Cloud Atlas

Interactive overview of the 10 WMO main cloud genera

The World Meteorological Organization (WMO) classifies clouds into 10 main genera. These are based on the height of their base and their shape. Each cloud type has its own meteorological significance and helps us understand and forecast the weather.

Browse all Identify a cloud yourself Identify clouds with AI

High clouds (5-13 km)

CirrocumulusCc

Cirrocumulus (lat. cirrus = curl of hair, cumulus = heap) is a high cloud genus consisting of many small, white cloud globules or platelets arranged regularly in groups or rows. These thin cloud fields are found at great altitude and consist predominantly of ice crystals, though they may also contain supercooled water droplets. Their characteristic arrangement creates the so-called "mackerel sky" pattern. The individual elements have an apparent width of less than one degree (approximately the width of a little finger held at arm's length). Cirrocumulus is among the rarer cloud genera and is often only briefly visible before transforming into cirrus or cirrostratus.

Height: 5.0 - 13.0 km

+4 subtypes

High clouds (5-13 km)

CirrostratusCs

Cirrostratus is a high cloud layer composed of ice crystals that partially or completely covers the sky with a thin, veil-like coating. The name combines "cirrus" (Latin: curl of hair) and "stratus" (Latin: spread out), describing an extensive, translucent ice cloud at high altitude. Cirrostratus is often so thin that the sun and moon shine through it, producing characteristic halo phenomena — ring-shaped refraction effects with a radius of 22° or 46°. The cloud can extend across large portions of the sky, giving it a milky-white, diffuse appearance. It typically forms through the large-scale gliding of warm, moist air over a cold air mass and is a classic harbinger of approaching frontal systems.

Height: 5.0 - 13.0 km

+2 subtypes

High clouds (5-13 km)

CirrusCi

Cirrus (lat. curl of hair, tuft of feathers) is a high cloud genus consisting of fine, white threads, streaks, or narrow bands. These delicate clouds are composed exclusively of ice crystals and often appear in the sky like faint brushstrokes. Cirrus clouds typically form at altitudes between 6,000 and 12,000 metres, where temperatures are well below freezing. Due to their thin, transparent structure, they allow sunlight to pass through largely unimpeded and cast no appreciable shadow on the Earth's surface. They are among the most frequently observed clouds and occur worldwide in all climate zones.

Height: 6.0 - 12.0 km

+5 subtypes

Mid-level clouds (2-7 km)

AltocumulusAc

Altocumulus (lat. altus = high, cumulus = heap) is a mid-level heap cloud that occurs in the middle troposphere. It typically appears as a layer or field of white or grey, rounded cloud masses, often arranged in regular patterns. The individual elements have an apparent width of approximately one to five degrees when observed at an angle of more than 30 degrees above the horizon. Altocumulus can occur in various species and varieties, including lenticularis (lens-shaped), castellanus (turret-shaped), or floccus (tufted). It is one of the most common cloud types in the mid-latitudes and often gives the sky an impressive, structured appearance.

Height: 2.0 - 6.0 km

+5 subtypes

Mid-level clouds (2-7 km)

AltostratusAs

Altostratus (lat. altus = high, stratus = spread out) is a gray or bluish-gray layer cloud that covers the sky extensively and often uniformly. It is thin enough to let the sun shine through like frosted glass, without any halo phenomena occurring. Altostratus consists of a mixture of water droplets and ice crystals and frequently extends over hundreds of kilometers. The cloud typically forms through large-scale, slow gliding of warm, moist air over a cold air mass, as occurs at warm fronts. As it thickens, altostratus can completely obscure the sun and transition into nimbostratus.

Height: 2.0 - 6.0 km

Mid-level clouds (2-7 km)

NimbostratusNs

The name Nimbostratus is derived from "nimbus" (Latin: rain cloud) and "stratus" (Latin: spread out, to cover with a layer) and describes a dense, grey cloud layer that completely covers the sky and brings persistent precipitation. Nimbostratus is one of the most distinctive foul-weather clouds and often extends over several kilometres in altitude, with the cloud base typically located in the middle level, while the cloud itself can reach both upward into the high level and downward into the low level. The cloud layer is so thick and opaque that the sun is completely obscured, producing a diffuse, dull light. Below the main cloud deck, ragged cloud fragments known as pannus clouds (Stratus fractus) frequently form, moving rapidly at low altitudes. Nimbostratus often develops through the overrunning of warm air over colder air masses along warm fronts or occluded fronts.

Height: 0.6 - 2.0 km

Low clouds (0-2 km)

StratocumulusSc

Stratocumulus (lat. stratus = spread out, cumulus = heap) is one of the most common cloud forms worldwide, covering approximately 20 percent of the Earth's surface at any given time. It is a low-level layer cloud composed of rounded, roll-shaped, or plate-like elements arranged in groups, rows, or waves. The individual cloud elements are gray to whitish and often show darker patches. Stratocumulus frequently forms through the spreading of cumulus clouds beneath an inversion layer, or through wave-like lifting processes in moist air masses. This cloud genus preferentially occurs in stable, slightly moist air masses and is especially widespread in maritime climate zones and during the colder season over continental areas.

Height: 0.6 - 2.0 km

+5 subtypes

Low clouds (0-2 km)

StratusSt

The name Stratus derives from the Latin word "stratus" (spread out, stretched) and aptly describes the appearance of this cloud genus. Stratus forms a uniform, grey cloud layer that often completely covers the sky, giving the landscape a dull, diffuse appearance. These clouds frequently form through large-scale, slow ascent of moist air or through the cooling of air layers near the ground, for example by nocturnal radiation or the flow of air over cold surfaces. The cloud base is often very low and can touch the ground as fog in hilly or mountainous terrain. Stratus is one of the most common cloud forms in the temperate latitudes, particularly during the cooler seasons and in coastal regions.

+2 subtypes

Clouds with large vertical extent

CumulonimbusCb

The name Cumulonimbus is derived from the Latin words "cumulus" (heap) and "nimbus" (rain cloud), describing a powerful, vertically extensive heap cloud with intense precipitation activity. The Cumulonimbus is the most imposing of all cloud genera and can extend from the level of low clouds up to the tropopause, reaching vertical extents of over 12 kilometres. It forms through intense convective lifting in an unstable atmosphere and is associated with severe weather phenomena such as heavy rain, hail, storm gusts, and thunderstorms. Characteristic is its anvil-shaped top (Incus), which develops when the rising air spreads out horizontally at the tropopause. The lower part of the cloud often appears dark and threatening, while the upper portion consists of ice crystals and can take on a smooth, fibrous, or streaked appearance.

Height: 0.3 - 2.0 km

+2 subtypes

Clouds with large vertical extent

CumulusCu

The name Cumulus (Latin for heap) describes the characteristic shape of this cloud, which appears as piled-up heaps or tufts in the sky. Cumulus clouds are free-standing, dense clouds with sharp outlines that develop vertically in the form of hills, domes, or towers. Their sun-lit upper surface is usually brilliantly white and cauliflower-shaped, while the base appears relatively flat and darker. They form through convective updrafts when near-surface air warms and rises until water vapor condenses at altitude. Cumulus clouds occur in various types, ranging from the shallow Cumulus humilis (fair-weather cloud) to the towering Cumulus congestus, which can develop further into thunderstorm clouds.

Height: 0.6 - 2.0 km

+4 subtypes

Loading cloud types...

Did you know?

• The WMO cloud classification was introduced in 1896 and is based on the work of Luke Howard (1802)
• In addition to the 10 main genera, there are 15 species and 9 varieties
• Clouds consist of either water droplets, ice crystals, or a mixture of both
• Cumulonimbus clouds can grow up to 18 km high and contain up to 500,000 tons of water

Species and varieties by genus

Not every species or variety occurs with every genus. The table shows the assignment according to the WMO International Cloud Atlas (2017). Click a dot to go to the detail page.

Cloud species and the genera with which they most commonly occur

Genus →Species ↓	Ci	Cc	Cs	Ac	As	Ns	Sc	St	Cu	Cb
fibratus (fib)
uncinus (unc)
spissatus (spi)
castellanus (cas)
floccus (flo)
stratiformis (str)
nebulosus (neb)
lenticularis (len)
volutus (vol)
fractus (fra)
humilis (hum)
mediocris (med)
congestus (con)
calvus (cal)
capillatus (cap)

Source: WMO International Cloud Atlas (2017)

Want to learn to read clouds?

At MeteoAcademy, you'll systematically learn all cloud genera and what they reveal about upcoming weather.

Explore MeteoAcademy