Jill Osaki, age 13, of Portland, Ore., for her question:

WHY DO RAINBOWS APPEAR ONLY IN SUNLIGHT?

Sometimes a rainbow appears at night, by the light of the silvery moon. We call it a moonbow or a lunar rainbow. Both daytime and nighttime rainbows are caused by the same sort of situation in the sky above. During the day, the necessary lighting equipment is provided by the sun. At night, the role of the vivid sun is replaced by the paler moon.

The time to see a lunar rainbow is on a bright moonlit night, when a partly clear ski is strewn with a few showery clouds. The moon must be fairly low and a cloud must be weeping on the opposite side of the sky. When you stand facing the cloud, the moonbeams coming over your shoulder may paint a ghostly moonbow. Its watery colors are the same as those in the brighter daytime rainbow.