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. 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
Rapid local ascent when heated air at the earth's
surface rises in the form of thermal currents (convection).
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'.
Upward motion associated with turbulent eddies
resulting from the frictional effect of the earth's
surface.
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.
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).
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 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:
lifting of air over land due to hills or 'bumping'
over rough ground;
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;
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 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 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.
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.
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.
