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Basic Forms

There are in fact only two basic forms of cloud - layered and convective, and only two basic constituents - water droplets and ice crystals.

Layered Clouds are much larger horizontally than vertically. They usually have a more or less smooth base and top and can extend for hundreds of kilometres, especially where associated with weather fronts. They can develop when the atmosphere is stable.

Convective Clouds are generally discrete and cellular. They are usually deeper than they are across. The base is quite uniform except sometimes when the cloud is dissolving, but the tops vary considerably with turrets of cloud projecting above the main mass. They form only when the atmosphere is unstable.

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Classification

Meteorologists use a classification that is similar to the Latin names employed for plants and animals. Clouds are arranged in genera (the ten main types described first), species and varieties.

Genus Type of cloud
species Cloud shape and structure
variety Arrangement of elements and transparency

Cloud Forms - The three basic cloud forms are

cu cumulus heap clouds
st stratus layer clouds
ci cirrus hair-like or feathery clouds

These are also the names of three specific types. There was once a fourth form, 'nimbus', meaning 'rain-bearing'. The name still remains in some types (such as nimbostratus) although other clouds also produce rain or snow.

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Cloud Height

Clouds are also classified by the height of their bases, with broad categories of high, medium and low. The names reflect the fact that specific types occur at particular levels. The range of heights and the maximum altitudes of medium and high level clouds are greater towards the equator than near the poles. Similarly, cloud heights tend to be lower in winter than in summer.

Cloud heights are difficult to estimate without a great deal of experience or equipment.

Height of base
Convective
Layered
Low (CL) - formed of water droplets

0 - 2.5 km
(0-8000 ft)

Cumulus (Cu)
Cumulonimbus (CB)
Stratus (St)
Stratocumulus (Sc)
Nimbostratus (Ns)
Medium (CM) - water droplets & ice crystals
2.5 - 6 km
(8000 - 20 000 ft)
Altocumulus (Ac)
Altostratus (As)
Altocumulus (Ac)
High (CH) - ice crystals
6 km (20 000 ft) or more
Cirrocumulus (Cc)
Cirrus (CI)
Cirrostratus (Cs)

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10 Main Types

Stratus | Stratocumulus | Cumulus | Cumulonimbus | Altostratus | Nimbostratus | Altocumulus | Cirrus | Cirrostratus | Cirrocumulus

Stratus - This is a low grey, uniform featureless cloud, usually with a base below 500 m always formed entirely of water droplets. It often has a ragged base and top and is identical to fog, which is stratus at ground level. It is sometimes shallow and tenuous. Generally there are no optical phenomena associated with it.

Stratus forms either by the slow uplift of moist air or when a gentle wind carries nearly saturated air across a cold land or sea surface. There is not much precipitation from stratus, because it is a shallow cloud, but it may produce slight to moderate drizzle.

Stratocumulus - Perhaps the most common cloud, it frequently forms in the lowest few kilometres when wind-driven turbulence lifts air above its condensation level. It is a low, grey or whitish sheet of cloud with a definite structure. These clumps, broader pancakes or rolls show dark shading and are outlined by thinner and paler regions of cloud or by blue sky.

It is formed completely of water droplets, and although not itself associated with rain, may markedly increase rain falling through it from higher cloud. Generally there are no optical phenomena associated with it.

Cumulus - Cumulus is easy to recognise, they are the fluffy clouds that float across the sky on a fine day and are often known as fair-weather clouds. They have rounded tops and flat, darker bases. They are formed when the unstable layer is shallow with plenty of blue sky between.

Cumulonimbus - This is the largest and most energetic of the cumulus family. They are the deepest and most vigorous convective clouds and produce not only showers but thunderstorms, hail, squally winds and occasionally tornadoes. They usually form by heating from below, but the most intense cumulonimbus are helped by cold winds at high levels.

Cumulonimbus are avoided where possible by aircraft because strong up and down currents within and near them create severe turbulence, while their high water content can rapidly produce thick layers of ice on cold airframes. Sometimes has rainbows in rain beneath cloud.

Altostratus - This is a dull, medium level, white or blueish grey cloud. It forms by slow ascent of air over a wide area, especially ahead of a warm front or occlusion where it is often a precursor of rain.

Although composed mostly of water droplets, thick altostratus can produce light rain, though most of this evaporates before reaching the ground. It may have optical phenomena such as coronae or iridescence when thin.

Nimbostratus - This is a dark, grey, heavy sheet of cloud from which rain or snow is falling. It appears black from below but as the rain or snow becomes heavier the base may become indistinguishable with just ragged scuds of low fractostratus to be seen. The rain or other precipitation usually lasts for a long time. At a warm front it may continue for several hours, unlike the relatively short, intense rain from showers.

Altocumulus - Altocumulus is a medium level cloud which occurs as an individual rounded masses with clear sky between them. It forms in rolls or patches with or without gaps between, by turbulent mixing often in moist layers remaining from dispersed fronts.

Altocumulus cloud may contain either water droplets, which are usually supercooled, or ice crystals, or both. They may therefore exhibit a range of optical phenomena, depending on which form of water is predominant (e.g. Iridescence, corona, mock suns, sun pillars)

Cirrus - This is high cloud composed entirely of ice crystals and takes many forms. It is non-uniform and often thin and wispy, sometimes with thicker bright sheaves, and all shapes between. It forms by ascent in the upper troposphere. Occasionally it is manufactured when condensation trails from high flying aircraft seed already moist or very cold air.

It has no precipitation which reaches the ground by it actually consists of falling ice crystals. Optical phenomena such as mock suns and circumzenithal arcs may be seen.

Cirrostratus - This is a sheet of high ice-cyrstal cloud, sometimes so thin that it goes completely unnoticed because it has little effect on sunlight. It is produced by the slow ascent of air and condensation or sublimation high in the troposphere, usually well ahead of weather fronts. It appears in meteorological folklore because it is often an early indication of rain.

Cirrocumulus - This attractive ice cloud is the high-level equivalent of stratocumulus or altocumulus, and much less common than either. It is formed of cells aligned in streets rather like ripples in the sand on the beach. Cirrocumulus forms by wave motion or turbulence through a moist layer in the high atmosphere.

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Species

The 14 cloud species are used to describe cloud shape and structure.

Species
Abbr.
Description
Class Types
Humilis
hum
Flattened
Cu
Mediocris
med
Moderate depth, tops with fairly small bulges
Cu
Congestus
con
Piled up. Markedly growing, often great vertical extent, with tops that resemble a cauliflower.
Cu
Fractus
fra
Broken, irregular or ragged shreds of cloud
Cu, St
Nebulosus
neb
Thin veil or layer with no distinct features
St, Ca
Stratiformis
str
Flat. Very extensive horizontal sheet or layer
Sc, Ac, Cc
Calvus
cal
Tops look smooth (bald). Losing cumuliform appearance but no obvious cirrus
Cb
Capillatus
cap
Having hair. Distinct icy regions with fibrous, striated appearance (anvil, plume, or disordered mass of cirrus)
Cb
Floccus
flo
Tufted. Small tufts of cloud, with ragged lower portion, and often virga
Ac, Cc, CI
Castellatus
cas
Castle-like battlements connected by a common base, sometimes arranged in lines
Sc, Ac, Cc, CI
Lenticularis
len
Lens or almond-shaped. Wave cloud
Sc, Ac, Cc
Fibratus
fib
Nearly straight, or more or less curved, no hooks
CI, Cs
Spissatus
spi
Dense enough to appear grey towards Sun
CI
Uncinus
unc
Comma or hook shaped, not rounded tuft of cloud
CI

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Variety

Just as cloud species describe shape and form varieties define cloud transparency or the arrangement of the individual elements in the specific type. Please note that most of the descriptions are self-explanatory so only those more obscure ones are detailed on the table below.

Variety
Abbr.
Description
Cloud Types
Intortus
in
Irregularly curved or apparently tangled
CI
Vertebratus
ve
Looking like ribs, vertebrae or fish bones
CI
Undulatus
un
Patches, sheets or layers with parallel undulations
Sc, Ac, As, Cc, Cs
Radiatus
ra
Broad parallel bands, appearing to converge by perspective
Cu, Sc, Ac, As, CI
Lacunosus
la
Thin cloud with regularly spaced holes, reticulated (like a net): rare
Ac, Cc
Duplicatus
du
More than one layer, at slightly different levels
Sc, Ac, As, CI, Cs
Translucidus
tr
Translucent enough to show position of Sun or Moon
St, Sc, Ac, As
Perlucidus
pe
Broad layers or patches, with spaces (occasionally very small) that allow blue sky, Sun or Moon to be seen
Sc, Ac
Opacus
op
Completely masks Sun or Moon
St, Sc, Ac, As
Other Varieties include: Billows, Cloud Streets

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Accessory Clouds

Certain forms of cloud are not true types but always occur in association with one or more of the ten main types. These three accessory clouds are

Accessory Cloud
Description
Types
pannus
shreds of cloud
Cu, Cb, As, Ns
pileus
cap cloud
Cu, Cb
velum
veil
Cu, Cb

There are also a number of other features which can describe the appearance of certain clouds. These are

Name
Description
arcus
arch cloud
incus
anvil cloud
mamma
pouches hanging from upper cloud
praecipitatio
precipitation reaching the surface
tuba
funnel clouds of any type
virga
fallstreaks

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Contrails

Condensation trails, or contrails are a familiar sight. They are lines of cloud that have formed from the water vapour emitted by aircraft engines. Initially the exhaust is very hot and the water vapour is invisible, so there is a clear gap behind the engines. Farther away mixing cools the exhaust sufficiently for condensation to occur.

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