Clouds

The nature of clouds
Types of cloud
Cumulus
Cumulonimbus
Stratus
Stratocumulus
Altocumulus
Altostratus
Nimbostratus
Cirriform clouds

 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).

Image: clouds. C.S. Broomfield (© Crown Copyright)
C.S. Broomfield (© Crown Copyright)

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. As air rises it expands – because the pressure decreases with height in the atmosphere – and this causes it to cool. Eventually it may become saturated and the water vapour then condenses into tiny water droplets, similar in size to those found in fog, and forms cloud. If the temperature falls below about minus 20 °C, many of the cloud droplets will have frozen so that the cloud is mainly composed of ice crystals.

The main ways in which air rises to form cloud

  1. Rapid local ascent when heated air at the earth’s surface rises in the form of thermal currents (convection).
  2. Slow, widespread, mass ascent where warm moist air is forced to rise above cold air. The region between warm and cold air is called a ‘front’.
  3. Upward motion associated with turbulent eddies resulting from the frictional effect of the earth’s surface.
  4. Air forced to rise over a barrier of mountains or hills.

The first of these tends to produce cumulus-type clouds, whereas the next two usually produce layered clouds. The last 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.

Types of cloud

The 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.

High clouds are usually composed solely of ice crystals and have a base between 18,000 and 45,000 feet (5,500 and 14,000 metres).

  • Cirrus – white filaments
  • Cirrocumulus – small rippled elements
  • Cirrostratus – transparent sheet, often with a halo

Medium clouds are usually composed of water droplets or a mixture of water droplets and ice crystals, and have a base between 6,500 and 18,000 feet (2,000 and 5,500 metres).

  • Altocumulus – layered, rippled elements, generally white with some shading
  • Altostratus – thin layer, grey, allows sun to appear as if through ground glass
  • Nimbostratus – thick layer, low base, dark. Rain or snow falling from it may sometimes be heavy

Low clouds are usually composed of water droplets – though cumulonimbus clouds include ice crystals – and have a base below 6,500 feet (2,000 metres).

  • Stratocumulus – layered, series of rounded rolls, generally white with some shading
  • Stratus – layered, uniform base, grey
  • Cumulus – individual cells, vertical rolls or towers, flat base
  • Cumulonimbus – large cauliflower-shaped towers, often ‘anvil tops’, sometimes giving thunderstorms or showers of rain or snow

Most of the main cloud types can be subdivided further on the basis of shape, structure and degree of transparency.

Cumulus

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.

Image: cumulus clouds
© Steve Jebson
Image: cumulus clouds
© Steve Jebson

Typically, the base of cumulus clouds will be about 2,000 feet (600 metres) above ground in winter, and perhaps 4,000 feet (1,200 metres) or more on a summer afternoon. Individual clouds are often short-lived, lasting only about 15 minutes. They tend to form as the ground heats up during the day and become less frequent as the sun’s heat wanes towards evening.

The cause of small cumulus clouds is usually 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 condenses out as tiny cloud droplets. A cumulus cloud is born.

The air within the cloud will continue to rise until it ceases to be buoyant. On some sunny days there is insufficient moisture or instability for moisture to form.

In hilly regions, a high, south-facing slope acts as a good source of thermals, and therefore of cumulus. Occasionally, a power station or factory will produce a cloud of its own.

When air rises in thermals there must be compensating downdraughts nearby. These create the clear areas between cumulus clouds and make it easier for glider pilots to find the thermals that they can use to gain height.

Cumulonimbus

Just as cumulus is heaped cloud, so cumulonimbus is a heaped rain cloud (nimbus means rain).

Image: cumulonimbus © N. Elkins
© N. Elkins

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 10 km 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 kilometres. Instead of a ball of cotton wool, a cumulonimbus will resemble a huge cauliflower of sprouting towers and bulging turrets.

But there is one important structural difference in that the uppermost levels of the cumulonimbus have turned to ice and become fibrous in appearance, whereas 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, and there will be showers of rain or, sometimes, hail. In winter, the showers may be of sleet or snow. The showers are often quite heavy and may be accompanied by lightning and thunder.

Sometimes cumulonimbus will be ’embedded’ or half hidden among 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 the day, but are most common inland during the afternoon in spring and summer, and frequently occur in the tropics. They develop where 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 managed to keep regenerating itself over one small area of London. About 170 mm of rain fell in three hours, causing severe flooding.

Stratus

Image: stratus Jim Glavin
Image: stratus A. Bushell
© Jim Glavin © A. Bushell

Stratus is a low-level layer cloud (not to be confused with altostratus and cirrostratus, which are much higher). In appearance, it is usually a featureless grey layer. Sometimes, when a sheet of stratus is affecting an area, the cloud base will be right down to the ground and will cause fog. However, the usual base is between the ground and 1,000 feet (300 metres), which means that hilltops may be obscured by cloud. Sometimes stratus will produce drizzle or light snow, particularly over hills.

Perhaps the most important indication of its low altitude is its apparent rapid movement across the sky in any wind stronger than a flat calm. For example, a stratus cloud at 500 feet (150 metres) moving at 20 miles per hour will appear to move much faster than altostratus with its base at 10,000 feet (3,000 metres) moving at 60 miles per hour.

An approximate guide to the height of stratus may be gained by measuring the relative humidity and subtracting it from 100. The resulting number gives some idea of the height of the low cloud in hundreds of feet. For example, 94% relative humidity would indicate that the stratus is about 600 feet (180 metres) above the ground.

Stratus forms as the result of condensation in moist air at low levels due to cooling. The cooling may be caused in a number of ways:

  1. lifting of air over land due to hills or ‘bumping’ over rough ground;
  2. warm air moving over a cold sea. If the cloud moves in over the land, it will readily cover any relatively high ground. In some cases, the base of the cloud falls to the sea surface, causing fog. This may drift in over the coast and is called sea fog, though it goes by the name of haar in the north and east of Scotland and fret in the east of England;
  3. temperature falling over land at night. The air may have been brought inland during the day on a sea breeze. There needs to be some wind, otherwise the cooling may lead to radiation fog.

Stratocumulus

Image: stratocumulus Jim Galvin
© Jim Galvin

Stratocumulus clouds usually form between 1,000 and 6,500 feet (300 and 2,000 metres).

Stratocumulus will often give a sheet of almost total cloud cover, with perhaps one or two breaks. The cloud elements are rounded and almost join up. Occasionally, the sheet is composed of a series of more or less parallel rolls, which often, but not always, lie ‘across the wind’. Stratocumulus sometimes produces light falls of rain or snow.

Stratocumulus is formed by weak convection currents, perhaps triggered by turbulent airflows aloft. The convection affects a shallow zone because dry, stable air above the cloud sheet prevents further upward development.

Sometimes there are huge sheets of stratocumulus covering thousands of square kilometres 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.

Altocumulus

Image: altocumulus Steve Jebson
© Steve Jebson

Altocumulus clouds usually form between 6,500 and 17,000 feet (2,000 and 5,000 metres) and are referred to as medium level clouds.

In most cases, there is little difference between the properties of stratocumulus and altocumulus, since both are composed of water droplets and are normally limited in vertical extent. The deciding factor between stratocumulus and altocumulus normally comes down to height as both types are formed in the same way.

Altocumulus also provides a sort of dappled pattern, but, since it is at a greater altitude, the cloud elements look smaller. One significantly different form is altocumulus castellanus, which is like a vigorous medium-level cumulus , sometimes with rain falling from their base, known as trailing virga. This type of cloud is sometimes an indication that thunderstorms will follow.

Altostratus

Altostratus clouds normally have a base between 8,000 and 17,000 feet (2,500 and 5,000 metres).

Altostratus appears as a uniform sheet either totally or partially covering the sky. Sometimes it is thin enough to just reveal the sun or moon. The sun appears as if through ground glass but shadows are not visible on the ground. Sometimes, if the base is below 10,000 feet (3,000 metres) it may give light rain or snow.

Image: altostratus C. S. Broomfield
© C. S. Broomfield

 

Nimbostratus

Nimbostratus clouds are found between 1,500 and 10,000ft (450 and 3,000 metres).

Nimbostratus forms a thick, diffuse layer of dark grey cloud covering all or most of the sky, which always obscures the sun or moon. It is accompanied by moderate or heavy rain or snow, occasionally ice pellets. Although classed as a medium cloud, its base frequently descends to low cloud levels. Nimbostratus may be partly or even totally obscured by stratus forming underneath in precipitation.

Image: nimbostratus C. S. Broomfield
© C. S. Broomfield

 

Cirriform clouds

Image: cirriform clouds © R.K. Pilsbury
Image: cirriform clouds © R.K. Pilsbury
Image: cirriform clouds © R.K. Pilsbury
All © R.K. Pilsbury

Cirriform clouds (i.e. clouds from the cirrus family) are found at high altitude, usually above 20,000 feet (6,000 metres). They are composed of ice crystals. Three types of cloud make up the group: cirrus, cirrostratus and cirrocumulus.

Cirrus itself 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 left over from the ‘anvil’ cloud of a cumulonimbus that 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.

Cirrostratus is a 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 a deterioration in the weather.

Cirrocumulus 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 pear-like cloud elements. Sometimes they occur in undulating patterns like tiny ripples.

This information sheet is based on a series of articles written by Dick File that appeared in The Guardian. Web page reproduced with the kind permission of the Met Office