Why Are Rain Clouds Dark?

Why Are Rain Clouds Dark?

It's pretty well-known that the majority of clouds are white, while rain clouds are usually a darker shade of gray. But why are rain clouds so dark? 

Unlike atmospheric particles that scatter more blue light than other colors (making the sky blue), the small cloud particles equally scatter all colors of sunshine, which together structure white light. However, rain clouds are gray rather than white due to their thickness, or height.

That is, a cloud gets thicker and denser because it gathers more water droplets and ice crystals the thicker it gets, the lighter it scatters, leading to less light penetrating all the way through it. The particles on the underside of the nimbus do not have tons of sunshine to scatter to your eyes, therefore the base appears gray as you look on from the bottom below. 

This effect becomes more pronounced the larger the water droplets get like right before they're large enough to fall from the sky as rain or snow because they become more efficient at absorbing light, instead of scattering it.