Ken Koncilja, age 12, of Euclid, Ohio, for his question:

How is rain formed?

It happens so often that we tend to take the silvery rain for granted. If we ponder its formation, most likely we suspect that the drops start splashing down when they grow too big and heavy to ride aloft in the clouds. This is true, but only a small part of the story. The mystery of how those heavy raindrops form from tiny droplets of mist is very complex. Modern meteorologists suggest theories, which means that they cannot explain the formation of rain in provable detail.

We know enough about it to try seeding the clouds    and sometimes this results in a shower, or even a deluge. If meteorologists understood exactly how rain is formed, they could coax the clouds to shed showers wherever they are needed. But precise weather control of this kind must wait for more research. At present, meteorologists suggest several possible ways in which clouds can form rain.

Such theories are based on the facts we already know about the behavior of moisture under laboratory conditions. One major factor seems to be temperature. In our everyday world, we expect water to freeze at 32 degrees Fahrenheit. The misty droplets of water in a cloud apparently make other arrangements. They can remain in the liquid state at temperatures as low as minus 40 degrees Fahrenheit. This odd behavior apparently governs the formation of most rain in our temperate zone.

One would suppose that it would be easy and natural for cloud moisture to gather into raindrops. After all, a cloud holds many tons of liquid water. However, this moisture is in widely separated droplets that measure perhaps one 4,OOOth of an inch in diameter. They are buoyantly suspended and calmly balanced in the air and it is not easy for these droplets to meet and gather together. In the tropics, apparently this meeting occurs in the turbulence of warm updrafts and cool currents~aloft.

Meteorologists suspect that in our temperate zone, rain formation depends more on the odd behavior of freezing moisture in the clouds. When a cloud droplet becomes a solid crystal of ice, it causes the droplets around it to evaporate. They become vapor and cling around the icy fragment, which grows bigger as it zooms through the misty moisture. If the temperature is warm enough, the nucleus becomes liquid water. And since water molecules attract each other, it gathers more droplets and finally becomes a raindrop, too heavy to stay aloft.

Other nuclei may cause raindrops to form at warmer temperatures. These include fragments of salt and soot, dust and solid specks of assorted pollutants. Meteorologists suspect that when conditions are dust right, these nuclei gather films of moisture that attract enough misty droplets to form raindrops of splash down size.

There may be more, much more, to the complicated story. But at present, hopeful cloud seeders depend on spraying likely clouds with either fragments of dry ice or silver iodide    which acts somewhat like ice. Sometimes it works. But unknown factors must still be present, because sometimes our modern method of cloud seeding does not work at all.