Why Does the Wind Blow?

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Illustration by Nate Skow based on NOAA graphic/www.srh.noaa.gov
Forces behind the wind are very different depending on your latitude.

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).

At the poles, a similar loop, called a polar cell, occurs in the same direction – cooling air at the poles moves toward the earth’s surface and then moves southward along the surface until it is heated and rises at about 60 degrees north latitude (most of Alaska and all but the very tip of Greenland are north of 60 degrees). This creates the “polar easterlies” (again, moving north to south and deflected to the right).

The cell in between is called a Ferrel cell or a mid-latitude cell. In this cell, the air moves south to north. This cell is essentially worked upon by the cells beside it. The cold air from the polar cell warming and moving upward, and the warm air from the Hadley cell cooling and moving downward cause the air in between to move in the opposite direction of the stronger Hadley and polar cells. Because of how it is formed, a Ferrel cell is less stable and more capable of variation. At the boundaries between the cells are jet streams (the polar jet stream often shows up on weather maps of North America), which also play a part in our weather.

This model begins to explain the large-scale movements of our atmosphere. Many other factors affect winds. What the wind flows over can have a major impact: Is it water or land? What is the altitude and heat potential of the land (is it snow, grass or concrete)? What ocean currents or temperatures are affecting the water? But I’m not out to make anyone a meteorologist, I’d just like to start everyone down the road to understanding the wind.

Web editor Jenn Nemec moved to Kansas at the tender age of 12; later, she returned by choice.

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