Trent Mitchelle-Borley, of Elmgrove School in Mosgiel, asks :-

Why are clouds white?

Esther Haines, a physicist at the University of Otago, responded.

The first thing we need to do to understand why clouds are white is to think about why we see differently coloured things at all. Light is made up of many different colours. When we see a rainbow we see sunlight split up into the colours red, orange, yellow, green, blue, indigo and violet and all the shades in between these colours. When we see a red ball, that ball is reflecting red light into our eyes. When we see something white it is reflecting all the colours together into our eyes.

Clouds are made up of droplets of water or ice. The droplets are small: about one hundred thousandth of a metre, though the biggest drops can be about one ten thousandth of a metre. This is similar to the diameter of a human hair. Even though water is transparent droplets this size can change the direction of light that hits them. This is called scattering. There are lots of droplets in clouds so light is scattered first by one droplet, then by another and then by another and so on. When this happens all the colours in the light are scattered by the same amount, on average, and we see a white cloud. If the cloud is very thick then there is so much scattering of the light that not much gets through the cloud and we see a grey or black cloud.

This is different from the scattering of light by molecules in the air, which are much, much smaller. Scattering of light by molecules in the air is much stronger for blue light than for the other colours, which is why the sky is blue.