Joe Kirby, age 11, of Phoenix, Arizona, for his question:

What is meant by cloud seeding?

The so called "seeds" are crystals of dry ice or silver iodide, sprayed into a cloud to change its misty moisture into rain or snow. Cloud moisture is in the form of liquid droplets, small enough to float suspended in the air. They are widely separated and apparently solid fragments are needed to amass them into sizeable raindrops, heavy enough to fall. This seems to occur when enough of these small solid nuclei are present in a moisture ladened cloud.

Many moist clouds may be unable to shed rain because they lack fragments of dust, salt or other natural nuclei. This is when cloud seeding may coax man made rain from the skies. But if the cloud and other weather factors are not just so, the operation is likely to fail. Meteorologists need to know much more about how nature forms precipitation. Then these complex methods can be copied to do a more dependable job of man made rain and snow.