Arthur Rathbun, age 13, of Staten Island, New York, for his question:

What does sodium chloride do to clouds?

A cloud is merely a mass of minuscule droplets of water floating aloft in the air. A raindrop large enough to fall forms when a zillion of these misty droplets gather to¬gether. This sounds simple enough, but when cloud material is viewed under a powerful magnifying glass, the congedling process seems downright impossible. The misty droplets are so small and so far apart that there seems no way for them to get together. We now know that cloud droplets do indeed need help in order to gather together in raindrops or snow¬flakes. They stick to dusty fragments that happen to be floating in the cloud's misty mixture of air and moisture.

These cores may be specks of soot. As they drift around, droplets of moisture gather around them. Molecules of sodium chloride also act as rain making nuclei. This chemical is, of course, ordinary table salt, and vast amounts of it are dissolved in the oceans. When surface sea water evaporates, most of its dissolved salt and other chemicals stay behind. But occasionally a few particles of sodium chloride from the ocean may Join the gaseous air. Later they become nuclei suitable for gathering misty cloud droplets. This airborne sodium chloride is a rainmaker.