Image: RONALD L. HOLLE, University of Illinois Cloud Catalog
Richard Brill, a professor at Honolulu Community College, gives this answer:
It is the thickness, or height of clouds, that makes them look gray. Clouds are made of tiny droplets of water or ice. They are formed when water vapor condenses within pockets of rising air. Under the right conditions, the air continues to be uplifted, causing the cloud to build higher and higher.
The tiny water droplets and ice crystals in clouds are just the right size to scatter all colors of light, compared with the smaller molecules of air that scatter blue light most effectively. When light contains all colors, we perceive it as white.
When clouds are thin, they let a large portion of the light through and appear white. But like any objects that transmit light, the thicker they are, the less light makes it through. As their thickness increases, the bottoms of clouds look darker but still scatter all colors. We perceive this as gray. If you look carefully, you will notice that the relatively flat bottoms of clouds are always a little grayer than their sides. The taller the clouds become, the grayer their bottoms look.
Answer originally posted January 24, 2000.