Bryn McDougal, age 12, of Tulsa, Okla. ,question:

Why does wind direction change at different altitudes?

Sometimes a flock of little gray clouds scoots along, while a stately parade of big white clouds at a higher level moves in a different direction. Since both groups are blown by winds, obviously the breezy traffic regu¬lations change at different altitudes. Actually, our winds are confined below about ten miles. But within this limit, upstairs and downstairs winds usually blow in different directions.

The winds are the global transportation system. They run the world¬wide circulation of the atmosphere. Actually they run two systems to mix and mingle the air to a height of about ten miles. One is a permanent system of permanent winds that circle the globe In six well defined belts. Each belt has an up and down elevator that circulates upstairs and down¬stairs wind, which blow in different directions.

Along certain latitudes, these airy elevators of low and high pressure lift and lower vast masses of air. This permanent system is governed by the sun, the rotating earth and the nature of gases. The spinning earth veers all winds north of the Equator to the right and those south of the Equator to the left. The three wind belts in each hemisphere match each other, though the wind directions are reversed.

The lower air is warmed or chilled by the surface. Warm air expands, spreads thin and rises aloft, where it cools. Chilly air is dense and tend to descend in heavy masses. Winds are set in motion because heavy, high¬pressure air blows into zones of light, low pressure air.

Above the warm Equator, currents of light air rise to about ten miles. At the surface, the northeast and southeast trade winds arrow into this wide belt of low pressure. They ascend the up elevator. Aloft, the light air cools and separates into two upstairs winds    blowing away from the Equator.

In our hemisphere, upper level southeast winds blow above the northeast trade winds. At latitude 30 degrees north ofthe Equator, the cool  winds upstairs pile up and descend in heavy masses. This creates a down elevator of high pressure. Some of the upper air ignores the elevator and continues on toward the North Pole.

Part of the descending air rejoins the surface trade winds and makes another trip toward the Equator, up the elevator and around the merry go round. The rest joins the prevailing westerlies that circle the globe between latitudes 30 and 60 degrees.

Along 1atitude 60 degrees, the westerlies clash with the polar easterlies, coming down from the frozen north. Their warmer air wedges above the masses of cold polar air, creating a high pressure zone. Here the clashing air masses upset the winds at all levels. These weathery conflicts cause blustery polar fronts and the prevailing westerlies carry them across North America.

This permanent system of breezy circulation whirls upstairs and down¬stairs winds in different directions. :4eantime, numerous highs and lows create pockets of turmoil in the global pattern. A monstrous hurricane spirals wild winds through the upper and lower levels. The turbulence of a major thunderstorm often reverses the upper winds over a wide area. Stormy weather fronts often cause upper and lower winds to blow in different directions.