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Clouds are accumulations of water droplets, ice particles, or both, that are suspended in the air. The ten types of cloud are explained below with the help of photographs.


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Clouds consist of water droplets or ice particles. For clouds to form, two conditions must be met:

  • The humidity must have reached saturation level. In other words, the relative humidity is 100%.

  • There must be sufficient condensation nuclei (aerosols) in the air mass for water droplets or ice particles to adhere to. Condensation nuclei include pollen, whirled-up dust and salt particles, as well as soot and other particles that are in the air as a result of human activities.

Clouds vary in appearance in terms of their structure, colour and distance from the Earth's surface. There are ten main internationally agreed definitions of cloud types (cloud genera). They can be identified using the cloud identification guide:

  1. Cirrus

  2. Cirrocumulus

  3. Cirrostratus

  4. Altocumulus

  5. Altostratus

  6. Nimbostratus

  7. Stratocumulus

  8. Stratus

  9. Cumulus

  10. Cumulonimbus

Other phenomena can be seen in the sky, including virga and halos.


Thin, detached clouds in the form of white, delicate filaments or mostly white patches or narrow bands. These clouds have a fibrous, hair-like appearance or a silky sheen, or both. After a thunderstorm, cirrus can be patchy and so dense that the sun behind it is veiled, looks grey or is completely obscured.


Thin, white patch, sheet or layer of cloud without shading, composed of very small elements in the form of grains, ripples, etc., merged or separate, and more or less regularly arranged. Most of the elements have an apparent width of less than 1 degree.

How is the apparent width of clouds determined?

Determining the apparent width of a cloud helps to determine its type. The width is given in degrees. One degree corresponds approximately to the width of the little finger when the arm is extended. Five degrees roughly corresponds to the width of the ring, middle and index fingers together when the arm is extended.


Transparent, whitish cloud veil of fibrous or smooth appearance, totally or partly covering the sky, and generally producing halo phenomena.


White and/or grey patches, sheets or layers of clouds, often with their own shadows. Their structure is flaky, sometimes fibrous and diffuse. Altocumulus consists of rounded masses or rolls that can be separate or merged. An altocumulus rarely appears alone. Most of the regularly arranged small cloud elements usually have an apparent width of between 1 and 5 degrees (two to three fingers wide).

Harbingers of thunderstorms: Altocumulus castellanus consists of whitish towers arranged in rows, with a relatively straight, horizontal base line, giving it a crenelated appearance. It usually persists for only a short time, a few hours at most, after which it dissipates again. It may be encountered in the early morning of a summer day, during the course of which cumulus and cumulonimbus clouds can form, with thunderstorm activity occurring.

What is striking about altocumulus lenticularis is that it is constantly changing. The changes may be limited to individual cloud elements but may also involve entire groups of cloud banks at different heights. The clouds may also dissolve in some places and immediately form again in another place. Between the individual clouds, bright sections or even blue skies can often be observed.

Weather situation: Altocumulus occur mainly in foehn conditions, at the end of a period of fine weather and in warm sectors, predominantly with southerly or south-westerly winds before the approach of a cold front. Similar clouds also form on the south side of the Alps with a northerly wind.


Greyish or bluish cloud sheet or layer of striated, fibrous or uniform appearance, totally or partly covering the sky, and having parts thin enough to reveal the sun as if through frosted glass. Altostratus clouds do not exhibit halo phenomena.


Thick, dark, grey cloud layer that appears diffuse during more or less persistent rain or snowfall. Nimbostratus is a sluggish cloud mass that hardly changes shape and is so dense that it blocks out the sun. It consists of supercooled water droplets, raindrops, snow crystals, snowflakes or a mixture of all of these. Low, ragged clouds often appear below this layer and can merge with it.


Grey and/or whitish patches, sheets or layers of cloud, often with dark areas. Cloud groups consist of mosaic-like as well as rounded masses or rolls that are clearly structured and may be merged together. Most of the regularly arranged cloud elements have an apparent width of more than 5 degrees.


Sometimes stratus appears in the form of ragged patches.
A closed, grey cloud layer with a fairly uniform base from which drizzle, ice prisms or snow grains may fall. The lower boundary of low stratus is usually clearly delineated. When the sun is visible through the cloud, its outline is clearly discernible.

Sometimes stratus occurs in the form of ragged swaths. May appear as an accompanying cloud below a nimbostratus (rain or snow cloud), on mountain slopes or when fog dissipates.


Detached, dense and sharply defined clouds that develop vertically in the form of mounds, domes or towers, of which the bulging upper part often resembles a cauliflower. The sunlit parts of these clouds are brilliant white. The inherent shadow makes for a dark base, which appears horizontal. Sometimes the cumulus clouds are ragged.


Heavy and dense cloud with considerable vertical extent in the form of a large mountain or high tower. A cumulonimbus develops from a cumulus. The clearly delineated, rounded bulges lose their sharp outlines towards the upper part of the cloud, where it becomes a fibrous, streaky cloud mass, which may eventually form an anvil shape (incus). Below the often dark cloud base, there are often low, ragged bands of cloud that may or may not merge with the main cloud. The precipitation from the anvil part occasionally falls as virga.



Precipitation that falls from the clouds but does not reach the ground is called virga. This occurs, for example, as a result of evaporation.

Halo phenomena

Halo phenomena are light phenomena that appear as circles around the sun or moon. They are created by light rays being deflected off ice crystals in the atmosphere.

Cloud identification guide

A cloud or group of clouds with similar structure, colour and height can be identified with the help of the diagram.