Everything You Need to Know About Clouds

Quick: Name these clouds!. detchana wangkheeree/Shutterstock

We stare at clouds all the time, whether trying to figure out what they look like or if they're bringing rain. Yet most of us know very little about clouds, let alone how to identify them.

The World Meteorological Organization (WMO) keeps a cloud atlas that divides clouds into genera, species and varieties. Some clouds have multiple "varieties" and some have "accessory" clouds that appear with or merge with bigger clouds. Specific conditions can even create special clouds of their own.

In short, clouds are a rich tapestry in the sky that changes every day.

Cloud Genera

These are the 10 most typical forms clouds take. The WMO notes that the definitions don't encompass all possible cloud permutations, but they do outline the essential traits to differentiate one cloud genus from another, especially those having similar appearances.

Cirrus clouds over Australia
Cirrus clouds have a silky, hair-like appearance. Fir0002/Flagstaffoto/Wikimedia Commons

1. Cirrus. Cirrus clouds are wispy and hair-like, and when viewed from below, they appear to have little to no structure. Inside, cirrus clouds are comprised almost entirely of ice crystals.

Cirrocumulus cloud spread out
Cirrocumulus clouds can sometimes appear a little patchworked. Indrajit Das/Wikimedia Commons

2. Cirrocumulus. Cirrocumulus clouds are similar to a well-worn basic sheet: thin and white. These clouds also have super-cooled water droplets within them. Technically, each individual cloud is referred to as cirrocumulus, but the term can also be used to refer to the entire sheet. If the term is used that way, each individual cloud is a cloudlet.

Cirrostratus have a way of making themselves known.
Cirrostratus have a way of making themselves known. The High Fin Sperm Whale/Wikimedia Commons

3. Cirrostratus. Cirrostratus clouds are a white-ish veil that totally or partially covers the sky. They often produce the halo effect you see above.

Altocumulus clouds floating in the sky
Altocumulus clouds occur in a few different types, not just these balls of fluff. Fir0002/Wikimedia Commons

4. Altocumulus. Altocumulus clouds come in several forms, though they mostly look like rounded masses. They can can appear as a sheet or a layer, like the above image.

Sun peeks through an altostratus cloud
Thicker layers of altostratus can be difficult to see through. Simon Eugster/Wikimedia Commons

5. Altostratus. This cloud sheet completely covers the sky, but will have sections thin enough that reveal the sun, "as through ground glass or frosted glass," according to the WMO. Unlike cirrostratus clouds, there is no halo produced.

Nimbostratus clouds over a city in Virginia
Nimbostratus clouds are thick enough to block out the sun. Eric T Gunther/Wikimedia Commons

6. Nimbostratus. While they don't have many distinct features, nimbostratus clouds are a gray cloud layer. They're thicker than altostratus clouds, and their bases often produce rain or snow.

Stratocumulus clouds in Sterling, Virginia
Stratocumulus clouds almost always have dark parts. Famartin/Wikimedia Commons

7. Stratocumulus. Characterized by dark, rounded masses, stratoculumus clouds appear either as a uniform sheet or layer, or they have a corrugated base.

Stratus clouds cover the sky
Stratus clouds look a lot like nimbostratus clouds. LivingShadow/Wikimedia Commons

8. Stratus. Stratus clouds are gray layers, sometimes with variances in their luminescence. If the sun is out, its brightness can help you to see the outline of the clouds. The bases of stratus clouds will produce light snow or drizzle.

Cumulus clouds in a blue sky
Cumulus clouds have a distinct outline. Korionov/Shutterstock

9. Cumulus. Quintessential clouds, cumulus clouds are detached and dense. The parts lit by sunlight are bright white while their bases tend to be a uniform dark color.

Cumulonimbus clouds have a flat top, somewhat anvil-shaped.
Cumulonimbus clouds have a flat top that is somewhat anvil-shaped. kazoka/Shutterstock

10. Cumulonimbus. Cumulonimbus clouds are heavy and dense, with often tall, vertical towers. They're referred to as thunderheads if they're observed during a storm. They're capable of producing lightning and tornadoes.

Cloud Species

Cloud genera are divided into species to account for their particular shape and internal structure. Certain species only appear within specific genera, but many species are common to multiple genera. Clouds are identified by their genus and then their species, e.g., cirrius fibratus or altocumulus stratiformis.

A cirrus fibratus over Norway
Cirrus fibratus are easy to pick out in the sky. Ximonic/Wikimedia Commons

1. Fibratus. A thin veil of clouds, fibratus clouds are either cirrus or cirrostratus clouds. Unlike most cirrus clouds, however, fibratus clouds do not have tufts or hooks at the end, and the strands are clearly separate from one another.

Cirrus uncinus clouds
Cirrus uncinus clouds are the commas of the the sky. HelloRF Zcool/Shutterstock

2. Uncinus. This species of cirrus cloud is distinct for its hook-at-the-end feature.

Dense cirrus spissatus
Cirrus spissatus clouds are often found in cumulonimbus clouds. Wikimedia Commons

3. Spissatus. A species of cirrus clouds, spisstaus clouds are the densest cirrus clouds you'll see. They're even able to hide the sun if they're dense enough.

Stratocumulus castellanus
Stratocumulus castellanus can be identified by their defined layers of clouds. Merikanto/Wikimedia Commons

4. Castellanus. This species of cloud appears in cirrus, cirrocumulus, attocumulus and stratocumulus clouds. The tops of castellanus clouds form turrets, which give it that castle-like appearance.

Altocumulus floccus at sunset
Floccus clouds have a ragged base trailing after them. Katarzyna Mazurowska/Shutterstock

5. Floccus. These clouds have small tufts at their tops with a ragged base. They often have a virga, or streak of precipitation, trailing after the tuft. The species manifests as cirrus, cirrocumulus, altocumulus (pictured) and stratocumulus clouds.

Stratocumulus stratiformis clouds over a river
Stratocumulus stratiformis clouds over a river. Leonardo Poletto/Wikimedia Commons

6. Stratiformis. A species found in altocumulus and stratocumulus clouds, stratiformis clouds are an extensive layer or sheet of their particular cloud.

Stratus nebulosus in winter
A stratus nebulosus cloud in winter. Simon Eugster/Wikimedia Commons

7. Nebulosus. This cloud species, found among stratus and cirrostratus clouds, is a veil without any distinct details.

Cirrocumulus lenticularis clouds over Torres del Paine National Park
Cirrocumulus lenticularis clouds over Torres del Paine National Park. Liam Quinn/Wikimedia Commons

8. Lenticularis. Appearing primarily as cirrocumulus, altocumulus and stratocumulus clouds, lenticularis clouds appear in almond- or lens-shaped arrangements. This also makes lenticularis clouds great as UFOs.

Roll clouds over the Southern Ocean
Volutus clouds are ominous-looking clouds to be sure. Joshua Stone/Wikimedia Commons

9. Volutus. It's hard to miss volutus clouds. Also known as roll clouds due to their distinct shape and movement, volutus clouds are typically stratocumulus clouds and are completely separated from any other clouds.

Cumulus fractus clouds against a blue sky.
Cumulus fractus clouds against a blue sky. Juanedc/Wikimedia Commons

10. Fractus. As their name implies, fractus clouds are stratus and cumulus clouds that have ragged, irregular shreds. These clouds have often broken away from another, larger cloud.

A collection of cumulus humilis against a blue sky.
Cumulus humilis lack the height of regular cumulus clouds. Thomas Bresson/Wikimedia Commons

11. Humilis. A species of cumulus clouds, humilis clouds are generally fairly flat as opposed to taller ordinary cumulus clouds.

Cumulus mediocris clouds over a sports field
Cumulus mediocris clouds have small bumps and sproutings at their tops. MarianaMigl/Wikimedia Commons

12. Mediocris. Another cumulus species, mediocris clouds are a bit taller than humilis clouds.

A cumulus congestus cloud over a town in Germany
A cumulus congestus cloud over a town in Germany. pilot_micha/Wikimedia COmmons

13. Congestus. Congestus clouds are the tallest species of cumulus clouds. They have sharp outlines and cauliflower-like tops.

Cumulonimbus calvus clouds over a farm in Austria
Cumulonimbus calvus clouds can lead to severe weather. Johann Jaritz/Wikimedia Commons

14. Calvus. Cumulonimbus clouds have two species, and the calvus is one of them. It's a moderately tall cloud with rounded tops but still with grooves or channels in them that direct the flow of air.

Cumulonimbus capillatus
This cumulonimbus capillatus cloud has a flat top but still has some dense cirrus clouds on top. Koichi Oda/Wikmedia Commons

15. Capillatus. The second species of cumulonimbus clouds, capillatus clouds have a flat, anvil-like structure near the top, with a mass of "hair" on top of it.


If we drill down further, the large scale arrangement of clouds give the genera and species a wide variety of presentation. Some clouds can exhibit multiple varieties at once, so the varieties are not mutually exclusive to one another, and many genera have a number of varieties. The exceptions to this are translucidus and opacus varieties; they cannot occur at the same time.

Cirrus intortus vertical clouds
Cirrus intortus clouds bend and twist in unusual ways. Bblanc/Wikimedia Commons

1. Intortus. This variety of cirrus clouds has irregularly curved and twisted filaments.

Cirrus vertebratus clouds
Cirrus vertebratus are bony-looking clouds. Laurent Julien/Wikimedia Commons

2. Vertebratus. Have you ever seen a cloud that looked like a fish skeleton? It was almost certainly a vertebratus cirrus cloud.

Undulatus clouds over Iceland
Wave on, undulatus clouds. Wave on. Axel Kristinsson/Wikimedia Commons

3. Undulatus. These sheets or layers of clouds display a wavy pattern. You can find undulatus varieties in cirrocumulus, cirrostratus, altocumulus, altostratus, stratocumulus and stratus clouds.

Altocumulus radiatus clouds over some trees
Radiatus clouds form a nice line in the sky. Unasia9/Wikimedia Commons

4. Radiatus. The bands of these separated clouds run parallel to one another and appear to merge on the horizon. Look for them when you spot cirrus, altocumulus (pictured), altostratus, stratocumulus and cumulus clouds.

Cirrocumulus lacunosus clouds
Cirrocumulus lacunosus clouds can cast a wide net in the sky. The High Fin Sperm Whale/Wikimedia Commons

5. Lacunosus. This cloud variety appears mostly in relation to cirrocumulus and altocumulus clouds. It is marked with small holes in the cloud layer, like a net or honeycomb.

Altocumulus lenticularis duplicatus clouds in Arizona
Altocumulus lenticularis duplicatus clouds float in the Arizona sky. Nicholas A. Tonelli/Wikimedia Commons

6. Duplicatus. These layers of cirrus, cirrostratus, altocumulus, altostratus or stratocumulus clouds appear in at least two slightly different layers.

The sun is hazy through an altostratus translucidus
Translucidus create a hazy shade of sunny. The Great Cloudwatcher/Wikimedia Commons

7. Translucidus. A large sheet of clouds — either altocumulus, altostratus (pictured), stratocumulus and stratus — that is translucent enough to allow the sun or the moon to shine through.

Perlucidus clouds
Perlucidus clouds make sure you don't lose the view of the sky. Sahil Kapoor/Wikimedia Commons

8. Perlucidus. Yet another variety of clouds in a sheet, these altocumulus and stratocumulus clouds have small spaces between each cloudlet that result in a visible sky.

Altostratus opacus looms over a horizon
This image of an altostratus opacus cloud demonstrates how completely it can cover the sky. The Great Cloudwatcher/Wikimedia Commons

9. Opacus. The opposite of the previous two varieties, these cloud layers are opaque enough to hide the sun or moon. This variety is found among altocumulus, altostratus (pictured), stratocumulus and stratus clouds.

Accessory Clouds

As their name implies, accessory clouds are smaller clouds associated with a larger cloud. They may be partially connected or separate from the main cloud.

A pileus cloud appears over a volcanic cloud produced by Sarychev Peak
A pileus cloud appears over a volcanic cloud produced by Sarychev Peak in the Kuril Islands in Russia. NASA Goddard Space Flight Center/Wikimedia Commons

1. Pileus. A small cap or hood that appears above the top of a cumulus and cumulonimbus cloud.

A velum accessory clouds form around a large cloud over Maracaibo, Venezuela
A velum accessory cloud forms around the middle of a large cloud over Maracaibo, Venezuela. Giancarlo Rossi/Wikimedia Commons

2. Velum. This veil is close above or attached to cumulus and cumulonimbus clouds.

Pannus clouds along the edge of a storm cloud
Pannus clouds form along the edge of a storm cloud. Anderson Mancini/Wikimedia Commons

3. Pannus. Appearing mostly along the bottoms of altostratus, nimbostratus, cumulus and cumulonimbus clouds, these are ragged shreds of the cloud that make up a continuous layer.

A wall cloud with a cauda cloud tail forms over Elmer, Oklahoma.
A wall cloud with a cauda cloud tail forms over Elmer, Oklahoma. The lowest middle section of this cloud is the flumen. Steve Willington/World Meteorological Organization

4. Flumen. These are bands of low clouds associated with supercell storm clouds, typically cumulonimbus. Some flumen clouds can resemble beaver tails due to their broad, flat appearances.

Special Clouds

Some clouds only form as a result of localized conditions or due to human activity.

Clouds produced by the Powerhouse Fire in May 2013
The 2013 Powerhouse Fire in California produced flammagenitus clouds. Chevy111/Wikimedia Commons

1. Flammagenitus. These clouds develop as a result of forest fires, wildfires and volcanic eruptions.

Coal-fired power plant in Greece
Homogenitus clouds, like the ones emitted from this coal-fired power plant, are created by human activity. thelefty/Shutterstock

2. Homogenitus. If you've ever driven by a factory with a kid and they've shouted "Cloud factory!", they have identified homogenitus clouds. This type of special cloud covers a range of man-made clouds, including rising thermals from power plants.

A contrail streaks out of some clouds
A contrail streaks out of some clouds. G. Larson/Wikimedia Commons

3. Aircraft condensation trails. Contrails are a special type of the homogenitus special cloud. They must have persisted for 10 minutes to be dubbed cirrus homogenitus.

Cirrus homomutatus, or a persistent contrail cloud, over Lille, France.
A cirrus homomutatus, or a persistent contrail cloud, over Lille, France. Lamiot/Wikimedia Commons

4. Homomutatus. If contrails persist and begin to grow and spread over a period of time thanks to strong winds, they become homomutatus clouds.

Clouds form near a waterfall in Iceland
Clouds form near a waterfall in Iceland. Francesco Carucci/Shutterstock

5. Cataractagenitus. These clouds form near waterfalls, the result of water broken up into a spray by the falls.

Silvagenitus clouds form over forests
Silvagenitus clouds form over forests. Glenn R. Specht-grs photo/Shutterstock

6. Silvagenitus. Clouds may form over a forest as the result of increased humidity and evaporation.

Supplementary Cloud Features

The final bit of cloud identification involves supplementary features that are attached to or merged with the cloud.

A large cumulonimbus cloud with an incus top
You could forge a horseshoe cloud on that thing. Simon Eugster/Wikimedia Commons

1. Incus. The spread-out, anvil-like portion at the top of a cumulonimbus cloud.

Mamma clouds over Leuven, Belgium
Mamma clouds appear over Leuven, Belgium. Bart De Bruyn/Wikimedia Commons

2. Mamma. Those hanging protuberances are called mamma, and they appear along the bottom of cirrus, cirrocumulus, altocumulus, altostratus, stratocumulus and cumulonimbus clouds.

Altocumulus clouds with virga features
These altocumulus clouds have virga trails along their bottoms. Kr-val/Wikimedia Commons

3. Virga. If a cirrocumulus, altocumulus, altostratus, nimbostratus, stratocumulus, cumulus or cumulonimbus cloud looks a bit like a jellyfish, chances are they have a virga feature. These are precipitation trails, or fallstreaks, and the precipitation never reaches the Earth's surface.

Praecipitatio features along the bottom of a cloud
Grab an umbrella, a cloud has a praecipitatio feature. Silar/Wikimedia Commons

4. Praecipitatio. If that precipitation makes it to Earth, however, then you have a praecipitatio feature on an altostratus, nimbostratus, stratocumulus, stratus, cumulus and cumulonimbus cloud.

Clouds with arcus features
Clouds with arcus features are pretty scary. Sensenmann/Wikimedia Commons

5. Arcus. These cumulonimbus clouds (and sometimes cumulus) feature dense horizontal rolls with tattered edges along the front. When the arcus feature is extensive, the roll can have a "dark, menacing arch."

A tuba accessory cloud extends from a cloud's base
Tuba accessory clouds look like funnels reaching out from the clouds. 7alaskan/Wikimedia Commons

6. Tuba. This cone protrudes from the cloud base and is the marker of a intense vortex. Like arcus clouds, tubas appear most often with cumulonimbus and sometimes with cumulus.

Asperitas clouds over Belgium
Varying levels of illumination and thickness of asperitas clouds can lead to dramatic visual effects. WikiRigaou/Wikimedia Commons

7. Asperitas. While they look like undulatus clouds, asperitas supplementary clouds are more chaotic and less horizontal. Still, these supplementary clouds for stratocumulus and altocumulus clouds make it look like the sky has become a rough and choppy sea.

Fluctus clouds appear on a sunny day
Fluctus clouds appear along the top of certain clouds. Grahamuk/Wikimedia Commons

8. Fluctus. These are short-lived, wave-looking supplementary clouds that appear with cirrus, altocumulus, stratocumulus, stratus and sometimes cumulus clouds.

A cavum cloud forms at dusk
Vigra or wispy cirrus clouds often fall from the central hole. H. Raab/Wikimedia Commons

9. Cavum. Also known as a fallstreak hole, cavum are supplementary clouds for altocumulus and cirrocumulus clouds. They're formed when the water temperature in the cloud is below freezing but the water itself has not frozen yet. When the ice does eventually form, water droplets around the crystals evaporate, leaving the large ring. Interaction with aircraft can result in a straight line cavum instead of a circular one.

A wall cloud extends from a cumulonimbus cloud in in Sant'Agata Bolognese, Bologna, Italy
Tuba clouds will sometimes spout from murus clouds. Giorgio Galeotti/Wikimedia Commons

10. Murus. Typically associated with supercell storms, murus (or wall clouds) develop in the rain-free portions of cumulonimbus clouds. They mark a place of strong updraft from which tornadoes can sometimes form.

A wall cloud with a tail extending from the base
A wall cloud with a tail cloud. NOAA/OAR/ERL/National Severe Storms Laboratory/Wikimedia Commons

11. Cauda. Cauda are an accessory cloud to an accessory cloud, appearing alongside murus clouds. These horizontal, tail-like clouds are attached to the murus, and they are roughly the same height. They should not be confused with a funnel.