The whiteness of clouds is due to a phenomenon called Mie scattering. Mie scattering is a type of elastic scattering that occurs when electromagnetic radiation (such as light) interacts with particles that are much larger than the wavelength of the radiation. In the case of clouds, the particles are water droplets, which are much larger than the wavelength of visible light.
When light hits a water droplet, it is scattered in all directions. The amount of scattering that occurs depends on the size of the droplet and the wavelength of the light. For water droplets that are about the same size as the wavelength of visible light, Mie scattering is most effective at scattering all colors of light equally. This is why clouds appear white, even though sunlight is made up of different colors of light.
Rayleigh scattering is another type of scattering that can occur in clouds. Rayleigh scattering is more effective at scattering blue light than other colors of light. This is why the sky appears blue during the day. However, Rayleigh scattering is less effective than Mie scattering, so it does not play a major role in the whiteness of clouds.
In addition to the size of the particles, the shape of the particles also affects the amount of scattering that occurs. Water droplets in clouds are not perfectly spherical, but they are typically much closer to spherical than to other shapes. This means that Mie scattering is more effective in clouds than it would be if the water droplets were perfectly spherical.
The whiteness of clouds is also affected by the concentration of water droplets in the cloud. The more water droplets there are in a cloud, the more scattering that occurs. This is why clouds appear whiter when they are thicker.
In conclusion, the whiteness of clouds is due to a combination of factors, including the size, shape, and concentration of water droplets in the cloud. Mie scattering is the most important factor, but Rayleigh scattering and the concentration of water droplets also play a role.