What are clouds and how do they form?

Clouds – we often look up and watch them drift across the sky, but what are they actually made of and how do they form?

The formation and movement of clouds around our planet plays a huge part in the water cycle and distribution of moisture from one place to another.

As well as playing an important part in our weather, they also provide a huge source of fascination – driven by how they look and how high in the sky they float.

The classification of clouds is based upon the work of a pharmacist and amateur meteorologist called Luke Howard (1772-1864), who wrote a book in 1803 which examined the various cloud structures that he had seen.

It was his work that was built upon by others and led to the first edition of the International Cloud Atlas being put together in 1896.

What are clouds and how do they form?

Clouds are tiny droplets of water or particles of ice that float in the air, with the height and way in which they form influencing how much of each is present.

Each drop of water is minute, having a diameter of a hundredth of a millimetre, with each cubic metre of air containing around 100 million droplets. Ice crystals are not quite as small, but are still tiny at a tenth of a millimetre long.

Clouds form when water vapour in the atmosphere condenses. The most common way in which this happens is when a parcel of air near the ground rises upwards in the sky and cools.

Eventually, the parcel of air is cooled to the point where it can no longer hold all of the moisture within it. As a result, it then condenses into tiny droplets of water or particles of ice and clouds appear.

What is really interesting is that there are five main ways in which air is lifted up into the sky in order for clouds to form and thus influence our weather.

Surface heating

This works by the sun heating the ground and the air just above it. As the parcel of air is heated, it becomes more buoyant and rises upwards. Eventually, the air rises high enough into the sky, cools and condenses, with clouds forming.

The most common clouds that result from this process are shower clouds – cumulus, towering cumulus or cumulonimbus.

These clouds are often most threatening in summer, when the sun is strong and moist air flows up from Iberia, providing the recipe for severe thunderstorms.

Weather fronts

When masses of air of different temperatures meet, weather fronts form. However, they don’t clash head-on, instead riding up over each other because of varying densities.

Warm air is less dense than cold air, so when warm air meets cold air, it slides upwards over it – like a gently sloping conveyor belt.

This means that the first signs of moisture appear high up in the sky when a weather front approaches, in the form of cirrus cloud. Then, as the weather front gets closer, the cloud lowers and thickens in layers, eventually producing rain.

Topography

When the wind blows, it pushes air along the surface of the earth. However, at some point, this air encounters hills and mountains.

As a result of this, air is forced up the side of mountains and cools with increasing height, causing the air to condense and clouds to form.

This is why you can be on low ground with clear blue sky and sunshine, yet look at mountains in the distance and see clouds looming above them.

Turbluence

When the air becomes unstable, sudden changes in wind speed and occur at different heights above the ground.

This can lead to turbulence forming, with parcels of air being forced to move up and down quickly.

If air is forced to move upwards enough, then it can cool and condense sufficiently for clouds to develop.

Convergence

Sometimes, winds blowing from different directions and collide, resulting in air being forced move upwards.

This is common along peninsulas in summer, where a sea breeze forms on along each coastline, then meets inland, causing cloud to form.

It was convergence that played a huge part in the intense thunderstorms that caused the Boscastle floods in 2004.

So, the next time you look up at the sky and stare at the clouds, see if you can work out which of these five processes played a part in their formation.

If you’ve got any other weather questions, feel free to contact me on Twitter – @liamdutton