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Cloud name trail
Are the clouds speckled/tiny tufts?
In general, clouds appear in two basic forms: extensive and
continuous sheets/layers, which may be featureless or patchy;
individual clouds with visible outlines and defined edges.
Note: when a number of these individual clouds are grouped
together, they may appear to form a continuous mass of cloud.
Follow the route which best describes the cloud that you are
observing.
Yes
Cirrocumulus
Are the clouds individual rather
than in an extensive sheet/layer?
No
Aircraft contrail cirrus
Yes
Yes
No
Other types of altocumulus
No
Yes
Altocumulus undulatus
Yes
No
Does the cloud form a veil with the sun casting shadows?
Is the cloud shaped like castle turrets?
Are the clouds high and
delicate/wispy/feathery?
Are the clouds in small
patches/broken?
No
Altostratus
Cirrostratus
No
Altocumulus castellanus
* If the patches are ragged rather
than rounded, it is probably
Fractostratus. Note: a well
broken layer of Stratocumulus
can look very similar to types of
Altocumulus.
No
Is the cloud tall and top heavy
with an anvil shaped top?
Cumulus
Yes
No
No
Yes
Is the cloud lens shaped?
Stratocumulus *
Stratus
Nimbostratus
Yes
Is the layer patchy?
High level clouds
cloud base above 6000m
Medium level clouds
cloud base 2000 - 6000 m
Low level clouds
cloud base below 2000m
Altocumulus lenticularis
Yes
Is the cloud rippled/stripy?
Cumulonimbus
Cirrus uncinus
Does the cloud follow a single linear path?
Yes
Yes
Yes
Yes
No
Yes
Are the clouds clumped or
cauliflower-like?
Are the clouds hook-shaped?
Other types of cirrus
Start here
No
No
Yes
No
Is the sun visible through the cloud?
No
No
Is there evidence of moderate to heavy rain?
This key has been designed as a practical
outdoor tool, for the identification of common cloud types.
Not all cloud forms are illustrated and some of the
clouds you observe may not be identical to the
formations shown in the pictures. Several cloud
types may be present at once. Therefore, you must
select and identify each cloud type separately.
Important:
The observer should never look directly into the
sun, as this can cause serious and permanent
damage to eye sight.
The cloud classification shown in this key is
essentially proposed by Luke Howard in 1803. This
classification is based upon the altitude and visual
appearance of the cloud. This was to become the
basis of of the international Cloud Atlas which was
first published in 1896.
Cirrus - fibrous, hair like
Cumulus - a heap or pile
Stratus - a horizontal sheet/layer
Nimbus - rain-bearing
By combining these terms and giving the medium
altitude clouds the prefix alto, Howard’s classification can be used to name common cloud types such
as nimbostratus, altocumulus, cirrostratus, stratocumulus etc.
Formation of clouds
Everywhere in the world, the air in the atmosphere
contains water vapour, most of which has evaporated from the oceans. Clouds are composed of water
droplets and/or ice crystals, which form when
water vapour condenses.
Condensation results from air being cooled
by one or more of the following processes
•• Convection
•• Frontal uplift
•• Orographic uplift
Convection
When the sun heats the ground surface, the air next
to it will in turn be heated. This warmer air will be
less dense than the air above it and so will rise.
The air rises in large bubbles known as thermals.
As thermals rise the air inside them cools and
condensation of water vapour occurs. Thus, clouds
are formed. The height at which the cloud forms,
known as the condensation level, depends upon the
amount of water vapour there is in the air, which
itself is a function of the temperature of the air.
Low clouds with precipitation
Outside of the tropics, fronts are associated with
low-pressure systems known as depressions.
These systems are characterised by a typical
sequence of cloud types, as shown hereafter:
Frontal uplift
Fronts are the boundaries between air masses of
different temperature and humidity. When these air
masses meet they do not readily mix. Instead, they
tend to remain seperate, with the cooler and more
dense air mass undercutting the warmer, less dense
air mass. This process leads to upward movement
of the warm air mass. As it rises, the warm air mass
cools, resulting in condensation and therefore
cloud formation.
Ns
Low clouds below 2000 metres
Cirrus
Cirrostratus
Warm air
Cold front
Altostratus
Nimbostratus
Warm front
Cold air
Cu
Sc
Cold air
Wind
Cumulus cloud types usually result from the
process of convection. The size of the cumulus
cloud which forms is a result of the strength of the
thermal, which can vary through the day. Typically,
in the early morning because the sun is too low in
the sky to heat the ground very much, the thermals
that form may be too weak to lift the air above the
condensation level. By late morning, some small
clouds may have formed and these usually get bigger during the afternoon.
In the late afternoon, when the sun sinks in the sky
and loses its heating power, the clouds also begin
to disappear.
If the thermals are very buoyant, the huge, dark,
towering cumulonimbus clouds will form.
Heavy rain or snow showers and sometimes thunder and lightning are associated with these clouds.
Cb
Cumulonimbus
When viewed from above:
Cirrostratus
Cumulus
Altostratus
Cumulonimbus
B
St
Sc und
A
Ac
Orographic uplift
When air is forced to rise over a topographic barrier such as a hill or a mountain, the air cools and
condensation occurs. The resulting cloud is often
a featureless layer known as stratus. If the stratus
gets very thick, it appears dark from the ground
and heavy rain can fall; the cloud is then called
nimbostratus.
As
Fractostratus (St fra)
Grey sheet/layer of cloud, which has an irregular
ragged/torn/shredded appearance.
Ac len
Altocumulus (other types) (Ac)
White or grey clumps, patches or sheets/layers
of cloud. Can occur in variable shapes and textures.
Altostratus (As)
A greyish sheet/layer, fibrous or uniform in appearance. Thin enough in parts to reveal the sun, as if
viewed through ground glass. No halo.
High clouds above 6000 metres
Ac cas
Ci
3000
2000
1000
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Stratocumulus (Sc)
A thin white patch, sheet or layer of cloud, which
may be dappled or rippled in appearance.
Altocumulus undulatus (Ac und)
White or grey patches or sheets/layers of cloud
with an undulating appearance.
Altocumulus
condensation level
Stratus (St)
Grey, featureless sheet/layer of cloud with a uniform base. Often associated with drizzle or snow
grains.
Medium clouds 2000 - 6000 metres
Stratocumulus
0
Cumulus (Cu)
Brilliant white to grey, dense, detached clouds.
Forms clumped/heaped (cauliflower) shapes,
usually with sharp outlines.
St fra
Stratus
4000
Nimbostratus (Ns)
Dark grey, featureless, thick layer of cloud.
Associated with prolonged precipitation.
Cirrus
Nimbostr.
Height (m)
Cumulonimbus (Cb)
Huge, towering cloud with dark base and white sides. Associated with heavy precipitation, thunderstorms and hail. Often has an anvil- shaped top.
Ci unc
Cc
Cs
Contrails
Altocumulus lenticularis (Ac len)
White or grey broken clouds, in lens or almond
shapes, usually with clear outlines. Formed by air
movement over a topographic barrier.
Altocumulus castellanus (Ac cas)
White or grey broken cumulus-like clouds,
with upper part appearing as a castle turret.
Sometimes the clouds are arranged in lines.
Cirrus (Ci)
White, delicate, silky and fibrous (hair-like) in appearance. Forms into patches or narrow bands.
Cirrocumulus (Cc)
A thin white patch, sheet or layer of cloud, which
may be dappled or rippled in appearance.
Cirrus uncinus (Ci unc)
White, delicate, fibrous strands, comma or hook
shaped.
Aircraft contrails
Caused by the passage of aircraft through the
atmosphere. Thin and white clouds which follow
a single linear path. Often short lived, although
sometimes very persistent.
Cirrostratus (Cs)
A thin, transparant, white veil through which the
sun is clearly visible and casting shadows. A halo
around the sun is sometimes visible. A halo can
sometimes be seen around the moon, especially a
full moon.