Garth MacPhee, age 11, of Peoria, Illinois, for his question:

Why do rain clouds get darker?

A snowy white cloud is made from droplets of moisture too small to be seen without the help of a microscope. Zillions of them are suspended in the air with plenty of space between them. Each is too small to cast a worthwhile shadow of its own and the darting sunbeams dodge between them. Masses of the suspended droplets of moisture mixed with the air look like frothy meringue and the cloud is white.

Millions of these droplets must get together to make even one small raindrop. As weathery conditions develop, the misty moisture condenses into bigger and bigger and still bigger blobs of liquid. These rain drops are big enough to cast worthwhile shadows. Their shadows fall upon each other and all through the cloud. The frothy while cloud turns darker and still darker as the raindrops grow bigger and bigger.

The basic ingredients of ordinary air are nitrogen and oxygen, argon and carbon dioxide; neon and xenon, krypton and helium. The particles of these substances get enough heat energy to separate and become gases at everyday temperatures. They are gases under normal conditions. Water particles need more than twice as much heat to speed up and separate. At 100 degrees Centigrade, the water boils and its separate molecules became gaseous vapor. Iron atoms need the heat energy of 3,000 degrees to separate from each other. With enough heat, almost every ingredient can reach the gaseous state. When this happens, its separate particles zoom off and mingle with the other gaseous ingredients of the air.