|MadSci Network: Physics|
Some clouds are white and some are "black" due to two chief reasons:
1) optical thickness and
2) viewpoint and/or background (contrast).
Concerning optical thickness: it is a technical term that just means that a thicker cloud has more droplets of water for the light to scatter off of. This can be due to either the physical thickness of the cloud or the density of the droplets, or both. For a sufficiently thick cloud (that has sufficient optical thickness) a bunch of the light is scattered back in the general direction from which it came as well as sideways and forwards, to there's just not as much light getting "through" the cloud to the ground. Up in the sky, thin clouds (like cirrus clouds) usually look white, but not thick clouds. The thicker ones look dark, but they are really just darker than the sky. It is the contrast that makes them look "black".
This optical thickness is different than saying that an optical component like a lens is thick. The optical thickness that I am talking about here refers only to those media that scatter light incoherently.
Concerning viewpoint: have you ever seen a smoke column (like from a burning pile of leaves) that looks light blue when it has a dark background (such as trees) but then it looks dark reddish if it has the sky behind it? It is the same "cloud", but the lighting is different. I have also seen water vapor columns (like from air conditioners or heat pumps) that look bright with a dark background but the very same column looks darker against a bright background (such as sky). It is like that with thick clouds, too. Even a very thick cloud has some light getting through, so if the whole sky is covered with this very thick cloud it looks gray, not black. The very same cloud looks very very white from up above (if you were in a jet plane, for instance).
You can see the same thing if you can walk around a column of water vapor. In the spring or fall you can see "mixing clouds" rising from ponds or lakes if the air temperature is very cool and the water is warm. If you can find such a "cloud", look at it with the sun at your back and it will look bright, but if you walk around it and view it more or less between you and the sun (or a bright sky) the "cloud" will look dark. It is a matter of contrast.
Well, I hope this helps. There are a couple of books that are really good about explaining this kind of thing. They are "Clouds in a Glass of Beer" and "What Light Through Yonder Window Breaks", both by Dr. Craig Bohren, and published by John Wiley & Sons.
Also, here is a nice Web site that has to do with scattering of all sorts: scattering
John Link, MadSci Physicist
Try the links in the MadSci Library for more information on Physics.