What Makes A Wind?

Sometimes when we are outdoors, a sudden and mysterious thing takes place. A wind begins to flow. We cannot see it, but we feel it, and we have no idea what started it.

A wind is simple the motion of air over the earth. What causes the air to move? All winds are caused by one thing— a change in temperature. Whenever air is heated it expands. This makes it lighter, and lighter air rises. As the warm air rises, cooler air flows in to take its place. And this movement of air is wind !

There are two kinds of winds, those that are part of a world-wide system of winds, and local winds. The major wind systems of the world begin at the Equator, where the sun's heat is greatest.

Here the heat rises to high altitudes and is pushed off towards the North and South Poles. When it had journeyed about one-third of the distances to the poles, it has cooled and begins to fall back to earth. Some of this air returns to the equator to be heated again, and some continues on to the poles.

These types of winds, which tend to flow in the same general direction all the year round, are called "prevailing winds". But these global winds are often broken up by local winds which flow from different directions.

Local winds may be caused by the coming of cold air masses with high pressure, or warmer air masses with low pressure. Local winds usually do not last long. After a few hours, or at most a few days, the prevailing wind pattern is present again.

Other local winds are caused by the daily heating and cooling of the ground. Land and sea breezes are examples of this kind of wind. In the daytime, the cool air over the ocean moves inland as the sea breeze. At night, the ocean is warmer than the ground, so the cooler air moves out to sea as the land breeze.