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Atlas of the clouds
Cumulus clouds with a cumulonimbus in the background
Cloud development in time lapse
Different types of clouds

A cloud ( going back to the Indo-European root u̯elg- “moist, wet” and therefore linguistically related to the adjective wilted and the river name Volga ) is a collection of very fine water droplets ( fog ) or ice crystals in the atmosphere .

Clouds are visible because light due to the Mie scattering scattered is causing the Tyndall effect occurs and the actually colorless droplets are visible.

The water droplets form around condensation nuclei when the relative humidity of the air slightly exceeds 100% (by at most 1%). This can be done either by cooling the air when ascending ( thermals , sliding on other layers of air or on the mountain slope) or by mixing two volumes of air ( Richard Mollier ). When the water condenses , the heat of evaporation of the water is released, which weakens the cooling as the air rises further. This allows the air to rise to greater heights. If the air is calm and there are few condensation nuclei, the air can become oversaturated with water vapor. Although the relative water content is then significantly more than 100%, there is still no condensation. The water content has to increase further before it condenses. If the air temperature is below −10 ° C, ice crystals ( snowflakes ) can form on the condensation nuclei through resublimation. Condensation nuclei are electrostatically charged and have a size of 1  nm to 1000 nm. They arise from private households, industry, car exhaust fumes, agriculture, nature and cosmic radiation ( e.g. cloud chamber ). After the start of condensation, more and more water vapor condenses at this point until it becomes a visible mist droplet. In the meteorological system, the clouds are assigned to the hydrometeors .

Clouds are mainly found in the troposphere , partly also in the stratosphere and mesosphere ( glowing night clouds ). They differ in their formation, in their properties and are easily observable features of the weather situation . By correctly interpreting the shape, appearance and height as well as changing the characteristics over time, statements can be made about local weather developments. In order to be able to transfer observations, clouds are classified. In practice, the division into cloud genera and cloud types is particularly important. In most areas, certain types of cloud are more common, especially when the weather is the same. Nevertheless, all types of clouds can occur almost anywhere on earth. The World Meteorological Organization therefore regulates the classification of clouds in an internationally uniform manner.

In addition to their optical properties and their beauty, which has always stimulated the imagination of people, clouds are important in numerous questions in science. This applies in particular to the earth's radiation budget , precipitation distribution and atmospheric chemistry. The cloud formation and climate change (clouds customer) is a rarely regarded as an independent subject area portion of meteorology; Luke Howard is considered to be its founder .

Physics and chemistry of clouds


A cloud consists of aerosol , a collection of finely dispersed particles in the gas mixture of the air (not water vapor , this is a gas and just as invisible as the rest of the air). Only after cooling below a certain temperature  - the dew point  - do tiny water droplets form from the water vapor, as do tiny floating ice crystals at great heights .

The diameter of the liquid droplets is typically in the range of two to ten micrometers , but can also be much larger , especially with rain clouds of up to two millimeters. Large droplets and the much larger hailstones can only arise if strong updrafts counteract gravity .

Education, development and dissolution

Dissolved anvil of a cumulonimbus
Cloud formation due to the flow over a mountain peak
Cumulus cloud

Cloud formation describes the process of the formation of clouds through condensation or also resublimation of water vapor on condensation nuclei in the troposphere and sometimes also in the stratosphere . When the temperature (density) and humidity of an air mass change, clouds form or dissolve. This can be done for example by

  • Elevation processes in the atmosphere when cold and warm fronts pass through , which transport air masses to higher layers and allow them to cool down there (e.g. in the jet stream ),
  • Thermal updrafts or slope updrafts,
  • Supply of colder air masses,
  • Supply of more humid air masses.

A visible cloud is created when the conditions for the formation of stable water droplets or crystals are met. It is less a matter of the air's ability to absorb water than the relationship between condensation and evaporation . On the surface of the water droplet within a cloud, there is a constant exchange of water molecules between the ambient air and the drop: A drop can only be if more water molecules accumulate on the drop than leave it at the same time, i.e. only if the condensation rate is higher than the evaporation rate grow and thus lead to cloud formation. Whether this can happen depends essentially on two factors:

  1. The number of water molecules in the vicinity of the drop: the more water vapor molecules surround the droplet, the more likely it is that one will stick to the droplet. The number of water vapor molecules can be expressed by the so-called water vapor partial pressure , which is the proportion of the total air pressure that is created by the water vapor.
  2. From the temperature of the water droplet : the warmer the droplet, the easier it is for water molecules to detach from the droplet.

The formation of a cloud is favored by low temperatures and a large number of water molecules or by high water vapor pressure, which is synonymous with high relative humidity .

The temperature at which condensation and evaporation equalize is called the dew point temperature . If this is not reached, stable droplets are created and grow under certain conditions. This temperature depends on the respective water vapor pressure. The height of this event in the atmosphere is called the cloud condensation zone . The water vapor pressure at which condensation and evaporation are in equilibrium is called the saturation vapor pressure . It depends on the temperature and is also determined by the effects of curvature and dissolution .

Droplets form in the earth's atmosphere only when there is a sufficient number of condensation nuclei . Such germs can be dust grains , for example , but also larger molecules, pollen or - by the sea - salt crystals (see aerosol ).

Above the oceans, the dimethyl sulfide (DMS) that is formed when algae decompose is often responsible for cloud formation.

Even at temperatures below 0 ° C, the majority of the cloud droplets can still be in the liquid state. When the temperature drops to around −12 ° C, ice crystals usually do not yet form, so that the cloud consists of so-called supercooled water droplets. Dissolved substances within the droplet can also lower the condensation temperature due to the lowering of the freezing point . As the temperature drops further, the proportion of ice continues to increase, until only ice crystals are present at around −40 ° C. At higher altitudes, cloud formation is therefore characterized by crystallization processes .

Droplets sink very slowly because of their small size - approximately 1 to 15  μm or 0.001 to 0.015 mm. Because their diameter is small, their Reynolds number is less than 0.1. There is therefore a laminar flow. According to Stokes' law, the rate of descent increases with the square of the diameter. A droplet with a diameter of 0.020 mm sinks about 1 cm per second. The rate of descent can reach values ​​of up to 15 cm / s. It is a purely aerodynamic value. The speed of fall must be distinguished from this. It results from the difference between the speed of the upwind or downwind and the rate of descent. Because the speed of the updrafts and downdrafts is much greater than the sinking speed of the droplets, the proportion of the sinking speed is usually insignificant. Since clouds are often formed by convective updrafts , they do not fall, but stay at the same height or rise (e.g. the cumulus). In rain clouds the drops are much larger (up to 3 mm) and therefore the speed of fall is also higher (with 1 mm drops approx. 1.8 m / s). The Stokes calculation no longer applies to this drop size. The droplets deform like an umbrella due to their air resistance. If a threshold value is exceeded, so that the updraft can no longer compensate for the sinking, it starts to rain . In the case of hail , there are very strong updrafts and downdrafts, which cause the hailstones to rise and fall several times, with their diameter growing layer by layer.

In meteorology, clouds are differentiated according to their shape and height above the ground. A cloud close to the ground is called a fog , but even if they differ only in their location, the fog is not considered a cloud type. In a broader sense, however, cloud formation is also understood to mean the formation of other types of cloud, such as dust clouds or methane clouds, although this is not limited to the earth and also includes cloud formation on other celestial bodies .

Significance for the radiation budget

Global distribution of the optical cloud thickness

Clouds have a major influence on the Earth's radiation budget and thus also on the air temperature , especially over the course of the day, but also on long-term climatic mean values. This is especially noticeable in summer. As soon as a cloud cover forms during the day and shields the solar radiation, i.e. the global radiation drops, the solar energy required to warm the air also decreases and it quickly becomes noticeably colder. However, this cloud cover also reflects the terrestrial radiation back to the ground to a certain extent. On a clear night it is consequently much colder than on an overcast night, as the terrestrial heat radiation escapes into space and can hardly be held back by the water vapor contained in the atmosphere.

These effects can be observed particularly in deserts, where clouds are usually rare: Much more heat is radiated at night than in more humid areas. The temperature differences between day and night are therefore much higher.

An important property of clouds is their optical thickness . It determines how much solar radiation can penetrate through a cloud cover and how much it absorbs or reflects on the other hand. The decisive influencing variables are the vertical extent of the cloud, the distribution of the droplet or ice crystal sizes and finally the amount and distribution of the clouds themselves. Clouds are somewhat more transparent to short-wave UV radiation than to the wavelengths of visible light. The scattering of direct solar radiation by the air particles causes their proportion to decrease with decreasing height and thus promotes this effect. Due to the additional scattering at the cloud droplets, the photon paths also increase, which favors absorption by ozone and reduces the transmission of light. With regard to UV radiation, absorption by the water droplets themselves is negligible, as long as they are not too heavily contaminated (e.g. by a volcanic eruption). On a global level, this has the long-term mean that clouds directly reflect back 20 percent of the short-wave solar radiation and at the same time absorb three percent.

However, as shown in the first paragraph, the effect of clouds in the radiation budget is not only linked to their properties, but is based on the interplay of many different factors. The effect of long-wave radiation from the earth's surface in connection with atmospheric counter-radiation is particularly important . This effect is the actual cause of the atmospheric greenhouse effect and plays an important role in relation to global warming . The radiation of the earth's surface is a consequence of the absorption of direct and diffuse solar radiation by the earth's surface and depends on its surface temperature. The optical thickness of the clouds, which in turn determines the global radiation , is now largely responsible for how much of this terrestrial radiation is absorbed in the atmosphere and reflected back onto the earth's surface, with multiple reflections between the underside of the cloud and the ground possible. This atmospheric counter-radiation increases the radiation directed towards the earth's surface and thus partially offsets the shielding effect of the clouds.

How big this compensation is in relation to large areas and long periods of time is difficult to determine, which is why it is also a central question of climate modeling .

Role in the water cycle

In the water cycle, clouds act as a mediator between evaporation and precipitation . Although the water they contain is quite insignificant in terms of quantity in relation to the water resources on earth, they convert the water quickly.


The appearance of a cloud is primarily determined by the type, size, number and spatial distribution of its components. It also depends on the intensity and color of the light hitting the cloud and on the position of the observer and the light source in relation to the cloud. The appearance of a cloud can best be described by information on size, shape, coarse and fine structure, brightness and color.

Shape and structure

Cloud in the shape of an elephant's trunk

Clouds can sometimes take on strange shapes that the human eye can associate with things from everyday life. Especially in stronger winds, which fray the clouds and let them form and deform again and again, you can "see" many things.


The brightness of a cloud is determined by the light reflected , scattered and transmitted by its particles . This light mostly represents direct or diffuse solar radiation , but it can also come from the moon or the earth's surface. Due to the large albedo of ice and snow surfaces in particular , the perceived brightness of the clouds can increase due to the reflected light.

The effect of haze or special light phenomena of the atmospheric optics , such as halos , rainbows , corons and glories , changes the cloud brightness. If there is haze between the observer and the cloud, the brightness of the cloud can be increased or decreased depending on the density of the cloud and the direction of the incident light. Haze also weakens the contrasts through which the shape as well as the coarse and fine structure of the cloud are only recognizable.

During the day, the brightness of the clouds is so strong that they can be observed without difficulty. On nights with moonlight, the clouds can be seen when the moon phase is more than a quarter. During the darker phases, the moonlight is not bright enough to reveal distant clouds. This is especially true when the clouds are thin. On moonless nights the clouds are generally not recognizable, but you can sometimes infer the presence of clouds due to the obscuration of the stars, the aurora , the zodiacal light or other effects.

In areas with sufficiently strong artificial lighting, clouds are also visible at night. Therefore, clouds can be seen over large cities as a result of the direct lighting coming from below. A cloud layer illuminated in this way can then form a light background, against which lower-lying cloud parts stand out plastically and darkly.


At sunset only the underside of the clouds is illuminated in a reddish color. The top is in their shadow.
Only the highest parts of the clouds are still illuminated by the setting sun.

The color of a cloud depends on the wavelength of the light that illuminates the cloud. The cloud itself can not change color because the droplet size in clouds is larger than the wavelength of the light (approx. 1 μm to 15 μm) and therefore the statements of Rayleigh scattering do not apply. This applies in particular to clouds up to a distance of about 20 km, because then there are too few air molecules to be able to cause color changes.

If there is mist or dust between the observer and the cloud, the color of the cloud can be changed slightly. Therefore, for example, very distant clouds can appear slightly yellow or orange.

  • When the sun is high enough, the clouds or parts of them appear white or gray in direct sunlight.
  • Those parts that receive the light preferentially from the blue sky have a blue-gray appearance.
  • When the sun approaches the horizon , i.e. at sunrise and sunset , its color can change from yellow to orange to red, because a large part of the high-frequency light components (blue) is scattered sideways due to the very long path of light through the earth's atmosphere ( see Rayleigh scattering ). Mostly light with long wavelengths remains and the color impression of the sun shifts strongly towards red. The sky around the sun and the clouds can only reproduce this color.

The cloud colors are also dependent on the height of the clouds and their position in relation to the observer and the sun. If the sun is just above or below the horizon, the high clouds can still look almost white, while the medium-high clouds show a strong orange or red color. Very low clouds lying in the arc of the earth's shadow look gray. These color differences allow an idea of ​​the respective cloud height. Clouds appear less red at the same altitude when looking towards the sun than in the opposite direction. At night the brightness of the clouds is usually too low to distinguish colors and all perceptible clouds then appear black to gray, with the exception of those that are illuminated by the moon and have a whitish appearance. Special lighting conditions, such as fires , city lights or northern lights , can sometimes give some clouds a more or less distinctive color at night.



Before the beginning of the 19th century, it was assumed that clouds were too diverse, complex and, above all, short-lived to be conceptually categorized. It was not customary to give them names; Rather, they were content to describe the clouds purely subjectively in terms of shape and color. There have been a few attempts to use them for weather forecasting, but mostly they have been limited to the degree of darkness. However, since the standardized differentiation between different cloud types is a prerequisite for their investigation, description and thus the understanding of the clouds, this could not be achieved by a merely roughly descriptive and also very inconsistent approach. A scientific approximation was hardly possible without such a basis. For this reason, clouds were only interpreted symbolically, if at all, or perceived as an aesthetic motif in art .

The change to today's cloud classification - and thus the scientific accessibility of clouds in general - goes back to Luke Howard and his 1802 publication On The Modification of Clouds . Jean-Baptiste de Lamarck took a different approach in the same year, independently of Howard and even a little earlier than him. Its publication in the third edition of the Annuaire Méteorologique , however, received no attention in the professional world of the time, if one can already speak of such.

Based on the taxonomy of living beings by Carl von Linné and in contrast to Lamarck, Howard used Latin names, which could be used worldwide according to the status of Latin as the language of science at the time. He divided clouds into the three basic forms stratus (layer clouds), cumulus (heap clouds) and cirrus (veil clouds). In addition, he differentiated the two intermediate forms cirrostratus and cirrocumulus as well as the two composite forms cumulustratus and cumulo-cirro-stratus and nimbus (rain clouds). The genus Cumulustratus was renamed Stratocumulus in 1840 with the consent of Howard by Ludwig Friedrich Kämtz, in 1855 Émilien Renou added the two genera Altocumulus and Altostratus .

International system

Main cloud types High clouds (cirro)   Clouds of great vertical extent
Medium high clouds (Alto)
Low clouds (no prefix)
Cloud family Polar regions Moderate latitudes Tropics
High clouds 3 to 8 km 5 to 13 km 6 to 18 km
Medium high clouds 2 to 4 km 2 to 7 km 2 to 8 km
Deep clouds 0 to 2 km 0 to 2 km 0 to 2 km
Vertical clouds 0 to 8 km 0 to 13 km 0 to 18 km

According to today's official classification of the World Meteorological Organization , recorded in the International Cloud Atlas , clouds are divided into four cloud families according to the height of their lower limit - high, medium-high, low and those that extend over several floors (vertical clouds). These four families comprise ten genera , which are represented in an overview with their 14 species types (with combinations of 27 species), 9 subspecies types and 9 special forms / accompanying clouds . A cloud can have the characteristics of one species and several subspecies.

It is of central importance that the clouds are classified according to their appearance. This is in contrast to the (genetic) classification systems in the natural sciences, which are usually based on origin, formation or relationship. How a cloud got a certain appearance does not play a role in its naming, even if many appearances can be interpreted in terms of the circumstances in which they were created.

The elevation of the cloud storeys vary with the geographical latitude , as the lowest layer of the atmosphere - the troposphere  - reaches around twice as high at the equator as at the poles . In winter the cloud levels are lower than in summer due to the lower temperature and thus higher air density. The heights are based on the location of the tropopause , which is variable in terms of location and time and does not rise uniformly from the poles to the equator. The following height information is therefore only a guide.

Clouds are named differently, for example the cirrus and the cirrus cloud or the cirrus and the cirrus cloud.

Often several cloud shapes are present at the same time, which can overlap each other.


The following presentation is based heavily on the International Cloud Atlas (p. 6). The letters of the respective abbreviations are clearly highlighted and are combined in the designation, for example Ci fib for Cirrus fibratus. German equivalents or descriptions of the Latin generic names are set in brackets. It should be noted that the classification of the cumulus cloud genus into the cloud families is not handled uniformly. This is due to the fact that the cloud types Cumulus humilis and Cumulus mediocris can be assigned to the deep clouds, while Cumulus congestus belongs to the vertical clouds. A similar picture emerges with Nimbostratus. These are classified here under the vertical clouds, but can also be counted among the medium-high clouds.

Genera species Subspecies Special forms, accompanying clouds Mother clouds (genitus) example
Ci rrus (Ci)
(spring cloud)
mostly not convective
fib ratus
unc inus
spi ssatus
cas tellanus
flo ccus
in tortus
ra diatus
ve rtebratus
du plicatus
mam ma Cirrocumulus
C irro c umulus (Cc)
(small fleecy cloud)
limited convective
str atiformis
len ticularis
cas tellanus
flo ccus
un dulatus
la cunosus
vir ga
mam ma
C irro s tratus (Cs)
(high cloud of veil)
not convective
fib ratus
neb ulosus
you plicatus
un dulatus
Cirrostratus stratiformis
A lto c umulus (Ac)
(large fleecy cloud)
limited convective
str atiformis
len ticularis
cas tellanus
flo ccus
pe rlucidus
tr anslucidus
op acus
du plicatus
un dulatus
ra diatus
la cunosus
vir ga
mam ma
A lto s tratus (As)
(medium-high layer cloud)
not convective
  tr anslucidus
op acus
du plicatus
un dulatus
ra diatus
vir ga
pra ecipitatio
pan nus
mam ma
S trato c umulus (Sc)
(pile layer cloud)
limited convectively
str atiformis
len ticularis
cas tellanus
pe rlucidus
tr anslucidus
op acus
du plicatus
un dulatus
ra diatus
la cunosus
mam ma
vir ga
pra ecipitatio
St ratus (St)
(deep layer clouds)
not convective
neb ulosus
fra ctus
op acus
tr anslucidus
un dulatus
pra ecipitatio Nimbostratus
Cu mulus (Cu)
(heap clouds)
freely convective
hum ilis
med iocris
con gestus
fra ctus
ra diatus pil eus
vel um
vir ga
pra ecipitatio
arc us
pan nus
tub a
N imbo s tratus (Ns)
(rain clouds)
not convective
    pra ecipitatio
vir ga
pan nus
C umulonim b us (Cb)
strongly convective
cal vus
cap illatus
  pra ecipitatio
vir ga
pan nus
inc us
mam ma
pil eus
vel um
arc us
tub a


The genera are the ten main groups of clouds. They indicate the height at which the clouds are and whether they are unstable or stably stratified.

With a stable atmospheric stratification, the (stratified) clouds are usually without contours if the humidity is high enough, otherwise they are torn or not present at all. An unstable stratification, in which there is updraft , leads to cumulus clouds like the cumulus or the cumulonimbus. The generic names are abbreviated with two letters, with the first letter capitalized.


When specifying the species, cloud genera are further subdivided according to their internal structure and shape. Types cannot be combined; a cloud can only have the characteristics of one species at a time (Cumulus congestus humilis would not be possible). Unlike in biology, the term species is not used for the entire name of the cloud (Cumulus congestus), but only for the species name (congestus).

Most species can be observed in several cloud genera, such as the species stratiformis , which occurs in Cirro-, Alto- and Stratocumulus. Others such as congestus or humilis , for example, only apply to cumulus clouds.

Types are abbreviated with three small letters: str, con etc.


The subspecies are used to describe the arrangement and the transparency of clouds and are abbreviated with two letters. In contrast to species, a cloud can have the characteristics of several subspecies, because the subspecies are generally not mutually exclusive. The only exceptions are opacus (opaque layer of cloud) and translucidus (fairly transparent layer of cloud).

Most subspecies can also occur in several genera, an example of this is the subspecies opacus , with which Altocumulus, Altostratus, Stratocumulus and Stratus can be described in more detail.

Examples of special arrangements of the clouds are the wave-like Altocumulus undulatus or the Cirrus vertebratus, which is reminiscent of a fish skeleton .

Special forms and accompanying clouds

Special forms and accompanying clouds do not necessarily have to be related to the main mass of the cloud, in particular the accompanying clouds are usually separate from it. For example, Cumulonimbus mamma (Cb mam) is a cumulonimbus with swellings “downwards” and Cumulus pannus (Cu pan) is a cumulus cloud with tattered parts of the cloud. The special forms and accompanying clouds are - like the species - abbreviated with three letters.

Mother clouds

The mother cloud is used to indicate from which genus a new cloud shape has formed. For this purpose, “genitus” is appended to the genus name of the mother cloud. They are abbreviated by adding “gen” to the generic abbreviation. When written out, replace the ending "-us" with an "o" and add a "genitus". A typical example is the cirrus cumulonimbogenitus (Ci cbgen), a cirrus that developed from the anvil of a Cb cloud.

Genetic classification

In addition to the international classification, which is based on the cloud height, there is also a genetic classification, which is based on the formation of the clouds. It goes back to Stüve, who published it in 1926.

Separate cloud shapes

In addition to the clouds included in the classification, there are a number of other types that have been given their own name for specific reasons. These are, for example, the wall clouds , which are very important for the development of a tornado, and the artificial contrails of aircraft (cirrus homogenitus). This includes the banner cloud , a not fully understood phenomenon that occurs on peaks and ridges.

Weather observation

As shown, clouds have a high level of momentum and react very quickly to the conditions in their environment. It is possible to establish a link between the observable properties of the clouds and the properties that cause them.

The spread of clouds with height is an important factor for assessing convective processes in the atmosphere. In many cases it is possible to use them to determine the stratification stability of the earth's atmosphere . Movements of the clouds provide information about the wind conditions at the corresponding altitude.

Front passage

Cloud formation in a warm front
Cloud formation with a cold front

With a few exceptions, clouds always appear on fronts . When a front passes through, one can therefore usually observe a very characteristic sequence of cloud types.

A slowly approaching warm front , on which the warm air slides over a large area onto the cold air lying in front of it, is initially noticeable with cirrus or cirrostratus. Altostratus follows later. Eventually Nimbostratus reached the observer with continued rain. After the warm front has passed through, the clouds in the warm sector loosen up, the weather improves and it becomes noticeably warmer. Sometimes, especially in winter or on the coasts, the warm sector can also be filled with low-hanging stratus, from which light rain or drizzle falls.

The cold front usually pulls faster than the warm front because the heavier cold air pushes itself under the warm air and displaces it. As an observer, one initially notices an increased formation of cumulus. Even in the warm sector, these can intensify to form large individual cumulonimbus clouds that bring showers or thunderstorms. The cold front itself often consists of a long chain of often very intense cumulonimbus clouds. But there are also less pronounced cold fronts, on which Stratocumulus or Cumulus then predominate. After the front has passed through, the sky tears open quickly, because the post-frontal warm-up zone ensures a temporary dissolution of the clouds. Then the deep cold air comes in, in which numerous cumulus clouds or cumulonimbus clouds with repeated showers and individual thunderstorms predominate.

Thunderstorms and storms

A roll cloud in Uruguay
A shelf cloud in Greece

Thunderstorms and storms can often be observed together with the characteristic cumulonimbus clouds, and they usually occur quickly and quickly disappear. Unless they appear in connection with fronts, the sky clears up very quickly.

In some cases, the clouds are completely isolated, meaning they form a single block in the otherwise clear sky. Thunderstorms are therefore particularly dangerous in the mountains. They can show up, rain down and move on locally within an hour.

Extremely large cumulonimbus clouds, so-called supercells , can hardly be distinguished with the eye from nimbostratus or a front due to their extent. They can bring cyclones with them and determine the weather for much longer than normal thunderstorms. The occurrence of gust fronts with roll or shelf clouds is also possible.

Cloud encryption

The codes C L , C M and C H are used to indicate the state of the sky. The advantage over the simple - and more precise - designation of clouds is that not every type of cloud has to be listed, but the total cloud cover for each floor can be specified with a number. The weather situation can also be determined from it.

The encryption takes place in the form:

C W = x

It means:

C. "Cloud" cloud
low clouds
medium clouds
high clouds
x Digit from zero to nine

If the state of the sky is not visible due to poor lighting conditions, fog, dust, sand or the like, this is marked with a slash instead of a number. For W one enters the respective cloud height. If the clouds cannot be clearly assigned to a number, the one that applies best is chosen, that is, the group that covers the largest part of the sky. In addition, there is a so-called priority rule that must be applied in cases when the image of the sky is not clear. Priority is always given to the clouds that are most important for aviation and / or synoptics (see for example the main cloud base ).

Encryption of the C L clouds

The cloud genera St ratus, S trato c umulus, Cu mulus and C umulonim b us belong to the deep clouds .

Encryption symbol description example
C L = 0 No deep (or C L -) clouds present. Trees-sky.jpg
C L = 1 Clouds CL 1.svg Cumulus humilis and / or Cumulus fractus present. No bad weather clouds.

The clouds encompassed by the code C L = 1 include cumuli that are in the developmental stage or in a final stage of disintegration, so that they still have small vertical dimensions. The fully developed cumuli are those without a cauliflower shape and with a small vertical extension ( Cumulus humilis ) or those disheveled by the wind (Cumulus fractus ).

Cumulus Cross.jpg
C L = 2 Clouds CL 2.svg Cumulus mediocris or Cumulus congestus , possibly with Cumulus fractus , Cumulus humilis or Stratocumulus. Lower limits in the same amount.

This code includes cumuli with a large vertical extension that are cauliflower-like in shape. Some of them can also display turret-like awards. They arise in strong winds with an irregular underside and can be tattered, or on days with a tendency to thunderstorms and thus strong convection . Then the underside is sharply defined. With larger cumulus clouds, a little rain can occasionally fall. In addition to the clouds mentioned above, C L = 1 clouds or Sc can also occur.

Cu congestus1.jpg
C L = 3 Clouds CL 3.svg Cumulonimbus calvus , possibly also Cumulus, Stratocumulus, Stratus

This includes the cumulonimbus calvus , i.e. a cumulonimbus without an anvil and without clearly fibrous or streaky-looking parts. Clouds of C L = 1 and C L = 2 and also St can also occur. For a more detailed description of the species calvus see here .

Large Stratocumulus.JPG
C L = 4 Clouds CL 4.svg Stratocumulus cumulogenitus are Stratocumulus clouds that have arisen from cumulus clouds. This happens when the incoming air reaches a thermally stable layer. It is now slowed down and spreads, a coherent stratocumulus layer forms. Occasionally the rising air can be so strong that the stable layer is broken and individual cumuli stand out between the Sc clouds. Clouds CL4.jpg
C L = 5 Clouds CL 5.svg Stratocumulus, which however has no mother cloud (that is, it did not arise from cumuli). It almost always has dark spots on the underside. In stronger winds it can look partially torn.
C L = 6 Clouds CL 6.svg Stratus nebulosus and / or Stratus fractus. No bad weather clouds.

This code includes the gray, regular-looking stratus ( nebulosus ) and stratus in the transition stage, i.e. either forming or dissolving stratus (stratus fractus ).

Clouds CL6.jpg
C L = 7 Clouds CL 7.svg Stratus fractus or Cumulus fractus and / or Cumulus pannus , usually below Altocumulus, Nimbostratus or Cumulonimbus. Bad weather clouds.

These are shredded cloud parts that, in contrast to the C L = 6 clouds, always occur under another cloud. They appear in a darker gray than the clouds above and can change their shape quickly. Most of the time, precipitation falls from the clouds above.

Clouds CL7.jpg
C L = 8 Clouds CL 8.svg Cumulus and Stratocumulus (not cugen) with bases at different heights.

Stratocumulus clouds (not formed from cumulus) that are pierced by the cumulus clouds below or with cumuli that are above the stratocumulus layer. The cumulus clouds do not spread to Stratocumulus, i.e. that is, no C L = 4 clouds arise .

Clouds CL8.jpg
C L = 9 Clouds CL 9.svg Cumulonimbus capillatus, possibly with Cumulonimbus calvus, Cumulus, Stratocumulus or Stratus.

At least one Cumulonimbus capillatus is visible, i.e. a Cumulonimbus with an anvil. If there is a cumulonimbus directly over the observation site and therefore it is not possible to clearly differentiate between C L = 3 and C L = 9, or if the anvil is covered by other clouds, the cloud cover is described as C L = 9 in case of doubt . Incidentally, thunderstorms are always an indication of the Cumulonimbus capillatus. In addition, clouds from C L = 3 can still be visible; the C L = 9 clouds also arise from the clouds of C L = 3.

Big Cumulonimbus.JPG

Encryption of the C M clouds

The cloud genera Altocumulus , Altostratus and Nimbostratus belong to the middle clouds .

Encryption symbol description example
C M = 1 CM 1.svg Altostratus translucidus.

Translucent altostratus through which the position of the sun or moon is visible. It usually arises on a warm front, when the cirrostratus becomes thicker.

Altostratus translucidus (2005) .jpg
C M = 2 Clouds CM 2.svg Altostratus opacus or Nimbostratus.

This code includes the very dense Altostratus (As opacus), which largely covers the sun or moon, and the Nimbostratus. The latter covers the sun everywhere and has a denser, darker and rather wet appearance. In addition, it is rather lower than the Altostratus.

Clouds CM2.jpg
C M = 3 Clouds CM 3.svg Altocumulus translucidus at the same level.

Altocumulus blanket or field that does not move across the sky. The sun, if it is covered, is visible as a bright, diffuse spot, the clouds are mostly translucent. They change very little themselves.

C M = 4 Clouds CM 4.svg Altocumulus ( lenticularis ) translucidus at different levels.

In the sky, for the most part, translucent banks of altocumuli are visible (Altocumulus translucidus), which are often lens or almond-shaped (lenticularis). They can be at different heights. The reason they show through is that they often dissolve and reform again.

Usually such clouds form in hilly or mountainous areas, see also the article on lenticularis .

C M = 5 Clouds CM 5.svg Altocumulus ( stratiformis ) perlucidus / translucidus radiatus ( undulatus ) or opacus

These include altocumulus clouds that come in from one direction - in German-speaking countries mostly from the west - and cover an ever larger part of the sky. In the direction from which they come, the sky is covered up to the horizon, where the layer of clouds is thickest.

At the front the clouds often dissolve a little, then wave-shaped clouds (undulatus) can appear, possibly with gaps in between (perlucidus) and arranged in parallel bands (radiatus).

The rear part can consist of several layers one on top of the other, but they are quite coherent. If the clouds touch the other side of the horizon, they no longer belong to the code C M = 5.

Partially illuminated Ac with shadows.JPG
C M = 6 Clouds CM 6.svg Altocumulus cumulogenitus or Altocumulus cumulonimbogenitus

This code is comparable to the C L  = 4. The altocumulus is created either by cumuli, the tops of which reach a thermally stable layer and spread to the side, or occurs in the cumulonimbus .

Cumulonimbus with giant anvil.JPG
C M = 7 Clouds CM 7.svg Altocumulus ( duplicatus ) opacus / translucidus , possibly with Altostratus or Nimbostratus

In contrast to the cloudiness of C M  = 5, these clouds do not move large across the sky. There can be a single altocumulus layer or several layers on top of each other (duplicatus), and the individual clouds change only slightly.

The cloud layer (s) are either translucent or mostly dark. At the same time, Altostratus or Nimbostratus clouds can occur.

Altocumulus undulatus duplicatus.jpg
C M = 8 Clouds CM 8.svg Altocumulus castellanus or Altocumulus floccus

Swelling altocumulus clouds. This is very clearly visible in the case of the species castellanus; Usually several turrets form from a cloud, which can often be observed in a row. The Altocumulus floccus looks similar to Cumulus fractus, but the individual clouds are smaller and rounded and slightly swollen at the top. Virga formation can also occur (falling strips).

C M = 9 Clouds CM 9.svg Chaotic sky with altocumulus at different heights

There is not much more to say about it. This code is used when all other codes do not apply the same or - often a lot of different genres clouds occur simultaneously here, even from the code C L and C H .

Encryption of the C H clouds

The genera Cirrus , Cirrostratus and Cirrocumulus belong to the high clouds .

All three cloud types: Ci, Cc, Cs
Encryption symbol description example
C H = 1 Clouds H1.svg Especially cirrus fibratus and possibly cirrus uncinus

If the greater part of the high clouds is cirrus fibratus or cirrus uncinus and the clouds do not condense or cover the sky, code 1 applies. This is usually a calm weather situation, also because there are not many clouds in between and obscure your view - otherwise it would be C H  = /.

C H = 2 Clouds H2.svg Cirrus spissatus , castellanus or floccus , not cumulonimbogenitus

This code also only includes cirrus clouds, but with a somewhat more turbulent atmosphere. This also includes the Cirrus castellanus, which can get small turrets through updrafts. The layers can become quite dense in places (spissatus), so that they can be similar to a cirrus layer formed from a cumulonimbus cloud (cbgen), but have formed differently. The clouds can coexist with those of C H  = 1, but exist in greater numbers.

C H = 3 Clouds H3.svg Cirrus spissatus cumulonimbogenitus. If at least one dense cirrus cloud (spissatus) has arisen from a cumulonimbus (cbgen), this code is used. There can be other clouds at the same time.

Since they are the remains of an anvil, they are often so dense that they can hide the sun completely and have frayed edges, as can be seen on the anvil. In the earlier stage of the dissolution one can still see the shape.

Clouds CH3.jpg
C H = 4 Clouds H4.svg Denser cirrus uncinus and / or fibratus .

This cloud cover gradually covers the whole sky as it thickens. The horizon is covered all the way down in the direction from which the thread-like or hook-shaped clusters of clouds come.

C H = 5 Clouds CH 5.svg Cirrostratus and possibly cirrus ( radiatus ) below 45 ° and becoming denser.

Cirrostratus is now added to the clouds C H = 4. The sky is not yet covered over 45 ° above the horizon, but it will soon be, because the layer of clouds is thickening and slowly covering the sky. The cirrus can occur in two parallel bands (radiatus), which seem to intersect at one point because of the perspective effect.

Clouds CH5-1.jpg
C H = 6 Clouds CH 6.svg Cirrostratus and possibly cirrus ( radiatus ) over 45 ° and becoming denser

This key digit follows the code C H = 5. The only thing that has changed compared to the above is the coverage: The sky is not yet completely covered, but the cloud layer has already exceeded the 45 ° limit above the horizon.

Clouds CH6.jpg
C H = 7 Clouds CH 7.svg Cirrostratus, covering the whole sky, possibly with cirrus and cirrocumulus

This code applies when the whole sky is covered by cirrostratus. It can be so thin that only the halo gives it away. Cirrus or cirrocumulus can occur at the same time.

Clouds CH7.jpg
C H = 8 Clouds CH 8.svg Cirrostratus, not covering the whole sky, possibly with cirrus and cirrocumulus

In contrast to C H  = 7, the cirrostratus does not or no longer covers the whole sky and does not cover it progressively. Here, too, cirrus and cirrocumulus can occur.

Close Cirrostratus.jpg
C H = 9 Clouds CH 9.svg Cirrocumulus ( undulatus ), possibly with cirrus and cirrostratus.

Most of the high clouds are cirrocumulus, which are often arranged in waves (undulatus).

Clouds CH9.jpg

Degree of coverage

The degree of cloud cover is often given in meteorology in eighths of the sky, the octa from 0 to 8.

Clouds and "farmer rules"


The easily observable move of the clouds is the basis of many peasant rules and has established their reputation as a weather messenger . A sufficient forecast quality of these farmer rules, which are based on observations passed on over decades, is only given regionally or even only locally. For example, a weather rule from the Vinschgau in South Tyrol reads :

If the clouds come from Schnals,
we have weather on our necks;
Pull into the Martell and
it will be light again; if
they come from mud,
it makes splashing;
if they come from Ulten,
you have to be patient! "

When Frau Hitt , a striking rock formation in the northern Alpine chain near Innsbruck, is surrounded by a cloud, this indicates that rain is about to occur:

" If Ms. Hitt a Koppen wears the Stadler by varnishing. "

Cirrus usually announce a warm front and thus a deterioration in the weather. However, one cannot be sure that it will actually reach the respective location. This is where the saying comes from: "In women and cirrus you can be wrong."

In Mittenwald , the Wetterstein (hence its name) is the mountain that predicts the weather:

If there is weather stoa on Sabi,
it will be Wetta misarabi.
If there is a
bad weather on Huat, it 'll be morning again good.

If the Wetterstein has a saber (elongated cloud below the summit), the weather will be miserable. If the Wetterstein has a hat (round cloud over the summit), the weather will be fine tomorrow. This saying can be found in many places in the Alpine region (e.g. on the Attersee).

Anomalies and extraterrestrial clouds

an unusual hole in a cloud layer (Hole Punch Cloud)

Anomalies are very unusual clouds that contradict the classical model in particular. These include, for example, polar stratospheric clouds , glowing night clouds and the Hole-Punch Cloud .

There are also clouds in the atmospheres of other celestial bodies, for example on the planet Venus and the Saturn moon Titan. These clouds can be of different densities and have different compositions.

Cultural history

Cloud Study (1822) by John Constable

The word "cloud" ( ahd . Wolkan , mhd . Clouds ) comes from common West Germanic * wulkana- from that possibly due to the Indo-European root * welg back 'wet'. Originally it is a neuter, only since Late Middle High German has "the cloud" been female.

Clouds were and are a popular motif in landscape painting and nature photography . To be mentioned here are Jacob Izaaksoon van Ruisdael , Jan van Goyen and Esaias van der Velde from the Dutch landscape painting as well as Ary Pleysier , William Turner , Caspar David Friedrich , Carl Blechen and above all John Constable from the Romantic period , Emil Nolde in the 20th century and Gerhard Richter's gray cloud images .

In China , clouds are a symbol of happiness and peace as well as the West. Under clouds and rain games refers to the sexual union.

The world of computer graphics has been simulating clouds with the help of 3D software since the 1990s. Since around 2000, the algorithms have been so sophisticated that the artificial clouds in films can no longer be distinguished from real ones. The software also takes into account the inner dynamics of real clouds and uses calculation methods from fluid mechanics .

The diamond pattern of the flag of Bavaria is often interpreted as a blue sky peppered with white clouds. In fact, the white and blue diamonds , also known as Wecken , originally come from the coat of arms of the Counts of Bogen , they were taken over in 1242 by the Wittelsbachers , the ruling family of Bavaria from the 12th to the 20th century. In the Bavarian hymn it says: "[...] and get the colors of His sky white and blue ".


In Old Testament history, the Hebrews interpret a pillar of cloud as a revelation of their God as a guide for their people.

And the Lord went before them, by day in a pillar of cloud, to guide them in the right way, and by night in a pillar of fire, to shine for them, that they might walk day and night. "( Ex 13,21  LUT )


James Pollard Espy (1785-1860) succeeded for the first time in describing the thermodynamics of cloud formation largely correctly by taking into account the role of latent heat in condensation .

Over 200 years ago, the German-English astronomer Sir Wilhelm Herschel established a connection between the wheat harvest in England and solar activity.

In Germany is open to researchers, the cloud simulation chamber AIDA for Cloud simulation experiments at the Karlsruhe Institute of Technology in Karlsruhe available.

"Cloud harvest" for water production

In the Chilean city ​​of Chungungo (La Higuera, Región de Coquimbo ), a project was funded that aims to harvest clouds from the Andes. Plastic nets were set up there to catch the fine water droplets from the clouds. These then run down the network and finally flow over seven kilometers of pipelines to Chungungo. Up to 110,000 liters can be tapped every day.


Web links

Wiktionary: cloud  - explanations of meanings, word origins, synonyms, translations
Commons : Clouds  - album with pictures, videos and audio files

Individual evidence

  1. ^ The dictionary of origin (=  Der Duden in twelve volumes . Volume 7 ). 5th edition. Dudenverlag, Berlin 2014 ( p. 932 ). See also DWDS ( "cloud" ) and Friedrich Kluge : Etymological dictionary of the German language . 7th edition. Trübner, Strasbourg 1910 ( p. 499 ).
  2. WMO: Homogenitus | International Cloud Atlas. (No longer available online.) Archived from the original on March 27, 2017 ; accessed on April 4, 2017 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot /
  3. James Pollard Espy . In: Encyclopaedia Britannica . Retrieved November 21, 2019.
  4. ^ JE McDonald: James Espy and the Beginnings of Cloud Thermodynamics . In: Bulletin of the American Meteorological Society , October 1963, doi : 10.1175 / 1520-0477-44.10.634 .
  5. Anja Roth: Investigations of aerosol particles and cloud residual particles using single particle mass spectrometry and optical methods - PDF file, accessed on July 12, 2019
  6. Harvest from the clouds . In: Der Spiegel . No. 2 , 1993 ( online ).
This version was added to the list of articles worth reading on January 13, 2006 .