Contents Double-click here to create/update the Table of Contents.
1. The nature of clouds
2. High clouds
3. Medium clouds
4. Low clouds
5. Further reading
1.The nature of clouds
A classification of clouds was introduced by Luke Howard (1772–1864) who used Latin words to describe their characteristics.
Cirrus – a tuft or filament (e.g. of hair)
Cumulus – a heap or pile
Stratus – a layer
Nimbus – rain bearing
There are now ten basic cloud types with names based on combinations of these words (the word alto meaning ‘high’, but now used to denote medium-level cloud, is also used).
Clouds form when moist air is cooled to such an extent that it becomes saturated. The main mechanism for cooling air is to force it to rise. Since pressure decreases with height up through the atmosphere, the rising air expands, and therefore cools. Eventually it may become saturated and the water vapour then condenses to form tiny water droplets – similar in size to those found in fog – to form a cloud. If the temperature falls below about -38ºC the cloud droplets freeze, so very cold clouds are composed largely of ice crystals. But note that clouds will generally be composed of a mixture of ‘supercooled’ cloud droplets and ice crystals.
There are four main ways in which air rises to form cloud:
Convection: rapid local ascent when heated air at the earth’s surface rises in the form of thermal currents.
Orographic ascent: air forced to rise over a barrier of mountains or hills.
Turbulence: upward motion associated with turbulent eddies resulting from the frictional effect of the earth’s surface.
Mass ascent: slow widespread ascent where warm, moist air is undercut by cold air. (The barrier between the warm and cold air is called a ‘front’.)
Convection tends to produce cumulus-type clouds, whereas mass ascent and turbulence usually produce layered clouds. Ascent over hills can produce either cumulus-type cloud or layered cloud depending upon the state of the atmosphere.
The range of ways in which clouds can be formed and the variable nature of the atmosphere give rise to the enormous variety of shapes, sizes and textures of clouds.
Figure 1. The four main ways in which air rises to form cloud: a) convection;
b)orographic ascent; c) turbulence; d) mass ascent.
he ten main types of cloud can be separated into three broad categories according to the height of their base above the ground: high clouds, medium clouds and low clouds. Table 1 summarises the main cloud types in terms of their shape and height.
Flat Bumpy Heaped Hairy
High Cirrostratus Cirrocumulus Cirrus
Medium Altostratus Altocumulus
Low Stratus Stratocumulus Cumulonimbus*
Table 1. A guide to the shape and height of the ten main cloud types. Those marked with a * produce most of the rain.
High clouds are usually composed solely of ice crystals and have a base between 16,500 ft and 45,000 ft. The three main high cloud types are:
Cirrus – white filaments;
Cirrocumulus – small, rippled elements.
Cirrus is very common in the British Isles and throughout most of the world. It is thin, wispy and white in appearance, and its name coming from the Latin word for ‘tuft of hair’ gives a good description of the cloud. Another name for the cloud, ‘mares tails’, also conjures up an accurate image.
Cirrus may be hooked or straight depending on the airflow aloft. Sometimes it comes as a very dense patch which is a left-over from the “anvil’ cloud of a cumulonimbus which has disappeared. On other occasions, cirrus may be quite extensive when associated with a jet stream – the cloud can then be seen moving across the sky despite its great altitude.
Aircraft condensation trails are a form of man-made cirrus. They can sometimes be seen in ‘historical’ films to the delight of film buffs who enjoy spotting technical inaccuracies!
This is fairly uniform sheet of thin cloud through which the sun or moon can be seen. Sometimes, if the cloud is thin, a bright ring of light (called a halo) surrounds the sun or moon. A layer of cirrostratus is often an indication of the weather deteriorating within the next few hours.
This is often present in small amounts along with cirrus, but rarely does it dominate the sky. On those occasions when it is widespread, a beautiful spectacle is created – especially at sunset. The individual clouds appear very small – often tiny rows of roughly spherical cloud elements. Sometimes they occur in undulating patterns like tiny ripples.
Medium clouds are usually composed of water droplets, or a mixture of water droplets and ice crystals, and have a base between 7000 ft and 16,500 ft. The three main medium cloud types are:
Altocumulus – layered, rippled elements, generally white with some shading;
Altostratus – thin, grey layer which allows sun to appear as if seen through ground glass;
Nimbostratus – thick layer, low, dark base, with rain or snow falling from it.
Altocumulus is a rippled cloud layer made up of small cloud elements, often appearing in a sort of dappled pattern. There are many variations including:
altocumulus castellanus, which is like a vigorous medium-level cumulus. This type of cloud is sometimes an indication that thunderstorms will follow.
altocumulus lenticularis, wave clouds. Such clouds appear most often in cloud formations of orographic origin.
Altocumulus is formed by weak convection currents, which may be triggered by turbulent air-flows aloft. The convection is confined to a shallow zone because dry, stable air above the cloud sheet prevents further upward development.
Altostratus is a flat cloud layer. It can be thin enough to allow the sun to shine through (as if looking through a piece of ground glass) or may be several thousands of feet thick, so that it appears grey and featureless.
Nimbostratus is a layered, rain-bearing cloud. Nimbostratus is classified as a medium cloud, and the base may initially be above 7,000 ft. However, continuous moderate or heavy precipitation over several hours will tend to lower the base – perhaps to as low as 1,000 ft. Rain or snow falling from this cloud tend to make the base diffuse and featureless.
Low clouds have a base below 7,000 ft and are usually composed of water droplets, though cumulonimbus clouds include ice crystals.
Stratocumulus – layered, series of rounded rolls, generally white with some shading;
Cumulus – individual cells, vertical rolls or towers, flat base, cauliflower-shaped tops;
Cumulonimbus – large, cauliflower-shaped towers, often anvil tops, sometimes giving thunderstorms, or showers of rain, snow or hail.
Stratocumulus is similar in form to altocumulus, though at a lower level. It is an undulating layered cloud formed by weak convection currents in the lower atmosphere, the vertical motion often restricted by a stable dry layer of air above. It often appears as an extensive sheet covering the whole sky with few, if any, breaks. Sometimes the sheet is composed of a series of more or less parallel rolls that are often, but not always, aligned at right angles to the wind.
Figure 2. A stratocumulus cloud sheet has regular elements which appear to get smaller in the distance.
Huge sheets of stratocumulus covering thousands of square kilometres are found around the flanks of a high pressure system, especially over the oceans. The weather below such sheets tends to be dry, but it may be rather dull if the cloud is two or three thousand feet thick.
Stratocumulus occasionally produces light rain or snow.
Stratus is a low-level cloud layer, not to be confused with altostratus and cirrostratus, which are much higher. In appearance it is usually featureless and grey. Sometimes, when a sheet of stratus is affecting an area, the cloud base will lower right down to the ground to give fog. However, typically the base will be between 100 ft and 1000 ft, which means that hill tops may be obscured by cloud. Sometimes stratus produces drizzle, snow or snow grains – particularly over hills.
Perhaps the most important indication of its low altitude is its apparent rapid movement across the sky in anything more than a flat calm. For example, a stratus cloud at 500 moving at 20 kn will appear to move much faster than altostratus with its base at 10,000 ft moving at 60 kn.
Inland, the base of any stratus will tend to lift slightly during the daytime, and in summer will usually disperse completely unless associated with rain. On the coast, when a moist south-westerly air stream persists, there may be no such respite.
The highest frequency of stratus occurs in the western parts of the British Isles where moist south-westerly air is forced to rise over the various ranges of hills.
Cumulus clouds are often said to look like lumps of cotton wool. With a stiff breeze, they march steadily across the sky; their speed of movement gives a clue to their low altitude. Cumulus clouds occasionally produce light showers of rain or snow.
Typically the base of the cumulus clouds will be about 2,000 ft above ground in winter and perhaps 4,000 ft or more on a summer’s afternoon. Individual clouds are often short-lived, often lasting only about
15 minutes. They tend to form as the ground heats up during the day and become less widespread as the sun’s heat wanes towards evening.
The cause of small cumulus clouds is thermal convection. Heat from the sun warms the ground, which in turn warms the air above. If a ‘parcel’ of warm air is less dense than the cooler air around it or above it, the parcel of air starts to rise – this is known as a ‘thermal’. As it rises it expands and cools, and if cooled sufficiently the water vapour within it condenses out as tiny cloud droplets. Thus a cumulus cloud is born.
igure 3. Cumulus clouds form due to thermals caused by a town or a south-facing slope.
The air within the cloud will continue to rise until it ceases to be buoyant, i.e. it reaches an inversion. Thus the vertical extent of cumulus clouds depends on the strength of heating at the surface, and the temperature profile of the air aloft. The strongest convection occurs over a warm surface when the air at high levels is particularly cold.
In hilly regions, a high, south-facing slope acts as a good source of thermals, and therefore of cumulus. Occasionally a power station, factory or a stubble fire will produce a cloud of its own.
When air rises in thermals there must be compensating down-draughts nearby, creating clear areas between cumulus clouds. Glider pilots like to fly on days with cumulus clouds as the thermals provide a reliable means of gaining height. Pilots look for the strongest lift beneath the centre of the cumulus clouds, whilst trying to avoid the descending air in between the clouds.
Just as cumulus is heaped cloud, so cumulonimbus is a heaped rain cloud (nimbus means rain). In many ways the rain-bearing variety can be considered as a bigger, better-organised version of the cumulus. A cumulonimbus may be 10 km across and extend 30,000 feet above the ground. This compares with a cumulus cloud, which is typically a few hundred metres across and reaches a height of only a few thousand feet. Instead of a ball of cotton wool, a cumulonimbus will resemble a huge cauliflower of sprouting towers and bulging turrets.
The most important structural difference in a cumulonimbus is that the uppermost levels of the cloud have turned to ice and become fibrous in appearance – cumulus clouds are composed entirely of water droplets. This icy section at the top may flatten out into an ‘anvil’ shape when the cloud is fully developed. When it reaches this stage the base is usually dark, with showers of rain, snow or hail falling. Ice or snow, however, will often melt before reaching the ground. Often the showers are quite heavy for short periods. Also thunder and lightning sometimes occur.
igure 4. The development of a cumulonimbus cloud, from humble cumulus, to towering cumulonimbus with anvil top.
Sometimes cumulonimbus will be ‘embedded’ or half-hidden amongst other clouds. On other occasions they will be well separated and the anvil may well be visible many miles away.
Cumulonimbus clouds may be seen at any time of day but are most common inland during the afternoon in spring and summer, when convection is at its strongest and most organised.
The lifetime of a cumulonimbus is usually less than one hour. There are exceptions, though. The ‘Hampstead Storm’ of 14 August 1975 was an example of a cumulonimbus cloud that kept regenerating itself over one small area of London. About 170 mm of rain fell in three hours, causing severe flooding.
This material was based on a series of articles by Dick File, which appeared in The Guardian. Further information on clouds and cloud observing may be found in the following publications, available in the Met. Office College library.
HMSO, Observer’s Handbook
HMSO, Cloud Types for Observers
Scorer, R., Spacious Skies
Crown Copyright. Permission to quote from this document must be obtained from The Principal, Met Office College, FitzRoy Road, Exeter, Devon. EX1 3PB. UK.
Last saved date: 6 February 2004