The Mystery Of The Wind

If you go past any tall smokestacks daily, you will notice that the air is very rarely still. It is almost always moving in one direction or another. Have you wondered why this is so? Why doesn't the air reach some state of equilibrium and then remain still? Why does it keep moving for millions of years without ever getting where it is going?

There is, of course, the night and day cycle. Air rises when the sun heats the ground during the day and then descends when it cools at night. This is most noticable near the seaside because since land is quicker to gain and shed heat than water is, we get a sea breeze by day and a land breeze by night.

The rotation of the earth is certainly a factor too. But yet there must be more to it than this. The earth always rotates in the same direction, eastward, and the configuration of land and sea on the earth remains the same. So then how do we explain why the wind direction continually shifts?

I think that I have a simple way of explaining it. The never-ending flow of wind is caused by the earth's tilt on it's axis. The total amount of energy that the earth receives over the course of a year remains the same. But due to the axial tilt, the distribution of that heat changes daily. The latitude on the earth's surface at which the sun is directly overhead moves between the Tropics of Cancer and Capricorn (23 1/2 degrees north and south of the equator) during the course of a year.

This means that we could consider the northern and southern hemisphere as two separate "heat hemispheres". Each hemisphere is continually either gaining or losing heat until the annual cycle begins again. Since air rises from where it is warm and descends where it is cold, this means that the atmosphere in each of the two "heat hemispheres" must continually redistribute itself to maintain equilibrium.

Compare it to a closed metal box. If we heat one half of the box and cool the other half, we will get what is known in meteorology as a "Hadley Cell" of air rising in one place, descending in another and, flowing back to the start. This is why there is higher winds at the earth's surface at night and in winter because it causes the air to descend. But if we suddenly change the areas of the box that are heated or cooled, we will get a much more complex flow of air within the box as it seeks an equilibrium.

This is the driving force behind the earth's winds aside from the daily cycle of heating and cooling and the earth's rotation. This means that wind would be at a minimum if there were no axial tilt and a maximum if the tilt were 45 degrees. We are just over the 50 per cent mark with a tilt of 23 1/2 degrees.