Atmospheric Circulation

As early as the 1730s, the English lawyer and amateur scientist George Hadley described an idealized model for the movement of air in the earth's atmosphere. It is well known, Hadley pointed out, that air at the equator is heated more strongly than at any other place on Earth. In comparison, air above the poles is cooler than at any other location. One can hypothesize, therefore, that surface air near the equator will rise into the upper atmosphere and, above the poles, sink from the upper atmosphere to ground level. In order to balance these vertical movements of air, it was also necessary to hypothesize that air flows across the earth's surface from each pole back to the equator and, in the upper atmosphere, from above the equator to above the poles.

The movement of air described by Hadley can be called a convection cell. The term convection refers to the transfer of heat as it is carried from place to place by a moving fluid, air in this case.

Hadley knew, of course, that surface winds do not blow from north to south in the northern hemisphere and from south to north in the southern hemisphere, as his simple model would require. He explained that winds actually tend to blow from the east or west because of Earth's rotation. The spinning planet causes air flows that would otherwise be from the north or south to be diverted to the east or west, Hadley said.

Atmospheric circulation results in prevailing global wind belts. Illustration by K. Lee Lerner, with Argosy. The Gale Group.

As an analogy of how this change could occur, suppose that you are sitting on a spinning merry-go-round trying to catch a ball thrown by a friend at the center of the platform. The ball will obviously travel in a straight line from the thrower to the intended catcher on the rim. But to the catcher, the ball will appear to follow a curved path, and he or she will have to reach out to catch the ball.

A century after Hadley's initial theory was proposed, a mathematical description of this "merry-go-round effect" was published by the French physicist Gaspard Gustave de Coriolis. Coriolis was able to prove mathematically that an object in motion on any rotating body always appears to follow a curved path in relation to any other body on the same rotating body. This discovery, now known as the Coriolis effect, provided a more exact explanation of the reason that surface winds are deflected to the east or west than did Hadley's original theory.