Donna Spoken, age 11, of W. St. Paul, Minnesota, for her question:

What makes clouds move?

In fairy tales of the Far East, the magical characters traveled aloft on flying carpets. Most of us don't believe this. But watching the clouds sail by on high it's easy to see where people got the delightful idea. For the cushiony clouds seem to drift across the sky like flying carpets. Actually, of course, there are less magical reasons why they move from here to there.

Most of the clouds look very peaceful, but looks can fool you. For one thing, all of them are on the move    up or down, east or west or some other direction. What's more, every cloud has a life cycle and no cloud lives forever. It may live for just a few stormy hours. Or it may lead a quiet life, drifting for several days over land and sea. But sooner or later it disappears    while other new clouds are born elsewhere.

A cloud, you would think, must be light as a feather to float aloft in the air. But actually it is quite heavy and even a small one weighs 100 tons. It is made of water, but its mini droplets of moisture are so tiny that it takes them a long, long time to sink down through the air.

The average cloud is born a mile or more above the ground. It is formed when cool air changes some of its gaseous water vapor into liquid droplets of moisture. The newborn cloud is a misty fog of separate droplets, with plenty of airy space between them. If nothing more happened, the cloud would sink slowly, slowly down to earth, pulled by the force of gravity.

But almost always there are weathery forces up there in the atmosphere. There is the wind, blowing gently or strongly from who¬ knows where. The wind is moving air and this moving air moves the clouds. Sometimes the wind aloft blows in the same direction as the ground level wind. Then all the clouds blow in the same direction as your windblown hair.

But often the low level winds and the high level winds blow in different directions. Then, for example, we may see the low level clouds moving east while a flock of upper level clouds is moving west. This strange event shows us which way the upstairs breezes are blowing  ¬for those traveling clouds must be blown along by the winds.

Clouds, of course, are very changeable. Lazy white summer clouds change their shapes as they drift along. Sometimes a flock of them pile up and build a wild, whirling thunderhead. Sometimes a sunny cloud turns dark and rainy. These weathery changes are caused by conflicting masses of air    dry or damp, warm or cool. And here, too, the wind plays a part, tossing the misty clouds around.

Since the wind drives the clouds, their speed must be related to its speed. This is why the clouds move faster on a breezy day. When grey clouds hang like quiet curtains overhead, we know that the air is calm, with hardly a breeze to ruffle the trees.