Prof. Dr. Thomas Mölg, Professor of Climatology at FAU, explains
Colours in the sky are the result of the scattering and refraction of sunlight as it travels to the earth’s surface. Certain everyday phenomena in atmospheric optics – such as the sky’s blue colour – are familiar to us, while others are more spectacular and occur only under special conditions – such as a rainbow or a Fata Morgana.
There are two factors that determine the colour of clouds. One is the size of the liquid and solid water particles in the cloud, which are known as hydrometeors. The other is the number of these hydrometeors. If the cloud consists mainly of small droplets and ice crystals, sunlight can enter it relatively easily and is scattered diffusely. These kinds of clouds appear bright white.
As the size and concentration of the hydrometeors increases, less and less sunlight penetrates the cloud, resulting in multiple scattering. We perceive these clouds as dark clouds in varying shades of grey. It makes sense that we pay most attention to dark clouds – large, heavy hydrometeors fall from clouds more easily, causing it to rain or snow.