Karen Wallner, age 11, of Columbus, Ohio, for her question:

What do they use to seed clouds?

Clouds can be seeded from above or from the ground below. When seeded from above, the chemical used is generally silver iodine crystals. An airplane goes up and sprays about a pound of these fine crystals into a likely cloud. If the cloud is right, the fragments of silver iodine cause the cloud mist to gel into sizeable drops of moisture and down tumbles the rain.

When clouds are seeded from below, the chemical generally used is dry ice, which is frozen carbon dioxide. Fine fragments of this material are sprayed aloft from guns on the ground. If the cloud is dust right, its moisture gels around the dry ice pellets and. forms rain drops. But cloud seeding at best is a tricky business. It will not work unless the cloud is almost ready to rain in any case, find even then it may not rain or it may rain in the wrong place.  It is not a successful way to produce rain and seldom used.