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Why do clouds become dark before it rains? Clouds are usually white or light grey, although sometimes there are red and purple clouds.

The colour of clouds is due to the reflection of sunlight. It also has to do with the size and surface area of the clouds, their thickness and the time they were formed.

On a nice day, there are not as many clouds and sunlight penetrates easily, so the clouds are white.

On a cloudy day, clouds cover a large area of the sky and sunlight cannot pass through, so the clouds appear to be grey.

In summer, before a thunderstorm, the sky is filled with clouds. These clouds which produce heavy rain have a large surface area and are very thick, so hardly any sunlight can penetrate. These clouds appear to be very dark.

At dusk and at dawn, clouds are usually bright orange or red because at sunrise and sunset, sunlight has to penetrate a very thick part of the atmosphere. The stronger and brighter rays will be cast on the clouds, turning them red or orange.