Clouds are made by a
system of evaporation and condensation.
In the air there is a
certain amount of water vapour, which you cannot see. When water in a kettle boils it begins to evaporate, which means that it turns into steam and is absorbed into the
If you hold a cold plate
above the steam as it is coming out of the kettle, the steam condenses and turns back into water.
This process of
evaporation and condensation is behind the story of how clouds are
Our atmosphere, the air
which we breathe, is warmest when it is closest to the earth. The
higher you go, the colder the atmosphere becomes.
As the water vapour in
the warmer air close to the earth rises, it meets colder air. The
vapour now condenses and turns into tiny drops of water or ice.
These drops do not fall
back down because the constant push upwards of warmer currents keeps them up.
These warmer currents of
air pushing up from the earth explains why some birds can float in the sky without beating their
wings. You may also have watched a glider sailing through the sky
without an engine, supported on the currents of air coming from below.
The small drops of water
and ice join together to form clouds.
The clouds look white
because of the sunlight from above reflecting from them. If the
cloud is very thick and heavy, the sunlight is scattered from above and the clouds look grey or
Clouds are divided into
three or four main types (scientists do not always agree).
· Cirrus, the highest of the cloud
· Cumulus, the clouds like soft white cushions that we
know best of all
· Stratus, the lowest of the cloud family
· Nimbus are clouds that bring us rain and
snow. The word nimbus is
added to the other cloud types. Nimbus, which simply means rain-bearing, is added to the other cloud
names. When we see a
cumulonimbus cloud, for example, we know there will be a thunderstorm and heavy rain.
(scientists who study the weather) divide these into several more cloud formations, such as cirrocumulus (a
mixture of cirrus and cumulus) or altocumulus (high cumulus) or stratocumulus (a mixture of stratus and
For now, we shall just
try to learn how to identify the main types.
· Cirrus clouds form the highest cloud
· They are made of particles of frozen vapour (little
drops of ice).
· They are thin, wispy clouds and are sometimes known as
· If the ends of the cirrus clouds are bent backwards,
as in the picture above, this is a sign that the wind is about to change to that
· Cumulus are the clouds that we see in
fairy-tales. They are white, puffy clouds, like
· Cumulus clouds mean either good weather or showery
weather, such as we often have in April.
This photograph shows a
good weather cumulus cloud. The sky is blue and the clouds do not
In the photograph above,
however, the cumulus clouds have become thicker and are rising like huge puffs of smoke from the cloud
base. You should be ready to shelter from a heavy
· Stratus clouds are much closer to the ground then
cirrus or cumulus.
· Stratus clouds are dark grey or black and
· They mean rain!
· Stratus often form around the tops of
· The atmosphere under a sheet of stratus clouds is
often very clear and distant objects such as hills 10 miles away or ships out at sea look much
· Nimbus is definitely a rain or snow
· A cumulonimbus cloud is a mixture of cumulus clouds
and nimbus clouds. This is a sign that a thunderstorm is
Clouds on other
· On the planet Earth our clouds are made of water and
· Mercury has almost no atmosphere and therefore no
· We cannot see the surface of the planet
Venus. It is covered with clouds of carbon dioxide (the gas you
· The atmosphere on Mars is too thin to form
· Jupiter, Saturn, Uranus and Neptune have clouds of
hydrogen, helium and frozen ammonia.
· The planet Pluto is probably made of frozen
nitrogen. It is too far away for scientists to know if there are
also frozen clouds.