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.
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.