Glenda Carpenter, age 13, of Charlotte, No. Carolina, for her question:

What causes the wind to blow?

The wind is mysterious because it is invisible. Actually, it is a stream of moving air and though we can feel it and see how it bends the boughs, we cannot see how it gets from here to there. However, scientists can explain why it blows in this or that direction. It seems that some masses of air are denser and heavier than others. And masses of dense, heavy air flow naturally into masses of thin, lighter air  ¬much as water flows down a slope.

As a rule, heavy air is cool and light air is warm. Cold, heavy air above the North Pole starts southward toward the equator. Warm air above the equator expands and rises and the trade winds blow into this region of light, low pressure. These are global winds. But the same forces of high and low pressure also set our local breezes in motion. The blowing winds move masses of heavier air to mix and mingle with masses of lighter air.