I have read this question:
High-scattering barriers are opaque for the same reasons that the undersides of clouds (which are made of transparent water droplets or ice crystals) are dark. There is some thickness of scatterer where the incident radiation is equally likely to be transmitted or backscattered. Many of these thicknesses, and the incident radiation is exponentially attenuated, even with negligible absorption.
Now I did not find any answer on this site about this topic specifically.
As clouds build vertically and get thicker, such as a cumulonimbus cloud, less light can pass through the cloud. This will give it a darker appearance. This is also the reason why the bottom of clouds sometimes appear darker than the top.
But this doesn’t specifically explain in detail why the bottom would be darker. One thought might be gravity, accumulating more mass (water molecules) to the bottom, but that still does not explain the darkening on the bottom completely. There is a hint of reasoning on this site though about why wet (more water molecules) objects are darker.
- Why are the undersides of clouds (which are made of transparent water droplets or ice crystals) dark?