Cloud spotting

Observing the clouds can tell us a great deal about what is happening in the atmosphere.

CM4 Altocumulus constantly changing in shape by Met OfficeMet Office

What is a cloud?

As air rises, it cools and condenses.
Clouds appear when there is too much water vapour (gas) for the air to hold. The vapour condenses to form tiny water droplets which are small enough to stay suspended in the air. It's these water droplets that make the clouds we can see.

Cloud classification, plate X (1863) by Robert FitzRoy (1805-1865)The Royal Society

Why do clouds have names?

Luke Howard first named the core cloud types in 1803. Howard was a Quaker and didn't want to use religious names for the clouds so he used latin and latinised terms. This worked well. 

Cumulus, Stratus, Nimbus and Cirrus are still the base of almost all cloud names to this day. 

CL5 Stratocumulus not formed by spreading Cumulus by Met OfficeMet Office

Low-level Clouds (Cl)

These are clouds with a base below 6500ft. There are four basic types: Stratus, Cumulonimbus, Cumulus, and Stratocumulus. Each of these is then broken down into further 'species'. 

CL6 Stratus or ragged Stratus by Met OfficeMet Office

Stratus - latin 'stratus' meaning flattened or spread out'

Stratus clouds tend to be flat and fairly featureless low level clouds with a base of 0 - 1,200ft. The cover the sky in a blanket of grey or white cloud. 

Stratus (St) are the lowest lying form of cloud and sometimes appear like mist or fog. 

CL7 Ragged Stratus or Cumulus by Met OfficeMet Office

Stratus clouds form in calm, stable conditions with gentle breezes. 

They usually mean dry conditions but if thick enough they can lead to light drizzle. 

There are two species of stratus: 'nebulosus' (a featureless and dark layer) and 'fractus' (starting to break up)

Stratus over Romania

CL3 Cumulonimbus no clear upper section by Met OfficeMet Office

Cumulonimbus - latin cumulus 'heap' and nimbus' raincloud'.

The King of Clouds these clouds are recognised for their icy anvil shaped tops. They tower through the troposphere with bases from 1,100 to 6,500ft. 

Cumulonimbus (Cb) clouds bring rain and thunderstorms and are the only clouds that can produce hail, thunder and lightning.

CL9 Cumulonimbus with clearly defined upper section by Met OfficeMet Office

Cumulonimbus clouds grow through convection, often from small cumulus clouds over hot land surfaces. They can store as much power as 10 Hiroshima-sized atom bombs. 

Cb clouds can bring extreme weather including torrential downpours, hail, lighting and tornadoes.

Cumulonimbus Sunset by Met OfficeMet Office

There are three types of Cumulonimbus cloud:

Calvus has a puffy top like a cumulus cloud.

Capillatus has a fibrous top but is contained. It indicates rain soon. 

Incus has an anvil shaped top. It indicates the cloud has reached the top of the atmosphere and must now spread out. 

Cumulonimbus over Papua New Guinea

CL1 Cumulus small by Met OfficeMet Office

Cumulus - latin 'heap'

Cumulus are fluffy cauliflower-shaped clouds and have bases between 1,200 and 6,500ft. 

Cumulus (Cu) clouds are fair weather clouds. They all develop through convection. This means that as air is heated at the surface it rises and cools and water vapour condenses to form clouds.

CL2 Cumulus moderate or great by Met OfficeMet Office

If cumulus clouds continue to grow during the day they can develop into towering cumulus or cumulonimbus and produce showers. 

There are four types of Cu clouds:

Humilis - wider than they are tall
Mediocris - as wide as they are tall
Congestus - towers
Fractus - breaking up

Cumulus over Uruguay

CL8 Cumulus and Stratocumulus with cloud bases different levels by Met OfficeMet Office

Stratocumulus - latin stratus 'flattened' and cumulus 'heap'

Stratocumulus clouds are low level clumps or patches of cloud varying from white to grey at the base. They are the most common clouds on earth and have bases between 1,200 and 6,500ft. 

Stratocumulus (Sc) clouds do not usually bring any form of rainfall. 

CL4 Stratocumulus formed by spreading Cumulus by Met OfficeMet Office

Stratocumulus indicate a change in the weather. They are usually found near a weather front. 

There are four forms:
Stratiformis - flat based 
Cumulogenitus - when rising cumulus spreads out
Castellanus - turreted tops, these can form cumulonimbus
Lenticularis - lens shaped

Stratocumulus over Poland

CM4 Altocumulus constantly changing in shape by Met OfficeMet Office

Mid-level Clouds (Cm)

These are clouds with a base between 6500ft and 20,000ft. There are three basic types: Nimbostratus, Altostratus and Altocumulus. 

Medium level clouds are made of ice and water droplets and can look more ethereal than low level clouds.  

CM1 Semi-transparent As by Met OfficeMet Office

Altostratus - latin altum (height) and stratus (flattened)

Altostratus are large mid-level sheets of thin cloud. 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 form ahead of weather fronts. 

Altostratus (As) has a base between 6,500 and 20,000ft. 

CM2 Opaque Altostratus or Nimbostratus by Met OfficeMet Office

Nimbostratus - latin nimbus (rainy) and stratus (flattened)

Nimbostratus (Ns) clouds are dark, grey, featureless clouds, which block out the Sun. They have bases between 2,000 and 10,000ft. 

Nimbostratus (Ns) is formed through the thickening of an Altostratus cloud. They are associated with fronts and bring persistent rain.

CM7 Altocumulus at two or more levels or opaque Altocumulus predominant by Met OfficeMet Office

Altocumulus - latin altum (height) and cumulus (heap)

Altocumulus clouds are small mid-level layers or patches of clouds, called cloudlets, they come in many varieties and shapes with bases 7,000 to 18,000ft. 

Altocumulus (Ac) indicate settled conditions. Any rain that does fall will not reach the ground and is seen as 'virga'.

CM8 Turreted Altocumulus or Altocumulus in tufts by Met OfficeMet Office

Altocumulus has many different 'species' including: 

Stratiformis - flat bottomed puffy clouds close together with small 'rivers' of sky in between. 

Lenticularis - lens shaped clouds over hilly areas, they can look like UFOs!

CH2 dense, turreted and tufts Ci more than other Ci by Met OfficeMet Office

High Clouds (Ch)

These are clouds with a base above 20,000ft. There are three basic types: Cirrus, Cirrocumulus and Cirrostratus. Each of these is then broken down into further 'species'. 

CH4 Cirrus invading the sky by Met OfficeMet Office

Cirrus - 'lock or tuft of hair'

All high clouds are forms of Cirrus. Cirrus are hair like clouds made of ice crystals that look bright white by day and can take the colours of the sunset. They have bases of 20,000 to 40,000ft. 

Cirrus (Ci) form before a warm front and indicate a change in the weather. 

CH9 Cirrocumulus more than Cirrus and Cirrostratus combined by Met OfficeMet Office

Cirrocumulus - cirrus 'tuft of hair' and cumulus 'heap'

Cirrocumulus clouds are made up of lots of small white clouds called cloudlets. They often look like ripples high in the sky with bases of 20,000 to 40,000ft. 

Cirrocumulus (Cc) appear in fair weather but are often a sign of stormy weather. 

CH5 Cirrostratus not over 45 degrees by Met OfficeMet Office

Cirrostratus - cirrus 'tuft of hair' and stratus 'flattened'

Cirrostratus are transparent high clouds with bases of 20,000 to 40,000ft. They cover large areas of the sky. They sometimes produce white or coloured rings around the Sun or Moon known as halos. 

Cirrostratus (Cs) are usually a sign that a front will arrive within 24hrs. 

Cloud types graphic, Met Office, From the collection of: Met Office
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