Atmospheric Circulation

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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.

An idealized model of 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.

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.

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.

Observed patterns of circulation

One of the implications of the Ferrell hypothesis is that there should be relatively little surface wind near the equator. In this region, surface winds should be flowing toward the equator from the Hadley cells and, when they meet, rising upward into the upper atmosphere. Equatorial regions would be expected to be characterized, therefore, by relatively low pressures with weak surface winds.

But these conditions are exactly what mariners have observed for centuries. Indeed, they long ago gave the name of the doldrums to the equatorial seas. For centuries, ship captains have feared and avoided equatorial waters because winds are so weak and unreliable there that they could easily become stranded for days or weeks at a time.

A second region of calm on the earth's surface, according to the three-cell model, would be around latitude 30°. In this region, air moving downward from both the Hadley and Ferrell cells collides as it reaches the earth's surface, producing regions of high pressure. As in the doldrums, the regions around latitude 30° are characterized by weak, unpredictable winds.

Again, such regions have long been feared and avoided by sailors, who have given them the name of the horse latitudes. The origin of this name comes from the fact that ships bringing horses to the Americas often became becalmed in the waters around 30°N latitude. As supplies ran low, ships were forced to throw their horses overboard. Many stories are told of the waters in these latitudes being littered with the carcasses of the unfortunate animals.

The regions between the horse latitudes and the doldrums (between 0° and 30° latitude) are those in which surface winds flow toward the equator. That flow is not directly from north to south or south to north, of course, because of the Coriolis effect. Instead, winds in these regions tend to blow from the northeast to the southwest in the northern hemisphere and from the southeast to the northwest in the southern hemisphere. Since the winds tend to be strong and dependable—the sorts of wind upon which sailing ships depend—these winds have long been known as the trade winds.

The intersection of the Ferrell and polar cells around latitude 60° is another region at which surface flows of air meet. One, from the Ferrell cell, consists of relatively warm air flowing toward the poles. The other, from the polar cell, consists of much colder air flowing toward the equator. The point at which these two systems meet is called the polar front and is characterized by some of the world's most dramatic storms.

The prevailing direction of surface winds with the Ferrell and polar cells is determined by the Coriolis effect. In the former cell, winds tend to blow from the southwest to the northeast in the northern hemisphere and from the northwest to the southeast in the southern hemisphere. To residents of North America , these prevailing westerlies are well known as the mechanism by which weather systems are carried across the continent from west to east.

In the polar cell, the predominant air movements are just the opposite of the prevailing westerlies: from northeast to southwest in the northern hemisphere and from southeast to northwest in the southern hemisphere.

Patterns of surface pressure

Any student of meteorology understands that conceptual models have only limited applicability to the real world. A number of factors in the real world differ from the ideal conditions used to construct a model. These factors insure that actual weather conditions will be far more complex than the general conditions described above.

For example, both the Hadley and Ferrell models assumed that the earth has a homogeneous composition and that the sun always shines directly over the equator. Neither condition, of course, is actually true. For example, most parts of the planet are covered with water, and land masses are distributed unequally among this watery background. The flow of air in any one cell, then, may be undisturbed for long stretches in one region (as across an ocean ), but highly disrupted in another region (as across a mountainous area).

Useful tools for meteorologists interested in studying air movements are charts of air pressure at various locations on the earth's surface. These charts are of value because, whatever models may predict, we known that in the real world air movements tend to occur from regions of higher pressure to those of lower pressure.

Such charts indicate that certain parts of the planet tend to be characterized by unusually high or low pressure centers at various times of the year. In general, about eight semipermanent high and low pressure cells have been identified. The term semipermanent is used for such cells because they seem to reappear every year on a regular basis.

For example, a semipermanent high pressure area occurs over the Bermuda Islands and persists throughout the year. A semipermanent low pressure—the Icelandic low—is usually found somewhat to the north of the Bermuda high, although it tends to shift from east to west and back again during various parts of the year. During the winter in the northern hemisphere, a semipermanent high exists over Siberia, although by summer it has disappeared and been replaced by a semipermanent low over India. The existence of these semipermanent highs and lows accounts for fairly predictable air movements over relatively large areas of the earth's surface.

The jet streams

During World War II, an especially dramatic type of atmospheric air movement was discovered: the jet streams. On a bombing raid over Japan, a sortie of B-29 bombers found themselves being carried along with a tail wind of about 186 MPH (300 km/h). After the war, meteorologists found that these winds were part of permanent air movements now known as the jet streams. Jet streams are currents of air located at altitudes of 30,000–45,000 ft (9,100–13,700 m) that generally move with speeds ranging from about 30–75 MPH (50–120 km/h). It is not uncommon, however, for the speed of jet streams to be much greater than these average figures, as high as 300 MPH (500 km/h) having been measured.

The jet streams discovered in 1944 are formed along the polar front between the Ferrell and polar cells. For this reason, they are usually known as polar jet streams. Polar jet streams usually travel on a west to east direction between 30°N and 50°N latitude. Commercial aircraft often take advantage of the extra push provided by the polar jet stream when they travel from west to east, although the same winds slow down planes going in the opposite direction.

The pathway followed by jet streams is quite variable. They may break apart into two separate streams and then rejoin, or not. They also tend to meander north and south from a central west-east axis. The movement of the jet streams is an important factor in determining weather conditions in mid-latitude regions.

Since the end of World War II, jet streams other than those along the polar front have been discovered. For example, a tropical easterly jet stream has been found to develop during the summer months over Africa , India, and southeast Asia . Some low-level jet streams have also been identified. One of these is located over the Central Plains in the United States, where topographic and climatic conditions favor the development of unusually severe wind systems.

Other violent wind systems

A number of air movements are not large enough to be described as forms of global circulation although they do cover extensive regions of the planet. Monsoons, for example, are heavy rain systems that sweep across the Indian subcontinent for about six months of each year. They are caused by a massive movement of air from Siberia to Africa by way of India and back again.

During the winter, cold, dry air from central Asia sweeps over India, out across the Indian Ocean, and into Africa. Relatively little moisture is transported out of Siberia during this time of the year. As summer approaches, however, the Asian land mass warms up, low pressures develop, and the winter air movement pattern is reversed. Winds blow out of Africa, across the Indian Ocean and the Indian peninsula , and back into Siberia. These winds pick up moisture from the ocean and bring nearly constant rains—the monsoons—to India for about six months.

Resources

books

Ahrens, C. David, Rachel Alvelais, and Nina Horne. Essentials of Meteorology: An Invitation to the Atmosphere. Belmont, CA: Brooks/Cole, 2000.

Ahrens, C. Donald. Meteorology Today. 2nd ed. St. Paul, MN: West Publishing Company, 1985.

Allen, Oliver E., and the Editors of Time-Life Books. PlanetEarth: Atmosphere. Alexandria, VA: Time-Life Books, 1983.

Eagleman, Joe R. Meteorology: The Atmosphere in Action. 2nd ed. Belmont, CA: Wadsworth Publishing Company, 1985.

Hamblin, W.K., and Christiansen, E.H. Earth's Dynamic Systems. 9th ed. Upper Saddle River: Prentice Hall, 2001.

Hodgson, Michael, and Devin Wick. Basic Essentials: WeatherForecasting. 2nd ed. Guilford, CT: Globe Pequot Press, 1999.

Houghton, John. The Physics of Atmospheres. 3rd ed. Cambridge: Cambridge University Press, 2002.

James, I. N. Introduction to Circulating Atmospheres. New York: Cambridge University Press, 1994.

Lorenz, Edward N. The Nature and Theory of the General Circulation of the Atmosphere. Geneva: World Meteorological Organization, 1967.

Lutgens, Frederick K., Edward J. Tarbuck, and Dennis Tasa. The Atmosphere: An Intorduction to Meteorology. 8th ed. New York: Prentice-Hall, 2000.

Wagner, A. James, "Persistent Circulation Patterns." Weather-wise (February 1989): 18-21.

David E. Newton

KEY TERMS

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Convection: —The transfer of heat by means of a moving fluid.
Coriolis effect: —An apparent force experienced by any object that is moving across the face of a rotating body.
Doldrums: —A region of the equatorial ocean where winds are light and unpredictable.
Horse latitudes: —A region of the oceans around 30° latitude where winds are light and unpredictable.
Jet stream: —A rapidly moving band of air in the upper atmosphere.
Polar front: —A relatively permanent front formed at the junction of the Ferrell and polar cells.
Trade winds: —Relatively constant wind patterns that blow toward the equator at about 30° latitude.