Larraine Riopel, age 12, of Westbr3ok, Maine, for her question:
What causes the wind?
Wind, of course, is no more than moving air and air is a mixture of gases. It must obey the laws which apply to all gases. When heated, it must expand. As it expands it becomes thinner and cooler. Any amount of gas must spread itself out as far as possible. The bigger the space, the thinner it becomes. When compressed into a smaller space, a gas becomes more dense.
It is not hard to imagine that a solid chair or a tree is made of tiny atoms and molecules. Even though we cannot touch it or see it, the air too is made of atoms and molecules. In gaseous form, these particles are separated and traveling at high speed. In a thimbleful of air there are millions of traffic collisions between the gas particles every second. On a grand scale, these rules and regulations are obeyed throughout the atmosphere which stretches hundreds of miles above our heads.
When heated, air molecules travel faster and spread out. Warm air expands. There is more space between the particles. Such a patch of warm, light air is likely to develop over the equator. Suppose this light air brushes with a patch of dense, heavy air. These crowded particles will rush right into the underpopulated area of light air„ When this happens on a grand scale, great masses of air surge from one place to another. The wind blows.
The wind, of course, is very fickle, It blows this way and that in a mighty gale or a gentle breeze. This is because the sun shines and the earth turns on its axis. The beaming sun heats up some parts of the earth more than others.
The air gets its heat from the earth or sea below it, so the atmosphere also heats up in patches. And where it becomes hot., it becomes light. Heavier air from nearby areas comes in and the wind blows.
If the earth did not rotate on its axis, all the winds would blow directly north and south. Warm air at the equator would rise straight up and cooler air from the poles would blow down to replace it. But the earth is a spinning globe. At the equator it moves eastward at 1,000 miles an hour. At the poles it does not move at all. Half way between it spins at 700 miles an hour. This variation in rotation speed puts a twist on the winds.
The great winds which are twisted by the spinning earth blow day and night clear around the globe. Around the equator they are the easterly trade winds. Further north and south, as the globe tapers towards the poles, the prevailing winds blow from the west. At the poles, cool, heavy air presses out in all directions and is twisted to blow eastward by the spinning globe,