Sandra i‑angloski, aged 11, of Colonial Heights,Va, , for her question:

What causes air currents?

The air is forever restless. Some of it blows in a ring around the globe. Some rises up, rolls over and tumbles down again. Sometimes the upper air travels in the opposite direction from the air below it. All this restless motion can be called air currents, And all of it caused. by the sun.

The sun heats the ground. And the ground heats the air above it. Warm air expands and rises, The air over a. desert on a warm day tends to be light end rising., A plane flying over such a spot might feel a rising current of air,

The desert may be near the sea. The sea heats more slowly than the land. The air above the sea world be cooler than the air above the desert. Cool air tends to be heavier and denser than warm air. It also tends to flow in to fill areas where the air is lighter and less dense. This kind of air current is the wind.

Currents of warm air forever rise from the equator. Cooler air flows in from the north and south. The spinning earth deflects these currents of air to form the trade winds that girdle the whole world.