Why Does the Wind Blow?

I had lived in Kansas less than a week the first time I heard why it was so windy. I moved in from Nebraska, and all I had to do was answer the question, “Where are you from?” to hear the joke that disparages the states to the north and south. Like many great turns of humor, variations exist for virtually every windy place on earth. I’m here to tell you the winds in Kansas have little to do with Oklahoma or Nebraska. So why does the wind blow?

Weather is incredibly complicated, and many factors affect the outcome, so any explanation of fewer than 50 pages must be over-simplified. While predicting the wind next Tuesday is difficult, overarching trends in air currents can be seen. For example, while the wind isn’t always from the south in Kansas, it is more often than not. Three main forces play a part in these prevailing winds.

First, wind is created by air flowing from high pressure to low pressure in an attempt to equalize the pressure. In the weather prediction game, this is called the pressure gradient force, and it’s a biggie in weather creation. The differences in pressure are caused primarily by heating and cooling of air.

Second, because the earth rotates, everything on it is influenced by the Coriolis effect. This “force” pushes everything on a rotating body. If the object rotates clockwise, it pushes to the left of the rotation direction; if counterclockwise, to the right. In the Northern Hemisphere, looking down from the North Pole, the earth rotates counterclockwise, and thus winds are pushed to the right by the Coriolis effect, and in the Southern Hemisphere to the left.

The third major force in wind creation is atmospheric circulation, which is a fancy way of talking about large-scale movement of air caused by the changing distance from the sun. Our atmosphere gets divided into six cells, three in each hemisphere. They cycle air from the earth’s surface into the higher atmosphere and then back toward the surface and along it.

Let’s use the Northern Hemisphere as an example. At the equator, air is heated and, because it is less dense than cold air, rises, thus causing a lower pressure zone at the equator. Air near the surface from farther north then moves south to equalize the pressure. As the air rises, it cools, and, pushed from below, it turns north toward the cooler areas, until it cools enough to return toward the surface. This creates a Hadley cell from the equator to about 30 degrees north latitude (in North America this cuts through the three most southern states, at the neck of Florida, Louisiana at New Orleans, and Texas at about Houston). The winds at the surface are the “trade winds,” and they flow toward the southwest (moving north to south and being deflected to the right by the Coriolis effect).