Craig Nims, age 10, of Des Moines, Iowa, for his question:

HOW DOES CLOUD SEEDING WORK?

Seeding the clouds is supposed to make their misty moisture turn into raindrops or snowflakes  and tumble down to the ground. The misty droplets in a cloud are very very small and they are separated by air spaces. It takes millions of them to form a single raindrop  and to do this they must'get together. A normal cloud can form raindrops only when it has lots of tiny fragments, such as dust or crystals of salt. These are called nuclei  and the misty droplets tend to cling to them and form raindrops.

Cloud seeders add their own nuclei fragments to a likely cloud. They may shoot crystals of dry ice or smoke from silver iodine into the misty moisture. Sometimes these fine fragments cause the tiny droplets to form raindrops  and the operation starts a shower.