Often we explain about dispersion of white light into seven colours having different wavelengths and how do we see colour.
The justification seems to be misleading in regard to white appearance of clouds. Most of the researchers previously got confused about the white appearance of sky clouds when they tried to understand it through Rayleigh scattering phenomenon.
The idea was later justified on the basis of non-selective scattering, which occurs when the diameter of the atmospheric particles are much larger (approximately 10 times) the wavelengths being sensed.
Non-selective scattering subjects to the popular physical phenomenon of white appearance of clouds. The effect of the scattering is wavelength dependent.
Firstly, a cloud is a visible mass of condensed water vapor floating in the atmosphere. Clouds are formed when the invisible water vapor in the air condenses into water droplets or ice crystals. Now the question is,
if clouds are formed by transparent water droplets then why can’t we see through them, why sky clouds appear white?
The clouds are composed of millions of tiny water droplets or ice crystals. The average size of one cloud droplet is 10 microns. This is pretty tiny but it is bigger than visible light wavelength. Since cloud droplet is bigger than any wavelength of visible light, all the different colours of light behave the same when white light hits a droplet- they scatter. Scattering means that light is diffused and made up of all colours and the net result clouds appear white.
In other words, as light moves through the clouds, it is dispersed by water droplets. All the wavelengths are scattered more or less uniformly to our eyes, no colour predominates. If we look at a cloud from different angles, we see a mixture of all the scattered colours, so it looks white. This effect is known as non-selective scattering.
For visible light (of wavelength 0.4-0.7μm), non-selective scattering is generally caused by water droplets which is having diameter commonly in the range of 5 to 100 μm. This scattering is nonselective with respect to wavelength since all visible and IR wavelengths get scattered equally giving white or even grey color to the clouds.The phenomenon occurs not only in water droplets in atmosphere but also in other situations such as latex particles in paints, droplet in emulsion etc.