Why are Clouds White

Discover the basic scientific principle that makes clouds white

Clouds are formed when humid air, or water vapour, rises and cools. The vapour expands and becomes tiny droplets. Clouds only get their white appearance if these droplets become large enough to scatter visible light in all directions; this is known as Mie scattering.

Visible light is a form of electromagnetic radiation, with each different colour that we can see having a different wavelength. White light, however, contains equal amounts of all colours of the spectrum. When sunlight hits the individual water droplets in a cloud all wavelengths of light are scattered evenly in all directions.

However, very thick clouds, which are made of very densely packed water droplets, will appear darker – like storm clouds – because less of the incoming light from the Sun can penetrate to the base. From above in an aeroplane, though, a storm cloud will still appear white – it only looks dark from the ground because little sunlight is not getting through.