Clouds

Cloud types

Credit: Center for Science Education

The three main types of clouds

A cloud is a visible accumulation of minute droplets of wate, ice crystals, or both, suspended in the air. Though they vary in shape and size, all clouds are basically formed in the same way through the vertical of air above the condensation level. Clouds may also form in contact with the ground surface, too. Such a cloud would be known as fog, ice fog, or mist.

The types of clouds can be divided into three levels, each in turn with its own main groups of clouds. All in all, there are ten fundamental types of clouds. Often, you’ll some places simply class clouds as cirrus, stratus, and cumulus because these clouds are the most common and representative for each altitude class.

• High-level clouds (5-13 km):  cirrocumulus, cirrus, and cirrostratus.
• Mid-level clouds (2-7 km): altocumulus, altostratus, and nimbostratus.
• Low-level clouds (0-2 km): stratus, cumulus, cumulonimbus, and stratocumulus.

High-level clouds

Cirrus

Cirrus clouds, El Calafate, Argentina. Credit: Flickr, Dimitry B.

Cirrus is one of the most common types of clouds that can be seen.

Cirrocumulus in Hong Kong. Credit: Wikimedia Commons.

Cirrocumulus clouds are among the most gorgeous out there. These usually form at about 5 km above the surface with small white fluff patterns that spread out for miles and miles over the sky. They’re sometimes called ‘mackerel skies’ because they can sometimes have a grayish color which makes the clouds look a bit like fish scales.

Cirrocumulus clouds exhibit features from both cumulus and cirrus clouds but should not be confused with altocumulus clouds. While the two can look similar, cirrocumulus does not have shading and some parts of altocumulus are darker than the rest. Cirrocumulus cloud comes after cirrus cloud during warm frontal system.

What’s worth keeping in mind about cirrocumulus clouds is that they never generate rainfall (but can mean cold weather) nor do they interact with other types of clouds to form larger cloud structures.

Cirrostratus

Cirrostratus clouds. Credit: Pixabay, jingoba.

Cirrostratus clouds have a sheet-like appearance that can look like a curly blanket covering the sky. They’re quite translucent which makes it easy for the sun or the moon to peer through. Their color varies from light gray to white and the fibrous bands can vary widely in thickness. Purely white cirrostratus clouds signify these have stored misture, indicating the presence of a warm frontal system.

Some of the best cloud pictures involve cirrostratus clouds because the ice crystals beautifully refract light from the sun or moon producing a dazzling halo effect. Cirrostratus clouds can turn into altostratus clouds if these descend to a lower altitude.

As a nice piece of trivia, cirrostratus clouds almost always move in a westerly direction. The sight of them usually means rainfall is imminent in the next 24 hours.

Mid-level clouds

Altocumulus

Altoculumus clouds are sometimes called ‘social clouds’ because they appear in groups. They have a grayish-white color with some portions darker than the others. Credit: Pixabay, MabelAmber.

Altocumulus clouds form at a lower altitude so they’re largely made of water droplets though they may retain ice crystals when forming higher up.  They usually appear between lower stratus clouds and higher cirrus clouds, and normally precede altostratus when a warm frontal system is advancing. When altocumulus appears with another cloud type at the same time, storm normally follows. Altocumulus clouds are common in most parts of the world.

Altocumulus clouds are quite common in most parts of the globe. They usually grow by convection, in most cases after damp air rises to mix with descending dry air. Altocumulus clouds may also form in combination with other types of clouds like cumulonimbus. The amount of rainfall from altocumulus is projected from light to moderate.

Altostratus

Altostratus clouds spelling a rainy day. Credit: Wikimedia Commons.

Altostratus often spread over thousands of square miles and are strongly linked to light rain or snow. Though they’re not capable of yielding heavy rain it’s common for altostratus clouds to morph into nimbostratus clouds which are packed with moisture and can deliver a pounding.

They’re uniformly gray, smooth, and mostly featureless which is why they’re sometimes called ‘boring clouds’. You’ll commonly see this types of clouds in an advancing warm frontal system, preceding nimbostratus clouds.

Nimbostratus

Nimbostratus — a gloomy sight. Credit: Wikimedia Commons.

The name Nimbostratus comes from the Latin words nimbus which means “rain” and stratus for “spread out”. These gloomy clouds are the heavy rain bearers out there forming thick and dark layers of clouds that can completely block out the sun. Though they belong to the middle-level category, they may sometimes descend to lower altitudes.

Nimbostratus clouds form as a result of the gradual accumulation of moist area over a large area as the warm frontal system lifts the warm and moist area higher up in the atmosphere where it condenses. As outlined earlier, a nimbostratus cloud can form from other types of clouds, like a descending altostratus. Spreading cumulonimbus clouds may also lead to the formation of nimbostratus.

Low-level clouds

Stratus

Sheets of stratus clouds. Credit: Pixabay, Bluesnap.

Stratus clouds are composed of thin layers of clouds covering a large area of the sky. This is simply mist or fog when it forms close to the ground. You can easily distinguish a stratus cloud by the long horizontal layers of cloud which have a fog-like appearance.

The clouds form from large air masses that rise to the atmosphere and later condense. These are pretty benign in terms of rainfall producing light showers or even light snow if the temperatures fall below freezing. However, if enough moisture is retained at the ground level, the cloud can transform into a nimbostratus. Stratus clouds are very common all over the world most especially in the coastal and mountainous regions.

Cumulus

Cumulus clouds are characterized by a white, fluffy appearance. People say these resemble cauliflowers. Credit: Pixabay, Hans.

It’s the most recognizable out of all the types of clouds.These adorable ‘piles of cotton’ form a large mass with a well-defined rounded edge, which explains the name ‘cumulus’ which is Latin for ‘heap’. Cumulus clouds

Cumulus clouds are a sign of fair weather, though they may discharge rains sometimes in form of a light shower. You can find them virtually everywhere in the world expected for the Polar regions.

Cumulonimbus

The cloud that produces showers and thunderstorms. Credit: Pixabay.

Cumulonimbus is fluffy and white like cumulus but the cloud formations are far larger. It’s a vertical developing type of cloud whose base grows from one to up to eight kilometers, hence it’s commonly called a tower cloud. For the same reason, cumulonimbus is both a low-level and high-level type of cloud. At the low-altitude base, the cloud is mostly made of water droplets but the high-altitude summit is dominated by ice crystals.

The rain comes and goes with this cloud but when it does, it can come pouring. When you see a cumulonimbus, you know there’s a thunderstorm waiting to happen somewhere.

Cumulonimbus clouds can be seen most commonly during the afternoons of summer and spring months when the Earth’s surface releases heat.

Stratocumulus

Stratocumulus clouds are low-lying clouds that have a wide horizontal structure. Credit: Pixabay, saragib

Stratocumulus looks like a thick white blanket of stretched out cotton. They resemble cumulus clouds except they’re far bigger. The base is well-defined and flat but the upper part of the cloud is ragged due to convection with the cloud itself.  Depending on the thickness of the cloud, a stratocumulus will have light to dark gray hues.

People often think rain is imminent when they see these clouds. In reality, you’ll be lucky to get a light drizzle out of them.

Cloud species and varieties

All of the above represents a broad classification as each type of cloud can be further grouped by species and varieties. The varieties are grouped and named based upon transparency and the arrangement of cloud elements, like so:

• duplicatus (du) – more than one layer at different levels;
• intortus (in) – irregular or tangled;
• lacunosus (la) – thin cloud with regularly spaced holes, net-like;
• opacus (op) – completely masks sun or moon;
• perlucidus (pe) – broad patches with some (small) gaps allowing blue sky to be seen;
• radiatus (ra) – broad parallel bands convergind owing to perspective;
• translucidus (tr) – translucent enough to permit the sun or moon to be seen;
• undulatus (un) – sheets with parallel undulations;
• vertebratus (ve) – looking like ribs or bones;

Cirrus species

• Cirrus fibratus – The most common type of cirrus cloud. Thin and fibrous, cirrus fibratus is often aligned with the high altitude wind direction. It appears as white parallel stripes which streak across the sky.
• Cirrus uncinus – Has a trademark hook shape.
• Cirrus spissatus – Thick and dense, cirrus spissatus tends to dominate the sky above.
• Cirrus floccus – These have a more cotton wool like appearance than any other cirrus.
• Cirrus castellanus – More vertically developed and have a turret-like summit.

Cirrocumulus species

• Cirrocumulus stratiformis – These are the famous ‘fish scale’ clouds.
• Cirrocumulus lenticularis –  Often larger than other clouds in the family with a rounded shape.
• Cirrocumulus floccus – Have a more ragged appearance than other species. The species often appears in smaller patches with other cirrocumulus clouds.
• Cirrocumulus castellanus – Taller than they are wide, these cute clouds resemble tiny towers in the sky

Cirrostratus species

• Cirrostratus fibratus – It looks a lot like cirrus only with more consistency. It has the look of an animal’s fur.
• Cirrostratus nebulosus – Has the appearance of a veil covering the sky. It’s featureless and sometimes unnoticeable.

Altocumulus species

• Altocumulus stratiformis – Looks like a bunch of flat-bottomed puffy clouds packed tightly together but separated by small streaks.These can sometimes extend over the whole sky.
• Altocumulus lenticularis – Lens-shaped clouds that usually form over hilly areas. These are often called spaceship clouds since they often resemble a UFO.
• Altocumulus castellanus – These often lead to cumulonimbus thunderstorms. They’re taller and more puffy looking than they are wide.
• Altocumulus floccus – Often spotted alongside altocumulus castellanus, altocumulus floccus is made of more rugged cloudlets.

Altostratus species

Mammatus clouds in the Nepal Himalayas. Also known as ‘mammary clouds’, Credit: Wikimedia Commons.

• Altostratus Undulatus – Characterized by thin layers that resemble waves. These are a sign of slight mid-atmospheric instability.
• Altostratus Duplicates – In this cloud formation, you will see two or more layers of altostratus clouds on top of each other.
• Altostratus Pannus –  Has chaotic layers that make it look like a shredded cloth.
• Altostratus Translucidus – It’s more transparent than other species allowing the contour of the sun to be visible through it.
• Altostratus Radiates  – Clouds come in wide parallel bands pointing towards the horizon.
• Altostratus Mamma – The name ‘mamma’ comes from the hanging pouches of this altostratus species which resemble a woman’s mammary glands.
• Altostratus Opacus – Seen in wet days, this is a gloomy species which, once it descends, transforms into the rain-bearing nimbostratus.

Stratocumulus species

• Stratocumulus stratiformis – This is the most common type of cloud out of all across the globe. Essentially, these are flat-based clouds with cracks in between.
• Stratocumulus cumulogenitus – These interestingly form when a cumulus encounters a temperature inversion.
• Stratocumulus castellanus – These are thicker, more drizzly stratocumulus clouds.
• Stratocumulus lenticularis – The rarest variety of stratocumulus, these are often spotted in hilly locations which produce atmospheric waves. These clouds have a lens-like shape.

Stratus species

• Stratus Fractus –  Cloud filaments whose appearance changes rapidly due to wind gusts.
• Stratus Nebulosus – Featureless gray stratus clouds that form in cool and stable conditions when moist air moves onto a water or cold ground surface.
• Stratus Opacus – These are the clouds that completely or partly covers the sun or moon.
• Stratus Undulatus – This variety displays a wave-like undulation.
• Stratus Praecipitatio – A form of stratus cloud that comes with precipitation through ice prisms, snow grains or light drizzles.
• Stratus Translucidus – Has a veil-like pattern that outlines the sun and moon.

Cumulus species

• Cumulus humilis – These cumulus clouds are wider than they are tall. You’ll often find more than one dotting the skyline.
• Cumulus mediocris – As the name implies, these clouds are just as wide as they are tall. You’ll usually see them amongst a variety of other cumulus species.
• Cumulus congestus – These taller than they are wide resembling long chimneys.
• Cumulus fractus – Simply the broken remnants of cumulus clouds that are dissipating.

Cumulonimbus species

• Cumulonimbus calvus – The top looks like a cumulus because the tower has not produced ice crystals yet.
• Cumulonimbus capillatus – The top-side of the tower cloud is fibrous. This time, the water droplets have started to freeze, indicating rainfall is to be expected.
• Cumulonimbus incus – Like in the case of cumulonimbus capillatus, the top of the cloud is fibrous but this time also anvil-shaped. This characteristic shape is the result of the cloud reaching the barrier of the troposphere and must now grow outward.

BONUS: Asperitas (Undulatus asperatus)

For years, Gavin Pretor-Pinney, who is the founder of The Cloud Appreciation Society, has been on a mission to convince the world that a new category of cloud deserves recognition. He called it Undulatus asperatus, an odd cloud formation with a distinct undulating and rolling motion. It’s characterized by localized waves in the cloud base, either smooth or dappled with smaller features, sometimes descending into sharp points, as if viewing a roughened sea surface from below. Varying levels of illuminations and thicknesses of cloud can lead to dramatic visual effects. Basically, this type of cloud looks as if it came straight from hell.

In March 2017, this very rare cloud formation was officially recognized as a distinct cloud by the International Cloud Atlas, marking the first cloud formation added since cirrus intortus in 1951. Its name was changed into Asperitas.

Asperatus Cloud, New Zealand. Photo: Merrick Davies.