Cloud Type Glossary

High-Level Clouds:

Base usually around 20,000 ft or above over the British Isles:

Cirrocumulus:

Height of base: 20,000- 40,000 ft

Latin: cirrus- Lock or tuft of hair; cumulus- heap

Cirrocumulus clouds are lots of small white clouds – called cloudlets – grouped together at high levels. Composed almost entirely from ice crystals, the little cloudlets are regularly spaced, often arranged as ripples in the sky. They are relatively rare, and unlike altocumulus clouds, never have any shading.

Cirrostratus:

Height of base: 18,000- 40,000 ft

Latin: cirrus- Lock or tuft of hair; stratus- flattened or spread out

Cirrostratus are transparent high clouds covering large areas of the sky. They sometimes produce white or coloured rings, spots or arcs of light around the sun or moon that are known as halo phenomena. Sometimes they are so thin that the halo is the only indication that a cirrostratus cloud is in the sky. Cirrostratus can span thousands of miles, and may be smooth or fibrous and are often fringed with cirrus clouds. Shadows will normally still be cast by the sun when shining through cirrostratus clouds, which can help distinguish them from similar nimbostratus clouds.

Cirrus:

Height of base: 18,000- 40,000 ft

Latin: cirrus- Lock or tuft of hair

Cirrus are short, detached, hair-like clouds found at high altitudes. These delicate clouds are wispy with a silky sheen or look like tufts of hair. In the day time, they are whiter than any other cloud in the sky. While the sun is setting or rising, they may take on the colours of the sunset.

Medium- Level Clouds:

Base usually between 6,500- 20,000 ft over the British Isles:

Altostratus:

Height of base: 7,000- 18,000 ft

Latin: Altum- height; stratus- flattened or spread out

Altostratus are large mid-level thin grey or blue coloured clouds. Usually composed of a mixture of water droplets and ice crystals, they are thin enough in parts to allow you to see the sun weakly through the cloud. The sun cannot cast shadows when shining through altostratus clouds, which is how you can differentiate between altostratus and nimbostratus. They are spread over a large area – up to thousands of square miles – and they are either featureless or can have parallel stripes.

Nimbostratus:

Height of base: 2,000- 10,000 ft

Latin: Nimbus- rainy clouds; stratus- flattened or spread out

Nimbostratus clouds are dark grey or bluish grey featureless layers of clouds, thick enough to block out the sun. These mid-level clouds are often accompanied by continuous heavy rain or snow and cover most of the sky. If there is hail, thunder or lightning it is a cumulonimbus cloud rather than nimbostratus.

Altocumulus:

Height of base: 2,000- 18,000

Latin: altum- height; cumulus heap

Altocumulus are small mid-level layers or patches of clouds – called cloudlets – in the shape of rounded clumps. These are white or grey, and the sides away from the Sun are shaded. Mostly found in settled weather, altocumulus are usually composed of droplets, but may also contain ice crystals. The presence of shading can help tell the difference between altocumulus and cirrocumulus. Cirrocumulus are white but altocumulus can be white or grey, and the sides will be shaded.

Low- Level Clouds:

The bases of these clouds are normally found at below 6,500 ft in the UK:

Cumulonimbus:

Height of base: 1,100- 6,500 ft

Latin: cumulus- heap; nimbus- rainy cloud

Cumulonimbus are heavy and dense low-level clouds, extending high into the sky in towers, plumes or mountain shaped peaks. Commonly known as thunderclouds, the base is often flat and very dark, and may only be a few hundred feet above the Earth’s surface. Cumulonimbus clouds are associated with extreme weather such as heavy torrential downpours, hail storms, lightning and tornados. If there is thunder, lightning or hail, it’s a cumulonimbus cloud rather than nimbostratus.

Cumulus:

Height of base: 1,200- 6,500 ft

Latin: cumulus- heap

Cumulus clouds are detached cauliflower shaped clouds usually spotted in fair weather. If they get bigger they can sometimes produce showers. The top of these clouds are mostly brilliant white when lit by the sun, although their base is usually relatively dark.

Stratocumulus:

Height of base: 1,200- 6,500

Latin: stratus- flattened or spread out; cumulus- heap

Low-level clumps or patches of cloud varying in colour from bright white to dark grey. They normally have well defined bases and some parts much darker than others. They can be joined together or have gaps between them. Stratocumulus clouds can be present in all types of weather conditions, from dry settled weather to light rain and snow.

Stratus:

Height of base:

Latin: stratus- flattened or spread out

Stratus clouds are very low-level grey layers or patches of clouds with fuzzy edges. They are the lowest clouds and sometimes appear at ground level in the form of mist or fog. Stratus clouds are a fairly uniform grey or white colour and may be accompanied by drizzle, snow or snow grains. If there are no other clouds above the layer of stratus cloud, the sun or moon may shine through.

Other Clouds:

Atlocumulus Castellanus:

Height: around 7,000- 18,000 ft

Their towering appearance is a sign of medium-level instability in the air and they often serve as a precursor to thunderstorms or heavy precipitation.  For this reason, they are sometimes also referred to as ‘thunder altocumulus’. The clouds usually appear as a collection of small individual closed circulations, known as cells, with the towers proceeding from the top of the cloud. If conditions are cool and moist enough, they can develop into large cumulonimbus clouds.

Arcus:

Height of base: Around 6,500 ft

Arcus comes from the latin word for ‘arch’. There are two types of arcus clouds: shelf clouds or roll clouds. They can normally be seen beneath powerful storm clouds or cumulonimbus clouds. Shelf clouds are wedge shaped and attached to the storm cloud whereas roll clouds are a horizontal column separated from the storm cloud that can appear to be rolling. They form within warm air from near the ground which has been pushed up by the cold air exiting out of the storm cloud in the downdraft.

Undulatus Asperatus:

Height of base: 4,000- 10,000 ft

Latin: undulus – waves; asperatus – rough

Whilst not yet an official cloud type, Undulatus Asperatus is currently being assessed by the World Meteorological Organisation to decide whether it should become a supplementary feature of mamma clouds in the International Cloud Atlas. If this goes ahead it will be the first new cloud type in over 50 years. These wave-like structures on the underside of the cloud makes it look like a rough sea surface viewed from below. We are not yet certain how they form but there have been many sited on the Plains of the United States.

Banner:

A banner cloud is one of a number of different cloud types that are grouped as orographic clouds. This defines them as being caused by the shaped of the land, with orographic (from the Greek óρος meaning ‘hill’) referring to the relief of land and mountains. When the wind blows against a hill or mountain it is forced to lift. Air cools as it rises and the water vapour within it condenses to form a cloud. As the name implies, these clouds form in a layer and then remain also stationary as the wind continuously flows from one direction. The prevailing wind determines the particular microclimates found around a hill or mountain. Areas to the leeward side of the hill can often experience shelter, with more sunshine, less rainfall and higher temperatures than areas to the windward side which is where banner clouds will develop. Particular mountains, such as the Matterhorn in the Alps in Switzerland, have become famous for the frequent appearance of a banner cloud as pictured below.

Fallstreak Hole:

Also known as hole punch cloud.

They form in clouds of supercooled water droplets, water below 0 °C but not yet frozen. These water droplets need a tiny particle, a nucleus, to freeze or to be cooled below -40 °C. Aircraft punching through this cloud layer can cause air to expand and cool as it passes over the aircraft wings or propeller. This change in temperature can be enough to encourage the supercooled droplets to freeze and fall from the cloud layer in this distinctive pattern.

Funnel:

A funnel cloud is a cone-shaped cloud which extends from the the base of a cloud towards the ground without reaching the ground. They are formed in the same way as a tornado (see How tornadoes are formed’) building around a localised area of intensely low pressure and are typically associated with the formation of cumulonimbus cloud (also known as thunderstorm clouds).Crucially, a funnel cloud does not reach the earth’s surface, at the point it reaches land it becomes a tornado, or if it reaches a body of water it becomes a waterspout.

Mammatus:

Mammatus clouds are some of the most unusual and distinctive clouds formations with a series of bulges or pouches emerging through the base of the cloud. Mammatus clouds are usually formed in association with large cumulonimbus clouds, particular when these are forming large thunderstorms. Typically turbulence within the cumulonimbus cloud will cause the mammatus to form, especially on the underside of the projecting anvil.

Noctilucent:

Height of base: Above 200,000 ft

Very rare cloud seen in the night sky, on clear summer nights.

Noctilucent clouds become visible about the same time as the brightest stars and are usually bluish or silvery, but sometimes orange or reddish. They most closely resemble thin streaky cirrus, and are thought to be made of ice crystals. These clouds are usually seen at latitudes between 45°N and 80°N in the Northern Hemisphere.

Virga:

Virga, from the Latin for ‘rod’ or ‘branch’ are a trail of precipitation that fall from the underside of a cloud but evaporate or sublime before it can reach the earth’s surface. They appear as light wisps which are attached to the base of a cloud and are often at their most striking when lit by a red sunset with a light wind extending the tail into a angled curve, as in the picture below. As a supplementary cloud feature, they occur most frequently with Cirrocumulus, Altocumulus, Altostratus, Nimbostratus, Stratocumulus, Cumulus and Cumulonimbus. Find out more about the different types of cloud in our cloud spotting guide.

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