What primarily causes clouds to appear white?

Clouds appear white primarily due to Mie scattering, which occurs when particles in the atmosphere are approximately the same size as the wavelength of light. In clouds, water droplets or ice crystals scatter sunlight in all directions. Since clouds usually contain a high concentration of these water droplets or ice crystals, they scatter all colors of light almost equally, leading to the perception of white.

In contrast to Mie scattering, Rayleigh scattering is more effective for smaller particles and is responsible for the blue color of the sky, as it scatters shorter wavelengths of light (blue and violet) more than the longer wavelengths (red and yellow). While absorption of light by particles can cause clouds to have a darker or different hue, it does not contribute to their white appearance. Reflection from the ground influences the color seen from below but does not directly affect the inherent color of the clouds themselves. Therefore, the predominant factor for the whiteness of clouds is indeed Mie scattering.