Why does the wind blow?
(Lansing State Journal, November 12, 1997)
The Earth is surrounded by a layer of gas molecules called the atmosphere. The atmosphere is made up of primarily nitrogen and oxygen gas. Winds, then are movements of many of these molecules. We often collectively refer to the molecules in the atmosphere as air. Though air may seem weightless, it is always pressing against us. In fact, air presses in on every square inch of our bodies with about 14.7 pounds of force.
Winds are caused by differences in air pressure between areas. The air pressure varies in different areas because of variations in temperature. Warmer air expands as its molecules spread far apart so it tends to weigh less and its pressure is lower. The molecules in cold air, however, press more tightly together which makes the air weigh more and thus exerts a higher pressure. The molecules in air generally flow from areas of higher pressure to areas of lower pressure. This flow of air is what we call wind.