Mary Clark, age 10, of Costa Mesa, California, for her question:

What causes air currents?

An air current may be a gentle breeze or a howling gale. It may blow from any compass direction, it may rise up from the ground or sink down from higher levels. It may be just a draft from an open window. All of these breezy events are streams of air on the move from one place to another place. And all of them are governed by more or less the same rules.

Air is a fluid substance and its gaseous molecules react to changes in temperature. When cool, they tend to crowd closer together. When warm, they tend to expand and spread farther apart. The whole atmosphere is a patchwork of different air pockets. As a rule, pockets of warmer air spread out and become lighter. Cooler, heavier air tends to flow and blow into regions of thinner, lighter air. As a rule, a breezy current is moving toward a less crowded pocket of air.