Paula Cook, age 10, of Spokane, Washington, for her question:

Why do some clouds contain water and others don't?

When we glance skyward, we may be rewarded by the sight of a soft white cloud or startled by a dark rain cloud. They look very different and arouse different feelings in us. But actually they are very much alike. One seems to be made of fluffy cotton; the other of sturdy lead. But looks can fool us. Both those clouds are made of water  ¬either in tiny ice crystals or in fine liquid droplets.

In most clouds the misty droplets are so small that it takes a million of them to make a single raindrop. Because they are so tiny, the earth's gravity cannot pull very hard to make them fall. As long as the cloud stays warm, these little wet, midgets rush around very fast    too fast to stick together and build sizable raindrops. But if the air cools, they have to slow down. If they collide with solid specks of dust, lots of droplets can stick together and form raindrops that are heavy enough to come tumbling down. So the dust, the misty moisture and the temperature all help to decide whether a cloud can promise a shower or a fine day for a picnic.