Atmospheric Circulation

Atmospheric circulation is the movement of air at all levels of the atmosphere over all parts of the planet. The driving force behind atmospheric circulation is solar energy, which heats the atmosphere with different intensities at the equator, the middle latitudes, and the poles. Differential heating causes air to rise in the atmosphere at some locations on the planet and then to sink back to the earth's surface at other locations. Earth's rotation on its axis and the unequal distribution of land and water masses on the planet also contribute to various features of atmospheric circulation.

The three-cell model

At about the time that Coriolis published his studies on rotating bodies, scientists were beginning to realize that Hadley's single convection cell model was too simple. Atmospheric pressure and wind measurements taken at many locations around the planet did not fit the predictions made by the Hadley model.

Some important modifications in the Hadley model were suggested in the 1850s, therefore, by the American meteorologist William Ferrell. Ferrell had, of course, much more data about wind patterns than had been available to Hadley. On the basis of these data, Ferrell proposed a three-cell model for atmospheric circulation.

Ferrell's model begins where Hadley's began, with the upward flow of air over the equator and its continued flow toward the poles along the upper atmosphere. At approximately 30° latitude, however, Ferrell hypothesized that this air had become sufficiently cooled so that it began to descend to the earth's surface. Once at surface level, some of this air would then flow back toward the equator, as in the Hadley model. Today this large convection current over the third of the globe above and below the equator is called a Hadley cell.

Ferrell's new idea, however, was that some of the air descending to the earth near latitude 30° would flow away from the equator and toward the poles along the earth's surface. It was this flow of air that made Ferrell's model more complex and more accurate than Hadley's. For at about 60° latitude, this surface flow of air collided with a flow of polar air to make two additional convection cells.

Ferrell had agreed with Hadley about the movement of air above the poles. That is, cool air would descend from higher altitudes and flow toward the equator along the earth's surface. At about 60° latitude, however, this flow of polar air would collide with air flowing toward it from the 30° latitude outflow.

The accumulation of air resulting from this collision along latitude 60° would produce a region of high pressure that could be relieved, Ferrell said, by massive updrafts that would carry air high into the atmosphere. There the air would split into two streams, one flowing toward the equator and descending to the earth's surface once more at about 30° latitude. This downward flow would complete a second convection cell covering the mid-latitudes and now known as the Ferrell cell. The second stream above 30° latitude would flow toward the poles and complete the third, or polar, cell.

One can hardly expect a model of the atmosphere developed nearly 150 years ago to be completely valid today. We know a great deal more about the atmosphere and have much more data than Ferrell knew or had. Still, his hypothesis is still valuable because it provides some general outlines about the nature of atmospheric circulation. It also explains a number of well-known circulation phenomena.