Everything You Need to Know About Clouds

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.

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.

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.

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.

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.

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.

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.

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

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.

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.

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.

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.

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

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.

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.

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.

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

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

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.

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.

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.

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

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

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

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.

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.

Varieties

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.

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

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

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.

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.

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.

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

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.

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.

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.

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

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

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.

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.

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

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.

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.

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

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

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.

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

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

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.

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.

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."

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.

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.

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

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.

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.

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.