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What are clouds?

Clouds are a collection of tiny droplets of water or ice crystals that settle on dust particles in the atmosphere. The droplets are so small - a diameter of about a hundredth of a millimetre - that each cubic metre of air will contain 100 million droplets.

Clouds will either be composed of ice or water droplets depending on the height of the cloud and the temperature of the atmosphere. Because the droplets are so small, they can remain in liquid form in temperatures as low as -30 °C. Extremely high clouds at temperatures below -30 °C are composed of ice crystals.

How do clouds form?

Clouds form when the invisible water vapour in the air condenses into visible water droplets or ice crystals. There is water around us all the time in the form of tiny gas particles, also known as water vapour. There are also tiny particles floating around in the air - such as salt and dust - these are called aerosols.

The water vapour and the aerosols are constantly bumping into each other. When the air is cooled, some of the water vapour sticks to the aerosols when they collide - this is condensation. Eventually, bigger water droplets form around the aerosol particles, and these water droplets start sticking together with other droplets, forming clouds.

Clouds form when the air is saturated and cannot hold any more water vapour, this can happen in two ways:

1. The amount of water in the air has increased - for example through evaporation - to the point that the air cannot hold any more water.

2. The air is cooled to its dew point - the point where condensation occurs - and the air is unable to hold any more water.

The warmer the air is, the more water vapour it can hold. Clouds are usually produced through condensation - as the air rises it will cool, and reducing the temperature of the air decreases its ability to hold water vapour so that condensation occurs. The height at which the dew point is reached and clouds form is called the condensation level.

What causes clouds to form?

1. Surface heating - This happens when the ground is heated by the sun which heats the air in contact with it causing it to rise. The rising columns are often called thermals. Surface heating tends to produce cumulus clouds.

2. Topography or orographic forcing - The topography - or shape and features of the area - can cause clouds to be formed. When air is forced to rise over a barrier of mountains or hills it cools as it rises. Layered clouds are often produced this way.

3. Frontal - Clouds are formed when a mass of warm air rises up over a mass of cold, dense air over large areas along fronts. A 'front' is the boundary between warm, moist air and cooler, drier air.

4. Convergence - Streams of air flowing from different directions are forced to rise where they flow together, or converge. This can cause cumulus cloud and showery conditions.

5. Turbulence - A sudden change in wind speed with height creating turbulent eddies in the air.

The range of ways in which clouds can be formed and the variable nature of the atmosphere results in an enormous variety of shapes, sizes and textures of clouds. To find out more about different types of clouds and how you can identify them, read our cloud spotting guide.

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