## A Cloud Encyclopaedia

Since its first publication in 1896, the International Cloud Atlas has become an important reference tool for people working in meteorological services, aviation and shipping.

Dating back to the 19th Century, the Cloud Atlas is the global reference book for observing and identifying clouds.  The present international system of Latin-based cloud classification dates back to 1803, when industrial chemist and amateur meteorologist Luke Howard $($1772-1864$)$ wrote a book, On the Modifications of Clouds.  WMO currently recognizes ten cloud genera $($basic classifications$)$, which describe where in the sky they form and their approximate appearance:

The original edition of the International Cloud Atlas featured 28 coloured photographs.  Source: thecloudproject.co.uk

The first edition contained 28 coloured photographs and set out detailed standards for classifying clouds.

With its last full edition published in 1975, and a revision twelve years later, the Manual on the Observation of Clouds and Other Meteors quickly became a collector's item.

The existing Cloud Atlas has two volumes and was originally published in 1956.  Volume I is a detailed technical manual of standards.  Volume II contains around 220 plates of photographs of clouds and certain other meteors, such as precipitation types, haze, rainbows, and lightning.

Each photograph is accompanied by explanatory text to enable the pictures in Volume II to be understood without the detailed technical definitions and descriptions contained in Volume I, and the International Cloud Atlas has been revised and updated on several occasions.

Most recently revised in 1987 with the addition of new photographs, the new fully-digital online edition of the International Cloud Atlas Manual on the Observation of Clouds and Other Meteors $($WMO-No. 407$)$ describes the classification system for clouds.

The Cloud Atlas provides a common language to communicate cloud observations, and it ensures consistency in reporting by observers, weather enthusiasts and cloud spotters around the World.  It serves as a training tool for meteorologists, as well as for those working in aeronautical and maritime environments.

Common cloud names, shapes and relative altitudes. Source: NOAA

## The Taxonomy of Clouds

The impact of technology has been of great importance to the evolution of the modern Cloud Atlas.  People all over the World are now able to use their smartphones to capture and exchange images of fleeting cloud formations.

Clouds and atmospheric formations come in many shapes and sizes.

Weather scientists at the National Oceanic and Atmospheric Administration $($NOAA$)$ distinguish between 10 major genera of clouds, which can be classed according to their relative altitudes in the atmosphere and their approximate appearance.

These genera are subdivided into 14 species $($secondary classifications$)$, which describe shape and internal structure, and 9 varieties $($tertiary classifications$)$, which describe the transparency and arrangement of clouds.  Not all genera have all species, and not all species have all varieties.

There are about 100 cloud combinations in total.

## High Clouds - Cirrus, Cirrostratus, Cirrocumulus

### Cirrus $($Ci$)$

Cirrus clouds are formed at very high altitude. When air masses reach these heights and the temperature drops drastically. Source: Typesofclouds.net

Cirrus are detached clouds in the form of white, delicate filaments, mostly white patches or narrow bands.  They may have a fibrous $($hair-like$)$ or silky sheen appearance.

Cirrus clouds are always composed of ice crystals.  Their transparent character depends upon the degree of separation of the crystals.

As a rule, when these clouds cross the Sun's disk, they hardly diminish its brightness.  Before sunrise and after sunset, cirrus are often coloured bright yellow or red.  These clouds are lit up long before other clouds and fade out much later.

Cirrostratus are high, thin clouds. They are precursors of approaching storms, and precipitation can be expected to follow them within a day or so. Their layer veils the Sun, creating the appearance of a halo.  Source: www.atmos.ucla.edu

### Cirrostratus $($Cs$)$

Cirrostratus are transparent, whitish veil clouds with a fibrous $($hair-like$)$ or smooth appearance.  Cirrostratus clouds lack definition and give the sky a hazy look.

A sheet of cirrostratus which is very extensive, nearly always ends by covering the whole sky.

A milky veil of fog $($or thin stratus$)$ is distinguishable from a veil of cirrostratus of a similar appearance by the halo phenomena which the sun or the moon nearly always produces in a layer of cirrostratus.

### Cirrocumulus $($Cc$)$

Cirrocumulus are thin, white patch, sheet, or layered of clouds without shading.  They are composed of very small elements in the form of more or less regularly arranged grains or ripples.

Generally, cirrocumulus clouds represent a degraded state of cirrus and cirrostratus - both of which may change into it.  It is an uncommon cloud.  There will be a connection with cirrus or cirrostratus and will show some characteristics of ice crystal clouds.

## Middle Clouds - Altostratus, Altocumulus, Nimbostratus

### Altostratus $($As$)$

Altostratus are gray or bluish cloud sheets or layers of striated or fibrous clouds that totally or partially covers the sky.  They are thin enough to regularly reveal the sun as if seen through ground glass.

Altostratus clouds do not produce a halo phenomenon nor are the shadows of objects on the ground visible.  Sometimes, virga - an observable streak or shaft of precipitation that falls from a cloud but evaporates or sublimates before reaching the ground - is seen hanging from Altostratus, and at times may even reach the ground causing very light precipitation.

### Altocumulus $($Ac$)$

Altocumulus clouds generally refer to clouds that are found in the middle range of the sky. Altocumulus is a family or genus of clouds that exist and this can then be divided into different varieties of clouds under this genus. The altocumulus floccus clouds generally indicate instability in altitude at the time of formation. Source: Namesofclouds.org

Altocumulus are white or gray patch, sheet or layered clouds, generally composed of laminae $($plates$)$, rounded masses or rolls.  They may be partly fibrous or diffuse.

When the edge or a thin semitransparent patch of altocumulus passes in front of the Sun or Moon, a corona appears.  This coloured ring has red on the outside and blue inside and occurs within a few degrees of the Sun or Moon.

The most common of mid clouds, more than one layer of altocumulus often appears at different levels at the same time.  Sometimes, altocumulus appear with other types of cloud.

### Nimbostratus $($Ns$)$

Nimbostratus are continuous rain clouds, resulting from thickening altostratus.  This is a dark gray cloud layer diffused by falling rain or snow.  It is thick enough throughout to blot out the Sun. The cloud base lowers into the low level of clouds as precipitation continues.

Low, ragged clouds frequently occur beneath this cloud which sometimes merges with its base.

## Low Clouds - Cumulus, Stratus, Cumulonimbus and Stratocumulus

### Cumulus $($Cu$)$

Cumulus clouds are the large fluffy type of clouds, which almost look like cotton balls or piles of cotton candy up in the sky. Source: Typesofclouds.net

Cumulus are detached, generally dense clouds, with sharp outlines that develop vertically in the form of rising mounds, domes or towers, with bulging upper parts often resembling a cauliflower.  The sunlit parts of these clouds are mostly brilliant white, while their bases are relatively dark and horizontal.

Over land, cumulus clouds develop on days of clear skies due to diurnal convection.  They appear in the morning, grow, then more or less dissolve again toward evening time.

### Stratus $($St$)$

Stratus usually form grey cloud layers with a uniform base which may, if thick enough, produce drizzle, ice prisms, or snow grains.  When the Sun is visible through this cloud, its outline is clearly discernible.

When a layer of stratus breaks up and dissipates, blue sky is often seen.

### Cumulonimbus $($Cb$)$

Cumulonimbus are thunderstorm clouds, heavy and dense, in the form of a mountain or huge tower.  The upper portion is usually smoothed, fibrous or striated and nearly always flattened in the shape of an anvil or vast plume.

Under the base of this cloud which is often very dark, there are often low ragged clouds that may or may not merge with the base.  They produce precipitation, sometimes in the form of virga.

Cumulonimbus clouds also produce hail and tornadoes.

### Stratocumulus $($Sc$)$

Stratocumulus clouds over the Central Belt of Scotland. Image: NaturPhilosophie

Stratocumulus are gray or whitish patch, sheet, or layered clouds which almost always have dark tessellations $($honeycomb appearance$)$, rounded masses or rolls.  Except for virga, they are non-fibrous and may or may not be merged.

They also have regularly arranged small elements with an apparent width of more than five degrees $($three fingers - at arm's length$)$.

One of my favourite childhood occupations was to lie down in the grass, stare at the sky on a beautiful day, and let my imagination run free.  And it still is.

Forget about bunny rabbits or sheep-shaped clouds...

## These Clouds will Feed Your Imagination

The atlas now includes asperitas clouds and 12 new kind of atmospheric phenomena after campaigns by citizen scientists.

Asperitas become the first addition of a new recognisable term since 1953.  Additionally, several other new entries have been added including cavum, cauda - also known as a tail cloud, contrails - the linear clouds formed from the vapour trail of aeroplanes, fluctus and murus - known as a wall cloud.

The WMO have also added one new "species" of clouds: the volutus - a low horizontal tube-shaped cloud mass that appears to roll about a horizontal axis.

Asperitas clouds over Ohio, United States.

Asperitas or Undulatus asperatus are cloud formations that were first popularised as a type of cloud by Gavin Pretor-Pinney of the Cloud Appreciation Society in 2009.  It is the first cloud formation added since cirrus intortus in 1951.  The name translates approximately as "roughness".

A tail cloud $($Cauda$)$ is sometimes seen attached to the wall cloud of a severe thunderstorm. The tail cloud originates from the area of the downdraft, which has a high moisture content due to the precipitation. Source: WMO

Cauda or 'tail cloud' is a horizontal, tail-shaped cloud at low levels extending from the main precipitation region of a supercell cumulonimbus to the murus $($wall cloud$)$.  It is typically attached to the wall cloud, and the bases of both are typically at the same height.  The cloud motion is away from the precipitation area and towards the murus, with rapid upward motion often observed near the junction of the tail and wall clouds.

A fallstreak hole cloud.  The Bergeron process causes water droplets around ice crystals to evaporate, which leaves a large circular hole in the cloud formation.  Image: Tony Sarno

Fallstreak hole $($punch hole or cavum$)$  is a large circular or elliptical gap that can appear in cirrocumulus or altocumulus clouds.  Such holes are formed when the water temperature in the clouds is below freezing, but the water, in a supercooled state, has not frozen yet due to the lack of ice nucleation.  When ice crystals do form, a domino effect is set off due to the Bergeron process, causing water droplets around the crystals to evaporate.

Temperature inversion clouds over the city of Glasgow in Scotland. Image: NaturPhilosophie

Inversion clouds occur when there is an increase of temperature with height in the atmosphere.  Thermal inversions and stable layers inhibit vertical motion, often limiting the vertical extent of cumuliform clouds.  If the air below an inversion is relatively moist, the inversion often caps a layer of stratiform cloud, increasing the potential for photochemical smog in busy capital cities.

A Kelvin-Helmholtz instability. These clouds look like ocean waves suspended in the sky.

Kelvin-Helmholtz clouds resemble breaking waves in the ocean.  These clouds are formed between two layers of air with different densities and travelling at different speeds.  If a warm, less dense layer of air exists over a layer of colder, denser air, and the wind shear across the two layers is strong enough, eddies will develop along the boundary.  Evaporation and condensation of the eddies render them visible as wave-shaped clouds.

Gibraltar's famous Levanter cloud. Source: Al Jazeera

Levanter clouds are characteristic banner clouds that form in moist, stable easterly winds over the Rock of Gibraltar.  The levant is an easterly mountain-gap wind that blows over the western Mediterranean Sea and southern France.  In the western Mediterranean, particularly when the wind blows through the Strait of Gibraltar, it is bears the name of "Viento de Levante" or "Levanter".

Mammatus clouds are rounded, smooth, sack-like protrusions hanging from the underside of a cloud $($usually a thunderstorm anvil$)$.  Mammatus clouds often accompany severe thunderstorms, but do not produce severe weather.  They may also accompany non-severe storms as well.  They only occur where cumulonimbus clouds are present, usually linked to the after-effect of tornadoes.

Murus or "Wall cloud" showing significant rotation and vertical motion, which results in the formation of a spout. Source: WMO

Murus is a localised, persistent, and often abrupt lowering of cloud from the base of a cumulonimbus from which tuba $($or spouts$)$ can sometimes form.  Usually, a murus is associated with a supercell or severe multi-cell storm, typically developing in the rain-free portion of a cumulonimbus cloud and indicating an area of strong updraft.  This type of clouds is commonly known as a 'wall cloud'.

Pile d’assiettes $($pile of plates$)$ clouds is the usual term for a series of lenticular clouds stacked one above the other, which is caused by wave motion in multiple humid layers of air.  The technical classification is Stratocumulus lenticularis duplicatus or AltocumuIus lenticularis duplicatus.  The photograph at the top shows this particular phenomenon photographed over Mount Fuji in Japan.

A volutus cloud over the ocean. Source: WMO

Volutus clouds are long, typically low, horizontal, detached, tube-shaped cloud masses, often appearing to roll slowly about a horizontal axis.  The roll cloud, volutus, is a soliton - it is not attached to other clouds.  It is an example of an undular bore.

## A Cloud Atlas of Meteors

The atlas also include descriptions of meteorological phenomena other than clouds: hydrometeors, lithometeors, photometeors, and electrometeors.

Non-stop lightning at Catatumbo, Venezuela

Electrometeors are visible or audible manifestations of atmospheric electricity.

An electrometeor can be:

• lightning or thunder $($related to discontinuous electrical discharges $)$
• Saint Elmo's fire or polar aurora $($related to more or less continuous electrical discharges$)$

This phenomenon is a Snow Devil or "snownado", photographed in Lessach, Austria. Image: Heinz Petelin/DailyPicksandFlicks

Hydrometeors consist of liquid or solid water particles.  They may be suspended in the atmosphere, fall through the atmosphere, be blown by the wind from the Earth’s surface or be deposited on other objects.

By convention, ground water or snow is not considered a hydrometeor.

The atlas lists five types of hydrometeors:

• suspended particles: fog, mist, haze
• precipitation $($i.e. falling particles$)$: rain, sleet, snow and hailstones
• particles raised by the wind from the Earth's surface: sea spray, snow devils and ground blizzard
• deposits of particles: dew, frost, white dew, hoar frost, glaze, clear ice and rime
• spouts.

Red hour. West Glasgow at twilight. Image: NaturPhilosophie

Photometeors are optical phenomena produced by the reflection, refraction, diffraction or interference of light from the Sun or the Moon.

A photometeor can be:

• on or inside clouds: halo phenomena, corona, irisations and glory
• on or inside hydrometeors or lithometeors: halo phenomena, corona, glory, rainbows, Bishop's ring and crepuscular rays
• in clear air: mirage, shimmer, scintillation, green flash and twilight colours.

Mount Redoubt eruption on 21 April 1990: The ash cloud shot over 14,000 metres $($45,000 feet$)$ into the sky, dropping over 5 cm $($2 inches$)$ of ash on the town of Soldona, Alaska, about 120 km $($80 miles$)$ to the east.

## Clouds of Fire

A mushroom cloud is a distinctive pyrocumulus mushroom-shaped cloud of debris/smoke and usually condensed water vapour resulting from a large explosion.  They can be caused by volcanic eruptions.

Nevertheless, the effect is most commonly associated with a nuclear explosion such as that associated with an atomic bomb.  However, any sufficiently energetic detonation or deflagration will produce the same sort of effect.

Mushroom clouds result from the sudden formation of a large volume of lower-density gases at any altitude, causing an atmospheric instability whereby a lighter fluid mass pushes a denser heavier fluid, known as a Rayleigh-Taylor instability.

Man-made "mushroom" cloud. The aftermath of an atomic bomb detonation. Beautiful, yet deadly...

The buoyant mass of gas rises rapidly, resulting in turbulent vortices curling downward around its edges, forming a temporary vortex ring that draws up a central column, possibly with smoke, debris and water vapour to form the "mushroom stem".  The mass of gas, along with drawn-up moist air, eventually reaches an altitude where it is no longer of lower density than the surrounding air.  At this point, the gas disperses, and any debris drawn upward from the ground scattering falls out.  The stabilisation altitude is dependent on the profiles of temperature, dew point, and wind shear in the air at and above the starting altitude.

The last picture is a sign of the times, perhaps the symbol of the Anthropocene - our current geological era.  Man-made and deadly, the atomic mushroom cloud is an infamous reminder of how humans have actively sought the destruction of their own kind and impacted on Nature...

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