Cloud names and types
High clouds (CH)
Base usually 20,000 ft or above, over the British Isles
High cloud types
Cirrus
All high clouds are a type of cirrus, a common cloud that can be seen at any time of the year.
Height of base: 20,000 - 40,000 ft
Shape: layered, tufty or patchy
Latin: cirrus - lock or tuft of hair
Precipitation: none
What are cirrus clouds?
Cirrus clouds 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 daytime, 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.
How do cirrus clouds form?
Cirrus clouds form from the ascent of dry air, making the small quantity of water vapour in the air undergo deposition into ice (to change from a gas directly into a solid). Cirrus is made up completely of ice crystals, which provides their white colour and form in a wide range of shapes and sizes.
Cirrus clouds can also form through contrails, the vapour trails left by planes as they fly through a dry upper troposphere. These streaks can spread out and become cirrus, cirrostratus and cirrocumulus.
What weather is associated with cirrus clouds?
They often form in advance of a warm front, where the air masses meet at high levels, indicating a change in the weather is on the way.
Technically these clouds produce precipitation but it never reaches the ground. Instead, it re-evaporates, creating virga clouds.
How do we categorise cirrus clouds?
Cirrus clouds have five defined 'species';
Cirrus fibratus - Thin and fibrous, cirrus fibratus are often aligned with the high altitude wind direction, making for white parallel stripes which streak across the sky. These are the most common type of cirrus cloud
Cirrus uncinus - With its trademark hook shape, cirrus uncinus is famous for looking like a horse's tail. These wispy streaks of cirrus cannot be seen without a characteristic 'flick' at the end of its tail
Cirrus spissatus - These clouds sit right at the top of the troposphere. A thick, dense cirrus layer that dominates much of the sky above, often formed by passing warm fronts or the remnants of a cumulonimbus incus
Cirrus floccus - Ragged cirrus patches which are much larger than cirrocumulus floccus. These have a more cotton wool-like appearance than the rest of the cirrus family
Cirrus castellanus - More vertically developed than cirrus floccus, cirrus castellanus have turret-like tops and are taller than they are wide.
Cirrocumulus
Cirrocumulus is a relatively rare cloud, forming ripples which may resemble honeycomb.
Height of base: 20,000 - 40,000 ft
Shape: layers or patches of cells
Latin: cirrus - lock or tuft of hair; cumulus - heap
Precipitation: none
What are cirrocumulus clouds?
Cirrocumulus clouds are made up of lots of small white clouds called cloudlets, which are usually grouped together at high levels. Composed almost entirely from ice crystals, the little cloudlets are regularly spaced, often arranged as ripples in the sky.
Cirrocumulus can sometimes appear to look like the scaly skin of a fish and is referred to as a mackerel sky.
How do cirrocumulus clouds form?
Cirrocumulus cloudlets are usually made up of both ice and 'supercooled' water. This means that the water remains a liquid, even at temperatures well below 0oC. They form when turbulent vertical currents meet a cirrus layer, creating the puffy cumulus shape.
Cirrocumulus clouds can also form through contrails, the vapour trails left by planes as they fly through a dry upper troposphere. These streaks can spread out and become cirrus, cirrostratus and cirrocumulus.
What weather is associated with cirrocumulus clouds?
Precipitation from cirrocumulus clouds never reaches the surface, meaning that these clouds are usually associated with fair weather. However, their appearance can often prelude stormy weather, meaning you should make the most of the Sun while you still can.
How do we categorise cirrocumulus clouds?
Cirrocumulus has four defined 'species' which describe its appearance;
Cirrocumulus stratiformis - flat sheets or patches of cirrocumulus, with fine separation leading to a fish scale-like appearance
Cirrocumulus lenticularis - high-level icy lenses, similar to lenticular clouds but much less common. These are often larger than the usual altocumulus cloudlet, with a rounded shape
Cirrocumulus floccus - fluffy tufts of cirrocumulus, with a more rugged appearance than other species. These often occur in smaller patches with a wide range of other cirroform clouds in the sky
Cirrocumulus castellanus - taller than they are wide, cirrocumulus castellanus resemble tiny towers sitting high in the sky
Cirrostratus
The thin, layered cirrostratus cloud is composed of ice crystals and forms a veil that covers all or part of the sky. Height of base: 20,000 - 40,000 ft Shape: Layered Latin: cirrus - lock or tuft of hair; stratus - flattened or spread out Precipitation: None
What are cirrostratus clouds?
Cirrostratus are transparent high clouds, which cover 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 clouds can span thousands of miles, can 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 altostratus clouds.
How do cirrostratus clouds form?
As a result of slowly rising air, cirrostratus cloud can form. Usually generated at the forefront of frontal weather systems, the movements of cirrostratus can be used to predict what the weather will do in the next 24 hours.
Cirrostratus clouds can also form through contrails, the vapour trails left by planes as they fly through a dry upper troposphere. These streaks can spread out and become cirrus, cirrostratus and cirrocumulus.
What weather is associated with cirrostratus clouds?
Though cirrostratus itself does not produce precipitation, it can indicate whether or not precipitation is likely. If cirrostratus nebulosus exists in the sky it is likely that an incoming warm front will bring persistent rain within a day. If cirrostratus fibratus is spotted, stratus may proceed it, bringing only light drizzle.
How do we categorise cirrostratus clouds?
There are two main 'species' of cirrostratus:
Cirrostratus fibratus
- similar to cirrus, but with more consistency. Wispy strands all tightly knitted together, making for more of an animal fur look
Cirrostratus nebulosus
- a uniform veil-like layer covering the sky. Sometimes almost unnoticeable, cirrostratus nebulosus is featureless, composed entirely of ice
Medium clouds (CM)
Base usually between 6,500 and 20,000 ft over the British Isles.
Medium cloud types
Altocumulus
Altocumulus clouds are generally associated with settled weather and will normally appear white or grey with shading.
Height of base: 7,000 - 18,000 ft
Shape: Bands or areas of individual cells
Latin: altum - height; cumulus - heap
Precipitation: None on its own
What are altocumulus clouds?
Altocumulus clouds are small mid-level layers or patches of clouds, called cloudlets, which most commonly exist in the shape of rounded clumps. There are many varieties of altocumulus, however, meaning they can appear in a range of shapes. Altocumulus are made up of a mix of ice and water, giving them a slightly more ethereal appearance than the big and fluffy lower level cumulus.
How do altocumulus clouds form?
Altocumulus clouds can form in several ways, such as,
Formation through the breakup of altostratus
The lifting of moist air pockets which are cooled by gentle turbulence
Mountainous terrain producing atmospheric waves from which clouds can form.
The presence of shading can help tell the difference between altocumulus and cirrocumulus. Cirrocumulus clouds are white and tiny, but altocumulus clouds can be white or grey with shaded sides.
What weather is associated with altocumulus clouds?
Mostly found in settled weather, altocumulus clouds are usually composed of droplets, but may also contain ice crystals. Precipitation from these clouds is rare, but even if rain does fall it doesn't reach the ground. This precipitation can be seen in the form of virga, where the rain is re-evaporated before reaching the surface.
How do we categorise altocumulus clouds?
Altocumulus clouds are one of the most diverse cloud types and have many different 'species'
Altocumulus stratiformis - The most common type of altocumulus, looking like flat-bottomed puffy clouds packed tightly together but separated by small rivers of sky. These can sometimes extend over the whole sky but are more common in smaller patches.
Altocumulus lenticularis - One of the most spectacular cloud types, altocumulus lenticularis (also known as lenticular clouds) are lens-shaped clouds that form over hilly areas. Sometimes referred to as 'spaceship clouds' they often resemble the shape of a UFO.
Altocumulus castellanus - An indicator of instability, altocumulus castellanus towers can lead to the formation of cumulonimbus thunderstorms. These are more puffy looking and are taller than they are wide, making them stand out from other altocumulus varieties.
Altocumulus floccus - Often spotted alongside altocumulus castellanus, altocumulus floccus is made up of slightly smaller, more ragged cloudlets. These are often found with virga clouds hanging below. What supplementary features are associated with altocumulus clouds? Being a highly varied cloud type there are many features often associated with altocumulus. As previously mentioned, virga is known to regularly hang from the bases of altocumulus clouds, making for a jellyfish-like appearance. Taking this up a level, a fallstreak hole looks like a hole has been punched right through an altocumulus stratiformis layer.
Altostratus
Altostratus evolves as a thin layer from a gradually thickening veil of cirrostratus and is usually grey or blue, with very few features.
Height of base: 6,500 - 20,000 ft
Shape: Layered and featureless
Latin: altum - height; stratus - flattened or spread out
Precipitation: None
What are altostratus clouds?
Altostratus are large mid-level sheets of thin cloud. 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. They are often spread over a very large area and are typically featureless.
How do altostratus clouds form?
Altostratus layers are often composed of both water and ice and usually form when a layer of cirrostratus descends from a higher level. The Sun often cannot cast shadows when shining through altostratus clouds. These layers can sometimes contribute to the formation of optical effects such as coronas and iridescence.
What weather is associated with altostratus clouds?
Altostratus clouds often form ahead of a warm or occluded front. As the front passes, the altostratus layer deepens and bulks out to become nimbostratus, which produces rain or snow. As a result, sighting it can usually indicate a change in the weather is on the way.
How do we categorise altostratus clouds?
Altostratus clouds are featureless with little character, so are not classified into 'species' like other cloud types. They are similar to nimbostratus in this way. They do have many pattern-based varieties though, such as undulatus, radiatus and duplicatus, and thickness based varieties; translucidus and opacus.
Nimbostratus
Nimbostratus clouds are dark, grey, featureless layers of cloud, thick enough to block out the Sun and produce persistent rain.
Height of base: 2,000 - 10,000 ft
Shape: Bands or areas of individual cells
Latin: nimbus - rainy cloud; stratus - flattened or spread out
Precipitation: Continuous rain or snow likely
What are nimbostratus clouds?
Nimbostratus clouds are dark, grey, featureless layers of cloud, thick enough to block out the Sun. Producing persistent rain, these clouds are often associated with frontal systems provided by mid-latitude cyclones. These are probably the least picturesque of all the main cloud types.
How do nimbostratus clouds form?
Nimbostratus clouds form through the deepening and thickening of an altostratus cloud, often along warm or occluded fronts. These clouds extend through the lower and mid-layers of the troposphere bringing rain to the surface below.
What weather is associated with nimbostratus clouds?
These mid-level clouds are often accompanied by continuous moderate rain or snow and appear to cover most of the sky. Nimbostratus will often bring precipitation which may last for several hours until the associated front passes over.
If there is hail, thunder or lightning present it is a cumulonimbus cloud rather than nimbostratus.
How do we categorise nimbostratus clouds?
Nimbostratus clouds are featureless, very dense and have little characterisation, so are not classified into 'species' like other cloud types. They are similar to altostratus in this way.
Low clouds (CL)
Base usually below 6,500 ft over the British Isles.
Low cloud types
Stratocumulus
Stratocumulus cloud consists of large, rounded masses of stratus that form groups, lines or waves.
Height of base: 1,200 - 6,500 ft
Shape: cumuliform "lump" at base
Latin: stratus - flattened; cumulus - heap
Precipitation: light
What are stratocumulus clouds?
Stratocumulus clouds are low-level clumps or patches of cloud varying in colour from bright white to dark grey. They are the most common clouds on earth recognised by their well-defined bases, with some parts often darker than others. They usually have gaps between them, but they can also be joined together.
How do stratocumulus clouds form?
Stratocumulus clouds usually form from a layer of stratus cloud breaking up. They are indicators of a change in the weather and are usually present near a warm, cold or occluded front.
What weather is associated with stratocumulus clouds?
Stratocumulus clouds can be present in all types of weather conditions, from dry settled weather to more rainy conditions, but they themselves are often not the culprit. Stratocumulus clouds are often mistaken for rain clouds when in reality, it is quite rare to get anything more than the lightest drizzle from them, if anything at all.
How do we categorise stratocumulus clouds?
Stratocumulus clouds are grouped into four different 'species'
Stratocumulus stratiformis - The most common cloud type across the globe, these are essentially flat-based layers of cloud often with a few cracks between.
Stratocumulus cumulogenitus- These form when rising cumulus clouds encounter a temperature inversion (a warming of the air above) and spread outwards, clumping together.
Stratocumulus castellanus - These are thicker, more drizzly stratocumulus clouds. Turreted tops form when convection initiates through the stable layer, allowing stratocumulus to grow upwards and potentially leading to the formation of cumulus congestus or even cumulonimbus.
Stratocumulus lenticularis - The rarest variety of stratocumulus, lenticularis is often spotted in hilly locations. Very different in appearance to the more spectacular altocumulus lenticularis, they form when hills produce atmospheric waves, which contribute to their lens-like shape.
Stratus
Stratus clouds tend to be featureless, low altitude clouds that cover the sky in a blanket of white or grey.
Height of base: 0 - 1,200 ft
Shape: layered
Latin: stratus - flattened or spread out
Precipitation: light
What are stratus clouds?
Stratus clouds are low-level layers with a fairly uniform grey or white colour. Often the scene of dull, overcast days in its 'nebulosus' form, they can persist for long periods of time. They are the lowest-lying cloud type and sometimes appear at the surface in the form of mist or fog.
How do stratus clouds form?
Stratus clouds form in calm, stable conditions when gentle breezes raise cool, moist air over colder land or ocean surfaces. These clouds can exist in a variety of thicknesses and are sometimes opaque enough to darken days, allowing for little light to pass through.
What weather is associated with stratus clouds?
Stratus is usually accompanied by little to no rainfall but if it is thick enough, it can produce light drizzle. This drizzle can also fall in the form of light snow if cold enough.
How do we categorise stratus clouds?
Due to the nature of stratus it only has two defined species
Stratus nebulosus - a featureless, dark layer which is capable of producing drizzle.
Stratus fractus - a stratus layer which is starting to break up or 'dissipate,' leading to breaks in the cloud.
Cumulus
The fluffy, cauliflower-shaped cumulus is one of the most common and distinctive types of cloud. All cumulus clouds develop as a result of convection.
Height of base: 1,200 - 6,500 ft
Shape: cauliflower of fluffy
Latin: cumulus - heap
Precipitation: occasional rain or snow showers
What are cumulus clouds?
Cumulus clouds are detached, individual, cauliflower-shaped clouds usually spotted in fair weather conditions. The tops of these clouds are mostly brilliant white tufts when lit by the Sun, although their base is usually relatively dark.
How do cumulus clouds form?
All cumulus clouds develop because of convection. As air heated at the surface is lifted, it cools and water vapour condenses to produce the cloud. Throughout the day, if conditions allow, these can grow in height and size and can eventually form into cumulonimbus clouds.
Along coastlines, cumulus may form over land during daylight hours as a sea breeze brings in moist air, which is then warmed by the surface. This effect reverses overnight as the sea becomes warmer than the land and cumulus form over the sea.
What weather is associated with cumulus clouds?
Mostly, cumulus indicates fair weather, often popping up on bright sunny days. Though if conditions allow, cumulus can grow into towering cumulus congestus or cumulonimbus clouds, which can produce showers.
How do we categorise cumulus clouds?
Cumulus clouds have four main categorisations or 'species'
Cumulus humilis - these are wider than they are tall, often numerous in the sky and indicate fair weather conditions
Cumulus mediocris - these are as wide as they are tall and are usually seen amongst a variety of other cumulus variations
Cumulus congestus - these are taller than they are wide, looking like long chimneys capable of producing light showers
Cumulus fractus - these are the broken remnants of cumulus clouds that are breaking up or 'dissipating.'
Cumulonimbus
The many possible variations in the shape of clouds and differences in their internal structure have led to the subdivision of most of the cloud genera into species.
Otherwise known as The King of Clouds, cumulonimbus clouds exist through the entire height of the troposphere, usually characterised by their icy, anvil-shaped top.
Height of base: 1,100 - 6,500 ft
Shape: fibrous upper edges, anvil top
Latin: cumulus - heap; nimbus - rain cloud
Precipitation: heavy rain and thunderstorms
What are cumulonimbus clouds?
Cumulonimbus clouds are menacing looking multi-level clouds, extending high into the sky in towers or plumes. More commonly known as thunderclouds, cumulonimbus is the only cloud type that can produce hail, thunder and lightning. The base of the cloud is often flat, with a very dark wall-like feature hanging underneath, and may only lie a few hundred feet above the Earth's surface.
How do cumulonimbus clouds form?
Cumulonimbus clouds are born through convection, often growing from small cumulus clouds over a hot surface. They get taller and taller until they represent huge powerhouses, storing the same amount of energy as 10 Hiroshima-sized atom bombs. They can also form along cold fronts as a result of forced convection, where milder air is forced to rise over the incoming cold air.
What weather is associated with cumulonimbus clouds?
Cumulonimbus clouds are associated with extreme weather such as heavy torrential downpours, hail storms, lightning and even tornadoes. Individual cumulonimbus cells will usually dissipate within an hour once showers start falling, making for short-lived, heavy rain. However, multicell or supercell storms contain many cumulonimbus clouds and the intense rainfall may last much longer.
If there is thunder, lightning or hail, the cloud is a cumulonimbus, rather than nimbostratus.
How do we categorise cumulonimbus clouds?
Cumulonimbus clouds have 3 distinct 'species' which describe the appearance of the head of the cloud
Cumulonimbus calvus - the top of the cumulonimbus is puffy, like a cumulus cloud. The water droplets at the top of the cloud tower have not frozen to become ice crystals.
Cumulonimbus capillatus - the top of the cloud is fibrous but relatively contained. Water droplets have started to freeze, usually indicating rain has begun or will begin soon.
Cumulonimbus incus - the top of the cloud is fibrous and anvil-shaped, as the cloud has continued to grow. If the cloud reaches the top of the troposphere and still wishes to grow, it must do so outwards, creating the picturesque anvil or 'incus'.
