Tropospheric clouds are divided into physical categories. They have Latin based names that indicate physical structure and process of formation. Clouds of the cirriform category are generally thin and occur mostly in the form of filaments. Two other basic categories are stratiform with non-convective clouds that are mostly sheet-like in structure, and limited or free-convective cumuliform that appear in heaps. Two additional categories derived from the cumuliform group are stratocumuliform which comprise rolled or rippled clouds of limited convection that combine cumuliform and stratiform characteristics, and cumulonimbiform, towering free-convective cumuliform clouds often with complex structures that include cirriform tops and multiple accessory clouds.
In the troposphere, ten genus types are derived by cross-classifying the physical categories into four families defined by altitude range; high, middle, low, and vertical or multi-level (with low to middle cloud base). The last of these can be subdivided into two sub-families or groups to distinguish between moderate and towering vertical types.
Cirriform category clouds are only found in the high-altitude family and therefore constitute a single genus cirrus. High stratiform and stratocumuliform types carry the prefix cirro- which yield the genus names cirrostratus and cirrocumulus. Clouds of the middle-altitude family have the prefix alto- (altostratus and altocumulus) to distinguish them from the high clouds. Strato- is dropped from high and middle stratocumuliform genus names to avoid double-prefixing. Low altitude stratiform, stratocumuliform, and cumuliform genera (stratus, stratocumulus, and small cumulus) carry no height-related prefixes.
The family of vertical clouds includes thick stratiform, cumuliform, and cumulonimbiform genera, all of which can produce precipitation of significant intensity. Within this family, the group of moderate vertical clouds comprise nimbostratus and cumulus mediocris that form in the low or middle altitude range. These genus types also have no height-related prefixes, but its stratiform genus carries the prefix nimbo- to denote its ability to produce widespread precipitation. The towering vertical group has no stratiform types, but rather comprises the genus cumulonimbus, and the species cumulus congestus, a towering variant of the genus cumulus whose other species belong to the low and moderate vertical clouds.
Most cloud genera are divided into species, varieties, or both (with species ranked above varieties), based on specific physical characteristics of the clouds. Species types and opacity-based varieties are always present with any genera that characteristically have them. However, pattern-based varieties are only seen with any particular genus when atmospheric conditions are favorable for their occurrence. A total of about ninety sub-types can be identified that are derived by this process of division and subdivision into species and varieties. Supplementary features of the main cloud types can take the form of precipitation or special cloud formations that are attached or located in close proximity to the main cloud. Although accessory clouds are most commonly seen with cumulonimbus, they are also occasionally seen with other genus and species types as well. They are not further subdivisions of the basic genera, species, and varieties, but are separately classified clouds associated with the main types..
Any genus type that undergoes a full or partial change into another genus is termed a mother cloud. If the change is only partial, the mother cloud is a genitus type indicating that some of its physical characteristics can be seen associated with the new genus type. If the change is complete, the mother cloud carries a mutatus designation to indicate its mutation into the new type. Changes in species and varieties usually accompany changes in the genus type, but the genitus and mutatus designations apply directly only to the genus level of classification.
Clouds that form above the troposphere have a generally cirriform structure, but are not given Latin names based on that characteristic. Polar stratospheric clouds form at very high altitudes in polar regions of the stratosphere. Those that show mother-of-pearlcolors are given the name nacreous. Both these and non-nacreous types are classified alpha-numerically according to their physical state and chemical makeup. Polar mesospheric clouds are the highest in the atmosphere and are given the Latin name noctilucent which refers to their illumination during deep twilight. They are sub-classified alpha-numerically according to specific details of their cirriform physical structure.
Mesospheric, stratospheric, and tropospheric classes are listed on this page in descending order of altitude range. Within the troposphere, families of non-vertical clouds are also listed in descending order of altitude. The genus types within each family are arranged in descending order of average cloud base height. Their constituent species, varieties, supplementary features and mother clouds are arranged in approximate order of frequency of occurrence. Vertical/multi-level cloud groups and their constituent genera and species are listed in ascending order of average altitude of cloud tops. Their varieties, supplementary features, and mother clouds are arranged in order of approximate frequency of occurrence.
A count of basic variants is shown as a number in parentheses after each variety, after nimbostratus that has no sub-types, and after certain species that are not always dividable into varieties.
High cirriform, stratocumuliform, and stratiform
High clouds form in the highest and coldest region of the troposphere from about 16,500 to 40,000 ft (5 to 12 km) in temperate latitudes. At this altitude water almost always freezes so high clouds are generally composed of ice crystals or supercooled water droplets.
Cirrus spissatus undulatus clouds
Cirrus uncinus clouds
Cirrus tends to be wispy, and are mostly transparent or translucent. Isolated cirrus clouds do not bring rain, however, large amounts of cirrus clouds can indicate an approaching storm system eventually followed by fair weather.
There are several variations of clouds of the cirrus genus based on species and varieties:
Cirrus fibratus (1) High clouds having the traditional "mare's tail" appearance. These clouds are long, fibrous, and curved, with no tufts or curls at the ends.
Clouds of the genus cirrocumulus form when moist air at high tropospheric altitude reaches saturation, creating ice crystals or supercooled water droplets. Limited convective instability at the cloud level gives the cloud a rolled or rippled appearance. Despite the lack of a strato- prefix, cirrocumulus is physically more closely related to stratocumulus than the more freely convective cumulus genus.
Clouds of the genus cirrostratus consist of mostly continuous, wide sheets of cloud that covers a large area of the sky. It is formed when convectively stable moist air cools to saturation at high altitude, forming ice crystals. Frontal cirrostratus is a precursor to rain or snow if it thickens into mid-level altostratus and eventually nimbostratus as the weather front moves closer to the observer.
Varieties are not commonly associated with Cs species nebulosus.
Supplementary features/accessory clouds
Not associated with cirrostratus.
Genitus mother clouds
Mutatus mother clouds
Middle stratocumuliform and stratiform
Middle cloud forms from 6,500 to about 23,000 ft (2 to 7 km) in temperate latitudes, and may be composed of water droplets or ice crystals depending on the temperature profile at that altitude range.
Clouds of the genus altocumulus are not always associated with a weather front but can still bring precipitation, usually in the form of virga which does not reach the ground. This genus is generally an indicator of limited convective instability, and is therefore structurally more closely related to stratocumulus than to the more freely convectice cumulus genus.
Altocumulus stratiformis (Always dividable into opacity-based varieties) Sheets or relatively flat patches of altocumulus.
Clouds of the genus altostratus form when a large convectively stable airmass is lifted to condensation in the mid-altitude 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 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.
Varieties are not commonly associated with St species fractus.
Precipitation-based supplementary feature
Praecipitatio Stratus (usually species nebulosus) producing precipitation.
Not usually seen with stratus.
Genitus mother clouds
Mutatus mother cloud
Genus cumulus (little vertical extent)
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.
Cumulus humilis (82) "Fair weather clouds" with flat light grey bases and small white domed tops.
None (always opaque except species fractus which is always translucent).
Humilis pattern-based variety
Cumulus humilis radiatus (83) Small cumulus clouds arranged in parallel lines that appear to converge at the horizon.
Supplementary features/accessory clouds
Not commonly seen with cumulus fractus or humilis.
Genitus mother clouds
Mutatus mother clouds
Vertical or multi-level stratiform, cumuliform, and cumulonimbiform (low to middle cloud base)
Clouds with upward-growing vertical development usually form below 6,500 feet (2.0 km), but can be based as high as 8,000 feet (2.4 km) in temperate climates, and often much higher in arid regions. Downward-growing cloud forms mostly above 6,500 feet (2.0 km) and achieves vertical extent as the base subsides into the low altitude range during precipitation.
Clouds of the genus nimbostratus tend to bring constant precipitation and low visibility. This cloud type normally forms above 6,500 feet (2.0 km) from altostratus cloud but tends to thicken into the lower levels during the occurrence of precipitation. The top of a nimbostratus deck is usually in the middle level of the troposphere.
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).
None (always opaque).
Cumulus congestus radiatus (88) Towering cumulus arranged in parallel lines that appear to converge at the horizon.
Precipitation-based supplementary features
Virga Accompanied by precipitation that evaporates before reaching the ground.
Praecipitatio Produces precipitation that reaches the ground.
Pannus Accompanied by a lower layer of fractus species cloud forming in precipitation.
Mamma Downward facing bubble-like protuberances caused by localized downdrafts within the cloud.
Pileus Small cap-like cloud over parent cumulus cloud.
Velum A thin horizontal sheet that forms around the middle of a cumulus cloud.
Arcus (including roll and shelf clouds) Low horizontal cloud formation associated with the leading edge of a thunderstorm outflow.
Tuba Column hanging from the cloud base which can develop into a small funnel cloud.
Genitus and mutatus types are the same as for small and moderate cumulus.
Clouds of the genus cumulonimbus have very dark gray to nearly black flat bases and very high tops that can penetrate the tropopause. They develop from cumulus when the airmass is convectively highly unstable. They generally produce thunderstorms, rain or showers, and sometimes hail, strong outflowwinds, and/or tornadoes at ground level.
Praecipitatio – falling – cloud whose precipitation reaches the ground.
Tuba – shaped like a tuba – column hanging from the bottom of cumulus.
Velum – a ship's sail – sail-like in appearance.
Informal terms related to clouds of limited convection
Aviaticus cloud - persistent condensation trails (contrails) formed by ice crystals originating from water vapor emitted by aircraft engines. May resemble cirrus, cirrocumulus, or cirrostratus depending on atmospheric stability and wind shear.
Fallstreak hole – thin cloud distinguished by holes (sometimes known as fallstreak holes) and ragged edges. See also lacunosus.
Kelvin-Helmholtz - Crested wave-like clouds formed by wind-shear instability that may occur at any altitude in the troposphere.
WMO and informal terms related to free-convective cloud types and storms
Accessory cloud (WMO supplementary feature) – cloud that is attached to and develops on body of main cloud.
Anvil (WMO supplementary feature incus) – the top flatter part of a cumulonimbus cloud.
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 – cumulus clouds formed by quickly generated ground heat; including forest fires, volcanic eruptions and low level nuclear detonation, generally of the WMO species mediocris or congestus.
Cumulus arcus roll cloud over Wisconsin
Roll cloud (may be informal term for WMO genus stratocumulus or supplementary feature arcus) – elongated, low-level, tube shaped, horizontal cloud.
Rope – (slang) narrow, sometimes twisted funnel type cloud seen after a tornado dissipates.
Rope cloud (informal term) – A narrow, long, elongated lines of cumulus cloud formation that develop at the leading edge of an advancing cold front or weather fronts that is often visible in satellite imagery.
Scud cloud (informal term for WMO species fractus) – ragged detached portions of cloud that usually form in precipitation.
Shelf cloud (informal term for WMO supplementary feature arcus) – wedge-shaped cloud often attached to the underside of Cb.
Stratus fractus (WMO genus and species) – ragged detached portions of stratus cloud that usually form in precipitation (see also scud cloud).
Striations (informal term for WMO supplementary feature 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.
Cloud decks in parallel latitudinal bands at and below the tropopause alternatingly composed of ammonia crystals and ammonium hydrosulfate.
Bands of cloud resembling cirrus located mainly in the highest of three main layers that cover Jupiter.
Stratiform and Stratocumuliform
Wave and haze clouds that are seen mostly in the middle layer.
Cumuliform and cumulonimbiform
Convective clouds in the lowest layer that are capable of producing thunderstorms and may be composed at least partly of water droplets. an intermediate deck of ammonium hydrosulfide, and an inner deck of cumulus water clouds.