| 
Black Clouds
Why can clouds look black from underneath?You will need:
- water
- milk
- black ink
- overhead projector
- 2 transparent dishes
What to do:
Put some water in each dish, and place them on the overhead projector. Their projected image should be identical.
Add a little milk to one dish, and a little ink to the other. If you look at the dishes directly, the difference is between them is obvious, but the projected images are still identical. Looking at the dishes directly, the milky dish appears white because a lot of the incident light is reflected (scattered). The inky dish appears dark because incident light is being absorbed (and reradiated at a different wavelength).
The image on the screen is identical but the darkness is caused by different mechanisms. Light incident on the inky water is not transmitted to the screen mostly because of absorption, whereas light incident on the milky screen is not transmitted mostly because of scattering.
If you look at a cloud from above, you see it mainly by the sunlight reflected off it (scattered by it), whereas if you see it from below, a lot of the incident sunlight has been scattered, making the cloud seem darker than the surrounding sky.
If you could only see the cloud from below, you would not know whether the darkness was due to absorption or scattering.
NB, if you put only a very little milk or ink in the dishes, you might see a reddish-brown image, because not all wavelengths are scattered an equal amount by milk, and not all wavelenths are absorbed an equal amount by ink.
More Information
"Clouds in a Glass of Beer", Bohren, Chapter 11
http://en.wikipedia.org/wiki/Cloud#Colors