Ben J. Losser, age 11, of Glendale, Arizona, for his question:

How can cloud seeding make it rain?

Last year in Florida, tests were made on a number of fluffy white cumulus clouds. Those that were seeded shed almost 33 times more rain than similar clouds that were not seeded. This sounds like success. However, a five year seeding experiment over a large inland area reduced the normal rainfall by 20 per cent. This effort in man¬made weather turned out to be a dismal failure.

The formation of raindrops is a very complicated weather event. Scientists started probing the mystery way back in the 1930s. They even managed to create miniature showers on a laboratory scale. Everybody had high hopes that at last mankind could control the weather. In the 1940s, the tests were taken outdoors. Clouds were seeded with crystals of dry ice, frozen carbon dioxide, and later with fragments of silver iodide. Sometimes the cloud seeding worked, sometimes it made no difference and sometimes it even stopped a likely cloud from raining.

Meteorologists have learned a lot more about cloud seeding in the past 20 years. But they still can't be sure whether it will work    because they are not sure of all the factors that make it work. A raincloud is loaded with misty droplets of water. Too small to fall, they float aloft, colliding and bouncing apart. Rain cannot fall until multitudes of these misty droplets merge together in big, heavy drops of water. In nature, we know of two things that tend to get this process started. One is an electric field. In the lab, for instance, a comb charged with static electricity sometimes makes drops form in a cloud of mist.

We also know that chilly cloud mist tends to gather around certain solid particles called nuclei. These raindrop cores may be particles of soot, dust or salt but usually they are fine crystals of ice. Cloud seeders inject zillions.of suitable nuclei particles into a likely cloud. If all goes well, the misty droplets freeze and stick to the seeds, then the icy gobs melt and become raindrops. But the operation is not always successful because many unknown factors may be present. Meteorologists know that success may be ruined by updrafts, by upper and lower wind directions. Atmospheric electricity may make a difference. And just recently, records revealed that thunderstorms occur most frequently on the second day after the full moon. It is suggested that the moon causes this indirectly when it passes through the earth's geo  magnetic field.

During this current period of trial and error, most cloud seeding is done by spraying fine silver iodide or dry ice crystals, either from the ground or from planes. If even a few of the seeds become coated with gobs of misty moisture they can start the complex rain making process within the whole cloud. But we need to know a very great more about cloud seeding before we can guarantee a shower of the right size in the right place at the right time.