Met Office College - Course Notes
Clouds
Contents
1. The nature of clouds
2. High clouds
2.1 Cirrus
2.2 Cirrostratus
2.3 Cirrocumulus
3. Medium clouds
3.1 Altocumulus
3.2 Altostratus
3.3 Nimbostratus
4. Low clouds
4.1 Stratocumulus
4.2 Stratus
4.3 Cumulus
4.4 Cumulonimbus
5. Further reading
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Principal, Met Office College, FitzRoy Road, Exeter, Devon. EX1 3PB. UK.
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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.
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Clouds
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.
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a)
b)
1500 ft
3250 ft
c)
d)
25,000 ft
1500 ft
Figure 1. The four main ways in which air rises to form cloud: a) convection;
b)orographic ascent; c) turbulence; d) mass ascent.
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. Table 1 summarises the main cloud
types in terms of their shape and height.
Flat
High
Medium
Low
Bumpy
Cirrostratus
Cirrocumulus
Altostratus
Nimbostratus*
Altocumulus
Stratus
Stratocumulus
Heaped
Hairy
Cirrus
Cumulonimbus*
Cumulus
Table 1. A guide to the shape and height of the ten main cloud types. Those
marked with a * produce most of the rain.
2. High clouds
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;

Cirrostratus – transparent sheet, often with a halo;

Cirrocumulus – small, rippled elements.
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Clouds
2.1 Cirrus
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!
2.2 Cirrostratus
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.
2.3 Cirrocumulus
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.
3. Medium clouds
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.
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3.1 Altocumulus
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.
3.2 Altostratus
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.
3.3 Nimbostratus
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.
4. Low clouds
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;

Stratus – layered, uniform base;

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.
4.1 Stratocumulus
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
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Clouds
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.
4.2 Stratus
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.
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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.
4.3 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.
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
Cumulus
Thermals
Cumulus
Town
South-facing
slope
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.
Figure 3. Cumulus clouds form due to thermals caused by a town or a southfacing slope.
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Clouds
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.
4.4 Cumulonimbus
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.
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Fibrous anvil
Fibrous appearance
Cumulus
Cumulonimbus
without anvil
Cumulonimbus
with anvil
Figure 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.
5. Further reading
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
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