Sheila Kerr, Age 11, of Downsview, Ont., Canada, for her question:
What causes the prevailing winds?
Local breezes blow soft and loud from all directions. But they are mere ruffles in the permanent wind system that whistles steadily around the globe. This global system of prevailing winds is governed mainly by the beaming sun and the rotating earth.
Most of North America is swept by the prevailing westerlies. These steady winds from the southwest circle the globe between latitudes 30 and 60 degrees. The westerly belt is sandwiched between the polar easterlies that howl around the top of the globe and the northeast trades that blow toward the equator. The Southern Hemisphere also is circled by three belts of prevailing winds.
We have a general idea of what causes the wind. The gaseous atmosphere is unstable. Warm air expands, forming a light mass of law pressure. It tends to rise aloft where it cools and condenses. Cool air is dense and heavy, forming masses of high pressure. It tends to sink, and masses of high pressure tend to blow toward masses of low pressure.
The air gets its heat from the land and sea, and the sun warms the earth unevenly. On a global scale, polar air is cool and dense while tropical air is warm and light. The winds, then, should circulate due north and south between the poles and the tropics. This neat plan is upset by the earth's rotation. What's more, currents of rising and sinking air create a surface level and an upstairs level of global winds.
The earth's rotation pushes at winds blowing due north and south. It shoves or deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This force, we suspect, causes the planetary winds to blow east and west around the globe.
At the equator, the light air rises a mile or two and begins to cool. It divides and two opposite streams blow away from the tropics. At latitude 30, some of this upstairs air is cool enough to sink and pile up heavy masses of high pressure. Some of this air turns back to the equator as the trade winds, and some turns north to join the westerlies. More dense masses of air sink at the poles and start for the equator. At latitude 60, these pcadex winds clash with the warmer westerlies in a restless weather front.
North of the equator, where the global winds are shoved to the right, the trade winds blow from the northeast. The westerlies blow from the southwest, and the polar winds blow from the northeast. South of the equator, where the global. Winds are shoved to the left, the trades blow from the southeast, the westerlies from the northwest and the polar winds from the southeast.