How clouds reveal the weather

Shaped by the landscapes they cross, clouds can tell us when a sunny day is on the horizon and warn us about incoming storms

The sky is often abundant with clouds. We usually take little notice of them hovering above our heads, but sometimes we fear the weather they may bring. Continuously appearing and fading out of sight, what are these unique shapes that parade across the sky?

Clouds are created when rising air cools down enough to release the water vapour it contains, and the condensation produced in this process makes clouds visible. Cloud masses consist of tiny droplets of liquid, frozen crystals and other suspended particles, such as sea salt, dust and dirt. These particles are known as cloud condensation nuclei.

The volume, source and occurrence of clouds differ around the world according to the climate conditions. Near the equator, hot air rises at the most rapid rate, which results in heavy, tropical rainfall. In desert areas, meanwhile, the extreme lack of moisture means clouds are scarce. With no shade, temperatures soar in the day. But the desert also demonstrates the role clouds play in retaining heat: at night, deserts can become bitterly cold because there is no moisture in the air to hold onto heat.

The mountainous regions of Earth act to obstruct wind flow, which has a fascinating impact on resulting clouds. When air reaches these barriers, it is pushed drastically upwards, creating a unique form of clouds called lenticular clouds. In appearance, these are often compared to a stack of plates, and they are even mistakenly reported as UFOs from time to time.

Reading the clouds

There are more than 100 cloud variations, grouped into ten types:

Cirrus

From the Latin word meaning
‘curl of hair’, these clouds are
thin, white and wispy. Acting as
storm warnings, they appear
before tropical cyclones.

Cirrostratus

These transparent clouds reveal
high levels of moisture in the air.
Formed of ice crystals, cirrostratus
indicate warm weather.

Cirrocumulus

Arranged in rows of small ‘cloudlets’,
they have a grain-like appearance.
These rarer formations can be seen
during cold, fair weather.

Altostratus

Obscuring the sky with thin
grey sheets, altostratus
clouds let little sunlight
through. They are usually
seen before warm weather.

Cumulonimbus

These multi-level clouds can be
found at heights of 21,000 metres
or more. They emerge before
heavy rain, hail and tornadoes.

Altocumulus

These common clouds appear on
warm summer mornings before
cooler afternoons. Also known as
‘sheep-backs’, the patches mimic
sheep’s woolly bodies.

Cumulus

These white, round and fluffy-looking
clouds are closest to the clouds you
probably drew as a child. They are
usually seen on clear, sunny mornings.

Stratocumulus

Stratocumulus look like dishevelled
cumulus clouds. Spaced out with blue
sky in between, they demonstrate
weak convection in the atmosphere.

Nimbostratus

Covering the sky in a thick,
dark grey sheet, these clouds
block out the sun and produce
widespread rain or snow.


This article was originally published in How It Works issue 130, written by Ailsa Harvey


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