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How 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. Aerosols make it easier for the water vapour to condense, and once the process starts, eventually bigger water droplets form around the aerosol particles, and these water droplets start to merge with other droplets, forming clouds.

Clouds form when the air is saturated which means it 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.

This video is from the free online course:

Learn About Weather

University of Exeter

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