supplementary Features
Supplementary features in cloud classification refer to distinct structural elements that attach to or modify the appearance of principal cloud genera, as defined by the World Meteorological Organization (WMO). These features provide additional diagnostic information about cloud dynamics, precipitation processes, and atmospheric conditions, aiding in weather forecasting and severe weather identification, but they do not constitute separate genera or species.[112] The WMO's International Cloud Atlas recognizes several such features, which are observed across various cloud types, particularly in cumulonimbus and stratocumulus formations.The following table summarizes key WMO supplementary features, including their descriptions and typical associations:
Feature
Latin Term
Description
Typical Associations
Virga
Virga
Detached precipitation trails falling from a cloud base that evaporate or sublime before reaching the ground, often appearing as wispy streaks.
A rotating, funnel- or tube-shaped cloud pendant from a cumulonimbus or cumulus base, indicating strong updrafts; if it reaches the ground, it becomes a tornado.
Cumulonimbus, cumulus congestus.
Anvil top
Incus
A spreading, anvil-shaped top on a cumulonimbus cloud, formed by high-level winds shearing the upper portion.
Cumulonimbus.
Mammatus
Mamma
Pouch-like cloud protrusions hanging from the base, formed by sinking cool air pockets in downdrafts.
Cumulonimbus, sometimes altocumulus or cirrocumulus.
Tail cloud
Cauda
An elongated, tail-like horizontal cloud band streaming from a cumulonimbus, resembling a flame-like extension due to strong winds.
Cumulonimbus.
Note that parhelion, or sundogs, are bright spots of light beside the sun caused by ice crystals in high clouds like cirrostratus, but they are optical phenomena rather than structural cloud features and thus not classified as supplementary by the WMO.In 2017, the WMO updated the International Cloud Atlas to include five new supplementary features: asperitas (wavy, undulating undersurfaces), fluctus (wave-like formations resembling standing waves), cavum (hole-punch clouds from aircraft), murus (wall cloud), and cauda (tail cloud), enhancing the ability to identify dynamic atmospheric processes via satellite and ground observations. These additions reflect advances in observational technology and have improved severe weather detection, such as identifying gust fronts in arcus or rotation in tuba formations by 2025. For instance, mammatus pouches signal post-thunderstorm stability, while tuba features indicate high tornado risk in cumulonimbus systems. Such features play a crucial role in aviation safety and meteorological modeling, allowing for precise differentiation from inherent species or varieties.
Genitus and Mutatus Formations
Genitus and mutatus formations represent supplementary classifications in the World Meteorological Organization's (WMO) cloud nomenclature, introduced in the 2017 edition of the International Cloud Atlas to account for clouds that develop or transform due to specific localized atmospheric or surface processes. These terms denote "offspring" (genitus) clouds formed from non-cloud origins, such as surface phenomena, or "altered" (mutatus) clouds that undergo significant modification from an existing parent cloud.[113] Unlike primary genera based on height and structure, these formations highlight causal mechanisms, aiding in the study of environmental influences on cloud development.[114]Genitus clouds arise when localized factors, such as heat, moisture, or mechanical effects, generate new cloud structures, often resembling established genera but tied to their origin. The WMO recognizes 11 special genitus types, named by combining the resulting genus with the suffix "-genitus" and a descriptor of the generating process.[115] Flammagenitus clouds, for instance, form from intense heat and updrafts over wildfires, typically appearing as cumuliform pyrocumulus or pyrocumulonimbus that can enhance fire spread by lofting embers.[8] Cataractagenitus develops from the evaporation of falling water in waterfalls or sea spray, manifesting as low-level stratus-like layers near sites like Niagara Falls.[116] Homogenitus encompasses clouds produced by human activities, including cumulus from industrial emissions or contrails that evolve into persistent cirrus homogenitus, contributing to radiative forcing and potential climate warming effects estimated at 0.05–0.1 W/m² globally.[117] Silvagenitus arises from transpiration over dense forests, forming stratus decks that influence local boundary layer humidity.[118] Other genitus include cumulogenitus from cumulus convection and altocumulogenitus from altocumulus growth, emphasizing the role of parent clouds in some cases.[113]Mutatus formations describe clouds where an existing cloud mass is substantially altered by local conditions, changing its genus without detaching new elements. These are denoted by the new genus followed by the original genus and "-mutatus" suffix, with the WMO listing 10 such types.[115] For example, stratocumulus cumulomutatus occurs when rising cumulus elements flatten under stable air, transitioning to a layered form.[113] Homomutatus specifically applies to human-induced alterations, such as aircraft exhaust modifying existing cirrus into spreading anvil-like structures.[119] Altocumulomutatus can result from orographic wave interactions, where altocumulus deforms into lenticular shapes over mountains, illustrating dynamic atmospheric forcing.[113] Cumulonimbus may occasionally serve as a host for mutatus developments, such as when anvil extensions transform under shear.[113]These classifications, formalized since 2017, enable precise tracking of anthropogenic and natural impacts on cloud evolution, with homogenitus and homomutatus particularly valuable for aviation and climate monitoring.[114] By integrating origin into nomenclature, they enhance observational consistency across meteorological networks.
Informal and Free-Convective Terms
Informal terms for clouds refer to colloquial or descriptive names used by meteorologists, pilots, and the public to describe cloud appearances or behaviors outside the strict Latin-based nomenclature of the World Meteorological Organization (WMO). These terms often highlight visual patterns or transient features that aid in quick recognition, particularly in aviation and weather forecasting. For instance, "mackerel sky" describes the rippled, scale-like arrangement of small, uniform cloudlets in cirrocumulus or altocumulus layers, evoking the skin of a mackerelfish; this pattern frequently signals approaching weather changes due to associated upper-level instability.[120] Similarly, "scud" is an informal synonym for fractus clouds, denoting ragged, low-level shreds of cumulus or stratus that form in turbulent winds near fronts or thunderstorms, often appearing as detached fragments racing across the sky.[121]Another common informal term is "pannus," which refers to the tattered, low-lying shreds of stratus fractus or cumulus fractus clinging beneath larger cloud layers like nimbostratus or altostratus, typically indicating deteriorating weather with imminent precipitation. These terms derive from practical observation rather than formal classification, with "fractus" rooted in Latin for "broken," emphasizing the irregular, torn morphology caused by wind shear or evaporation. While not genera or species in the WMO system, such descriptors enhance communication in operational meteorology, as seen in pilot reports and weather logs.[122]Free-convective terms describe cloud formations driven by buoyancy from surface heating or atmospheric instability, leading to vertical development without forced ascent like orographic lift. These primarily apply to cumuliform clouds within the cumulus genus, where warm, moist air parcels rise freely as thermals, condensing into distinct species based on maturity and height. Cumulus humilis, the smallest free-convective form, features flat bases and minimal vertical growth, often appearing as isolated "fair weather" puffs below 2 km altitude during light diurnal heating. As convection strengthens, these evolve into cumulus mediocris, with moderate bulging tops reaching 2-3 km and equal horizontal-vertical extent, signaling potential for further development but rarely producing precipitation.[123]More vigorous free convection produces cumulus congestus, towering heaps with cauliflower-like tops extending to 6 km or higher, often accompanied by virga (evaporating precipitation trails) and marking the transition to cumulonimbus; these can span 1-5 km in width and indicate unstable conditions conducive to showers.[2] Unlike forced-convective clouds, free-convective types rely on convective available potential energy (CAPE) from solar heating over land, typically forming in clusters during afternoons in tropical or mid-latitude summers. The WMO classifies these species to quantify convective intensity, aiding forecasts of thunderstorm potential.
Cloud Nomenclature and Etymology
Genera Origins
The nomenclature of the ten principal cloud genera recognized by the World Meteorological Organization (WMO) originates from the foundational work of Luke Howard, a British pharmacist and amateur meteorologist, who in 1803 published "On the Modifications of Clouds," introducing a systematic classification using Latin terms to describe cloud forms based on their appearance and structure.[124] Howard's system identified core types such as cirrus, cumulus, and stratus, along with combinations like cirrocumulus and cumulonimbus, drawing on descriptive Latin roots to ensure universality in meteorological observation.[124]This Latin-based framework was further standardized and internationalized in 1896 through the efforts of the International Meteorological Committee, which adopted Howard's terms in the first edition of the International Cloud Atlas, promoting a consistent global nomenclature for scientific communication.[124] The high-level genera include cirrus, derived from the Latin cirrus meaning a lock of hair, tuft of horsehair, or bird's tuft, evoking wispy filaments; cirrocumulus, combining cirrus and cumulus for layered heaps with fibrous elements; and cirrostratus, merging cirrus and stratus to denote a high, veil-like sheet with curl-like features.[125] Mid-level genera are altocumulus, from altum (height or upper air) and cumulus, indicating elevated heaps; and altostratus, from altum and stratus, describing a high, uniform layer.[125]The low- and vertical-development genera encompass cumulus, from the Latin cumulus meaning an accumulation, heap, or pile, an ancient term for piled masses; cumulonimbus, combining cumulus and nimbus (rainy or storm cloud) for towering rain-bearing heaps; nimbostratus, from nimbus and stratus for a thick, rain-producing layer; stratus, from stratus (past participle of sternere, to extend, spread out, or flatten), signifying a spread-out blanket; and stratocumulus, blending stratus and cumulus for layered heaps.[125] The core genera have remained unchanged since their formalization, with the last significant refinements to the classification system occurring in 1951 by the WMO's International Commission for the Study of Clouds and Other Meteors, which focused on stabilizing definitions without altering the genera themselves.[124] The 2017 edition of the International Cloud Atlas, released as a digital resource by the WMO, clarified and expanded supplementary terms while preserving the original ten genera for ongoing meteorological use.[124]
Species, Varieties, and Features Etymologies
The etymologies of cloud species, varieties, and supplementary features in the World Meteorological Organization (WMO) classification system derive from descriptive Latin terms, selected for their precision in capturing the visual and structural characteristics of cloud subdivisions. These terms, established through historical meteorological nomenclature, emphasize observable traits such as texture, shape, and opacity, facilitating standardized identification across scientific observations.[125]Cloud species names highlight primary morphological forms within genera. For instance, fibratus originates from the Latin for "fibrous," denoting thin, filament-like threads; uncinus means "hooked," referring to cirrus clouds with curved, hook-shaped tips; spissatus indicates "thickened" or "condensed," for dense, opaque patches; and castellanus derives from castellum, meaning "castle-like," evoking turreted or cumuliform protuberances that suggest fortified structures. These choices reflect a deliberate intent to use evocative yet precise descriptors rooted in classical Latin to convey shape and arrangement.[125]Varieties further refine species by describing transparency, pattern, or density. Duplicatus translates to "doubled" or "repeated," describing layered repetitions; intortus signifies "twisted" or "entangled," for convoluted shapes. Translucidus comes from Latin for "transparent" or "translucent," applied to layers allowing partial sunlight penetration; opacus means "shady" or "opaque," for thick, sun-obscuring formations; perlucidus denotes "brightly clear" or "allowing light through," emphasizing luminous gaps; lacunosus translates to "having holes" or "pitted," for clouds with distinct openings; and undulatus signifies "wavy" or "undulated," capturing ripple-like patterns. This nomenclature prioritizes optical and textural qualities to differentiate subtle variations within broader cloud types.[125]Supplementary features name distinctive accessory elements. Virga derives from Latin for "rod," "twig," or "branch," describing dangling precipitation streaks; arcus means "bow" or "arch," for elongated, horizontal roll clouds; mammatus originates from "breast" or "udder," referring to pouch-like protrusions; incus translates to "anvil," for the anvil-shaped top of cumulonimbus; and tuba signifies "trumpet" or "tube," denoting funnel or tornado-like appendages. These terms draw from everyday Latin objects or forms to vividly illustrate dynamic or irregular components.[125]In 2017, the WMO updated the International Cloud Atlas to include new terms like flammagenitus, a hybrid from flamma ("fire") and genitus ("generated" or "born"), describing clouds formed by intense heat sources such as wildfires, highlighting evolving observational needs in modern meteorology.[125]
Special Formation Etymologies
The etymologies of special cloud formation names derive primarily from Latin roots, reflecting the processes or agents responsible for their generation or alteration, as standardized by the World Meteorological Organization (WMO) in its International Cloud Atlas. These names use suffixes like "genitus" and "mutatus" to denote origin and change, respectively, allowing for precise description of non-standard cloud development influenced by localized factors such as fire, water, or human activity. This nomenclature evolved to accommodate observations of emerging phenomena, particularly with the 2017 update to the Atlas, which incorporated citizen science contributions and advanced imaging.[125][126]The suffix "genitus," from the Latin past participle of "gignere" meaning "born" or "generated," is appended to roots describing the generating mechanism, indicating clouds formed or extended due to specific, often localized, processes rather than typical synoptic conditions. For instance, flammagenitus combines "flamma" (fire) with genitus, describing cumulus-type clouds generated by intense heat from wildfires or volcanic activity, which can evolve into cumulonimbus flammagenitus under extreme conditions. Cataractagenitus merges "cataracta" (waterfall or cascade) with genitus, referring to low-level clouds produced by the evaporation and upward motion of water droplets from falling water, such as near waterfalls or sea waves. Homogenitus joins "homo" (man or human) with genitus, encompassing anthropogenic clouds like persistent contrails from aircraft exhaust or small cumulus from industrial emissions. Silvagenitus pairs "silva" (forest or woods) with genitus, for clouds arising from enhanced evaporation in forested areas, often resembling homogenitus but tied to natural vegetation. Cumulogenitus indicates clouds formed from a cumulus mother cloud, such as altocumulus cumulogenitus generated below a developing cumulus. These genitus terms were formalized in the 2017 International Cloud Atlas to address gaps in describing human-influenced and localized formations.[125][127][126]In contrast, the suffix "mutatus," from the Latin "mutare" meaning "changed" or "altered," denotes transformations of existing clouds due to localized influences, without implying a completely new generation. Examples include homomutatus, combining homo with mutatus, for cirriform clouds that develop from spreading aircraft contrails, altering their original homogenitus form into a more diffuse state. For spreading transformations, such as when cumulus fragments into stratocumulus under wind shear or stable layers, the term stratocumulus cumulomutatus is used, emphasizing the change from the cumulus mother cloud. These mutatus designations highlight dynamic changes, as outlined in WMO guidelines for identifying mother clouds and transformations.[113][128][125]Supplementary features with special etymologies include asperitas, from the Latin "asper" (rough) and "-itas" (state or quality), denoting undulating, wave-like textures in altocumulus or other layers that create a rough, chaotic appearance, first proposed in 2008 and accepted in 2017. Fluctus, directly from Latin "fluctus" (wave or surge), refers to wave-like formations such as Kelvin-Helmholtz billows or undulatus patterns in various cloud genera. These terms provide Latin equivalents for informal descriptions; for example, the non-Latin "mackerel sky"—a folk name for the scaly, rippled appearance of cirrocumulus or altocumulus undulatus—aligns with fluctus or undulatus (from "unda," wave), capturing the fish-scale resemblance without altering core genus nomenclature. The 2017 expansions, including these features, marked the first major update since 1956, integrating over 100 new images to reflect contemporary environmental influences like increased wildfire activity and aviation.[125][124][126]
Clouds on Other Planets
Venusian Clouds
The clouds of Venus form a global deck spanning altitudes of 48 to 70 km, consisting of droplets with bulk composition approximately 22% sulfuric acid, 62% water from hydrates, and 16% iron sulfate by weight in the main cloud deck, creating a thick layer 20 to 30 km deep and imparting a characteristic yellowish haze to the planet's appearance.[129][130] These clouds encircle the entire planet, reflecting about 75% of incoming sunlight while trapping infrared radiation, which contributes significantly to Venus's extreme surface temperatures exceeding 460°C through a runaway greenhouse effect.[131] Unlike Earth's water-based clouds, Venusian clouds contain significant water in the form of hydrated sulfates, comprising about 62% of aerosol mass, despite the overall arid atmosphere with low free water vapor.[129][132]The cloud structure is divided into distinct layers: an upper haze extending from 70 to 90 km altitude composed of fine submicron aerosols; a main cloud deck from 48 to 70 km subdivided into three strata—upper (56–70 km), middle (50.5–56.5 km), and lower (47.5–50.5 km) clouds—with progressively larger droplets toward the base; and a lower haze from 30 to 48 km featuring coarser particles.[133][134] Droplet sizes range from 0.2 to 3 micrometers in the main deck, with the aerosols including sulfuric acid, water hydrates, and iron sulfates as major components, as revealed by reanalysis of 1978 Pioneer Venus probe data.[129] These layers exhibit dynamic variability, with Akatsuki observations from 2024–2025 identifying gravity wave patterns and transient clearings in ultraviolet imagery, indicating localized instabilities in the cloud tops.[135] A 2024 reanalysis of Pioneer Venus data, discussed in 2025 studies, highlights that the clouds are less acidic than previously thought, with substantial water reservoirs potentially influencing photochemical processes and aerosol stability.[129][136]Cloud formation arises from photochemical reactions in the upper atmosphere, where solar ultraviolet radiation dissociates sulfur dioxide (SO₂) and water vapor to produce sulfuric acid vapor, which then condenses into droplets transported downward by convection.[137][138] The clouds are sustained by Venus's superrotating winds, reaching speeds of up to 100 m/s at cloud-top altitudes around 65–70 km, which circle the planet in just four Earth days—60 times faster than the surface rotation—distributing aerosols globally and maintaining the opaque veil.[139] Observations from NASA's Pioneer Venus mission in 1978 first confirmed the cloud composition through in situ sampling and polarimetry, while JAXA's Akatsuki orbiter (2015–2025) provided high-resolution infrared and ultraviolet imaging to track these dynamics, including wave-induced clearings that reveal underlying atmospheric structures, until its mission ended in September 2025.[140][141] This veil, superficially analogous to Earth's stratocumulus layers in opacity but with hydrated acidic aerosols and heat-trapping properties, underscores Venus's role as a cautionary model for greenhouse-dominated climates.[142]
Martian Clouds
Martian clouds form in the planet's thin carbon dioxide-dominated atmosphere, primarily consisting of water ice crystals, carbon dioxideice particles, and suspended dust, influenced by seasonal temperature variations and topographic features.[143] These clouds are generally thinner and more transient than those on Earth due to the low atmospheric pressure, which is about 1% of Earth's at sea level, limiting the amount of condensable material available.[144]Waterice cirrus clouds typically occur at altitudes between 20 and 40 kilometers, appearing as wispy, high-altitude formations similar to terrestrial cirrus, and include orographic types over elevated regions like the Hellas Basin as well as polar hood clouds that encircle the poles during winter transitions.[145]Carbon dioxideice clouds, often referred to as "dry ice" clouds, predominantly form over the winter poles through the condensation of atmospheric CO2 as temperatures drop below sublimation thresholds, contributing to seasonal polar hoods and occasional snowfall events.[146]Dust storm clouds, composed of fine regolith particles lifted from the surface, range from localized hazes to planet-encircling events that occur approximately every three Martian years (about 5.5 Earth years), dramatically altering the planet's albedo and atmospheric opacity.[147]The characteristics of Martian clouds emphasize their delicate and dynamic nature; water ice cirrus are thin and feathery, often exhibiting wave patterns due to atmospheric gravity waves, while CO2 ice clouds can appear denser near the poles but remain optically thin overall.[148] Orographic clouds in the Hellas Basin, the deepest impact basin on Mars, form when moist air rises over the basin's topography during northern summer, creating persistent water ice layers up to several kilometers thick.[145] In summer hemispheres, high-altitude noctilucent-like clouds, composed of CO2 ice at around 60 kilometers, display iridescent colors from sunlightscattering and are visible during twilight, resembling Earth's polar mesospheric clouds but adapted to Mars' colder conditions.[149] Dust clouds, by contrast, are opaque and reddish, with global storms raising particles to heights of 50 kilometers or more, sometimes lofting water ice particles indirectly through increased atmospheric heating.[150]Cloud formation on Mars relies on the sublimation and condensation cycles of water vapor and CO2 in the sparse atmosphere, where temperatures range from -140°C at poles to 20°C equatorially, driving diurnal and seasonal variations.[151]Water ice clouds nucleate when humidity exceeds saturation over topographic highs or in upwelling regions, while CO2 clouds form via direct condensation in the cold winter stratosphere, often enhanced by dust acting as condensation nuclei.[143]Dust lifting occurs through wind speeds exceeding 10-20 meters per second, common during perihelion seasons, which erode surface particles and sustain hazes or storms until gravitational settling or precipitation removes them.[152]Early observations of Martian clouds were provided by NASA's Viking orbiters in 1976, which imaged water ice fogs, polar hoods, and initial dust hazes using visual and infrared cameras, revealing cloud opacities and seasonal distributions.[153] The Mars Reconnaissance Orbiter (MRO), launched in 2005 and operational since 2006, has extensively mapped clouds with instruments like the Mars Color Imager (MARCI) and Compact Reconnaissance Imaging Spectrometer (CRISM), identifying water ice signatures at 1.5 and 2.0 micrometers and tracking dust storm evolution over multiple Mars years.[154] In 2023, NASA's Perseverance rover captured time-lapse images of drifting wave clouds composed of water ice just before sunrise, formed by convective lifting in the Jezero Crater region at altitudes up to 40 kilometers.[155] More recently, in early 2025, the Curiosity rover documented iridescent noctilucent clouds during southern autumn twilight, highlighting their seasonal visibility and composition of sub-micrometer CO2 ice particles illuminated by the setting Sun.[149]
Gas Giant Clouds
The atmospheres of the gas giants Jupiter and Saturn feature prominent banded cloud systems primarily composed of ammonia-based condensates, driven by their internal heat and rapid rotation. In Jupiter, the upper cloud deck consists of ammonia cirrus clouds at approximately 0.5 bar pressure, forming the bright white zones where upwelling air creates cooler, reflective layers. Beneath these, at around 1-2 bar, lie clouds of ammonium hydrosulfide, which appear as the reddish-brown belts due to chemical reactions and haze absorption. Deeper still, at pressures exceeding 5 bar, water ice and liquid clouds form, contributing to the planet's weather dynamics. Saturn's cloud structure is analogous but hazier, with ammonia ice crystals dominating at about 1 bar, obscured by thicker photochemical hazes that reduce visibility of underlying layers. Seasonal polar clouds on Saturn, such as the northern polar hood, emerge due to its 26.7-degree axial tilt, introducing hemispheric asymmetries absent in Jupiter's nearly equatorial 3-degree tilt.These banded patterns arise from powerful zonal jet streams, alternating eastward and westward winds that shear the atmosphere into parallel light and dark bands, with speeds reaching 100 m/s on Jupiter and up to 500 m/s near Saturn's equator. Lightning activity, detected primarily in the deeper water cloud layers, indicates convective storms penetrating below the ammonia decks, as observed during spacecraft flybys. On Jupiter, the Great Red Spot exemplifies such dynamics: a massive, persistent anticyclone spanning 16,000 km, featuring towering cumuliform clouds that rise to 0.1 bar, with vertical extents up to 50 km driven by moist convection analogous to Earth's severe thunderstorms. Saturn's north polar hexagon, a six-sided jet stream vortex spanning 30,000 km, hosts stratified cloud layers with embedded thunderstorms, revealing wave patterns in the stratiform ammonia cirrus.Key observations of these clouds stem from pioneering missions: Voyager 1 and 2 in 1979 provided the first close-up images, revealing the banded structures and initial composition hints through infrared spectroscopy. The Juno spacecraft, orbiting Jupiter since 2016, has mapped microwave emissions to probe deeper ammonium hydrosulfide and water layers, confirming ammonia's role in zone-belt contrasts. Cassini's 2004-2017 tenure at Saturn imaged the hazy ammonia decks and polar features, including the hexagon's evolution. Recent James Webb Space Telescope observations in 2024 have unveiled details of deeper phosphine-bearing clouds on both planets, highlighting chemical mixing in the tropospheres. These findings underscore the gas giants' distinct yet comparable cloud regimes, shaped by composition, rotation, and thermal gradients.
Ice Giant Clouds
Ice giant clouds, primarily found in the atmospheres of Uranus and Neptune, are dominated by methane ice particles and extensive hazes that contribute to the planets' distinctive bluish hues through selective absorption of red light by methane gas.[156] These hazes, formed from photochemical reactions and condensation in the cold upper troposphere, often obscure underlying atmospheric features, making detailed observations challenging.[156] Unlike the more prominent ammonia-based cloud bands of gas giants like Jupiter and Saturn, ice giant clouds exhibit minimal vertical development and are confined to pressures around 1-2 bars, where temperatures drop to approximately -200°C, allowing methane to condense into ice crystals.[157]On Uranus, clouds consist mainly of methaneice and haze layers at the 1-2 bar level, manifesting as faint latitudinal bands with subdued activity due to the planet's weak internal heat flux, which limits deep convection.[157] Seasonal storms occasionally brighten these features, as seen in the 2023 northern polar hood enhancement captured by the Hubble Space Telescope, where aerosol layers increased reflectivity without significant methane concentration changes.[158] Voyager 2's 1986 flyby revealed these hazy structures but few discrete clouds, highlighting the planet's quiescent atmosphere with minimal vertical extent.[159] Ongoing ground-based and space observations from Hubble and Keck telescopes, including 2025 studies detecting evolving cloud wave patterns, continue to track these subtle dynamics as Uranus approaches northern solstice.[160]Neptune's clouds, in contrast, feature deeper methane cirrus layers near the 1 bar level, driven by stronger internal convection that sustains dynamic weather despite similar cold interior temperatures around -200°C.[157] Prominent storms, such as the 1989 Great Dark Spot observed by Voyager 2—with associated cumuliform cloud structures and companion cirrus of frozen methane—exemplify this activity, accompanied by extreme winds reaching 600 m/s, the fastest in the solar system.[161][162] Hubble observations in 1994 confirmed the spot's dissipation but revealed similar transient vortices with high-altitude methane ice clouds, underscoring Neptune's volatile atmospheric circulation.[163] These features, obscured intermittently by hazes, highlight convection's role in clearing aerosols more efficiently on Neptune than on Uranus.[156]
