Kathy Bingham, age 10, of Portage, lnd., for her question:

What makes clouds light or dark?

No two clouds are exactly the same shape or color. What's more, most of them are likely to change their shapes from moment to moment as they move across the sky. A cloud is a mass of misty moisture  at the mercy of all sorts of things up above the ground. The breezes blow in all directions, dancing sunbeams warm up patches of the air. These restless events cause never ending changes in the clouds.

All clouds are frothy mixtures of air and either tiny ice crystals or minidroplets of water. In a puffy white cloud, the droplets are very, very small and far apart. Sunbeams dance through the spaces, and the whole thing looks like the white meringue on a pie. When the air up there gets very cool, the droplets tend to cling together. These drops of water are big enough to cast shadows all through the cloud. This shuts out the sunbeams, and the cloud turns shadowy gray.