Medios y técnicas para la comunicación externa

2.0 Objectives 3.0 Main Content

3.1 An Idealized Model of Atmospheric Circulation 3.2 The Three Cell Model

4.0 Conclusion 5.0 Summary

6.0 Tutor-Marked Assignment 7.0 References/Further Readings

1.0 Introduction

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 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 atmospheric circulation.

2.0 Objectives

At the end of this unit student should be able to:

i. Explain an idealized model of atmospheric circulation

ii. Enumerate the three-cell model of atmospheric circulation.

3.0 Main Content

3.1 An Idealized Model of Atmospheric Circulation

George Hadley, an English lawyer and amateur scientist in 1730s described an idealized model for the movement of air in Earth’s atmosphere. Hadley recognized 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. Therefore, surface air near the equator will rise into the upper atmosphere and sink from the upper atmosphere to ground level near the poles. In order to balance these vertical movements of air, it was also necessary to hypothesize that air flows across Earth’s surface from each pole back to the equator and, in the upper atmosphere, from above the equator to the poles. This is illustrated in figure 5.1 below:

Fig 5.1 An Idealized View of Three Large Circulation showing Surface Winds

The circular movement of air described by Hadley represents 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 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 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.

A century after Hadley’s initial theory was proposed, a mathematical description of this circular motion 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 description of the way in which surface winds are deflected to the east or west than did Hadley’s original theory.

Self-Assessment Question

 Discuss the George Hadley theory of atmospheric circulation?

3.2 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 by the American meteorologist William Ferrell. Ferrell had much more data about wind patterns than had been available to Hadley. On the basis of the data, Ferrell proposed a three-cell model for atmospheric circulation. Ferrell’s model begins, like Hadley’s, with the upward movement of air over the equator and lateral flow toward the poles along the upper atmosphere. At approximately 30° latitude, Ferrell suggested, the air becomes cool enough to descend to Earth’s surface. Once at the surface, some of the air would 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 known as a Hadley cell.

Ferrell’s new idea was that some of the air descending to Earth near latitude 30° would flow away from the equator and toward the poles along Earth’s surface. It was this flow of air that made Ferrell’s model more elaborate 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. Cool air would descend from higher altitudes and flow toward the equator along Earth’s surface. At about 60° latitude, however, the 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, according to Ferrell, by 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 Earth’s surface once more at about 30° latitude. This downward flow would complete a second convection cell covering the mid-latitudes; it is now known as the Ferrell cell. The second stream above 30° latitude would flow toward the poles and complete the third or polar cell.

Self-Assessment Question

 What were the major improvements of William Ferrell Three Cell Law of Atmospheric circulation over the Hadley original theory?

4.0 Conclusion

The Earth's atmospheric circulation varies from year to year, but the large-scale structure of its circulation remains fairly constant. The smaller scale weather systems- mid-latitude depressions or tropical convective cells- occur "randomly", and long-range weather predictions of those cannot be made beyond ten days in practice, or a month in theory.

5.0 Summary

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.

Hadley idealized model for the movement of air in Earth’s atmosphere posited that, air at the equator is heated more strongly than at any other place on earth, therefore, causing surface air near the equator to rise into the upper atmosphere and sink from the upper atmosphere to ground level near the poles. On the basis of more data Ferrell proposed a three-cell model for atmospheric circulation, in which he opined that that some of the air

descending to Earth near latitude 30° would flow away from the equator and toward the poles along Earth’s surface. It was this flow of air that made Ferrell’s model more elaborate and more accurate than Hadley’s, principle.

6.0 Tutor-Marked Assignment

a) Discuss the George Hadley idealize theory of atmospheric circulation?

b) Enumerate the major improvements of William Ferrell Three Cell Law of Atmospheric circulation over the Hadley original theory?

7.0 References/Further Readings

Ahrens, D. C. (2006), Meteorology Today. Pacific Grove, Calif.: Brooks Cole.

Palmer, T. and Renate H. eds. (2006), Predictability of Weather and Climate. New York: Cambridge University Press, 2006.

Unit 2: Patterns of Circulation and Surface Pressure