Atmospheric circulation

An idealized model of atmospheric circulation

Observed patterns of circulation

Patterns of surface pressure

The jet streams

Other violent wind systems

Resources

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 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 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.

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.

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.

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 flow toward the equator from the Hadley cells and, when they meet, rise into the upper atmosphere. Equatorial regions would be expected to be characterized, therefore, by relatively low pressures with weak surface winds. Those conditions had been observed for centuries by mariners, who long ago gave the name of the doldrums to the equatorial seas. Sailing ship captains feared and avoided equatorial waters because winds are so weak and unreliable that they could easily become stranded for days or weeks at a time.

A second region of calm on 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 Earth’s surface, producing regions of high pressure. As in the doldrums, the regions around latitude 30° are characterized by weak and unpredictable winds. Sailors named these regions the horse latitudes because ships carrying horses to the Americas often became becalmed in the waters around 30°N latitude; as supplies ran low, sailors sometimes threw the horses overboard.

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 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. Because 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 Ferrell 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 carry weather systems 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

Conceptual models of meteorologic phenomena have only limited applicability in the real world because a number of factors depart from the ideal conditions used to develop models. These factors ensure that actual weather conditions will be far more complicated than the general conditions described above.

For example, both the Hadley and Ferrell models assume that Earth has a homogeneous composition and that the sun always shines directly over the equator. Neither condition is strictly true. Most parts of the planet are covered with water, and land masses are distributed unevenly. The flow of air in any one cell, therefore, may be undisturbed for long stretches in one region (as across an ocean), but highly disrupted in another region (as across a mountain range).

Charts showing air pressure at various locations on Earth’s surface are useful tools for meteorologists, because air flows 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. Eight semipermanent high- and low-pressure cells that reappear every year on a regular basis have been identified.

A semipermanent high pressure zone persists in Bermuda throughout the year. A semipermanent low pressure zone—the Icelandic low—is usually found north of the Bermuda high and tends to shift from east to west and back again during the year. During the winter in the northern hemisphere, a semipermanent high that exists over Siberia disappears and is replaced by a semipermanent low over India each summer. The existence of these semipermanent highs and lows accounts for fairly predictable air movements over relatively large areas of 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, and velocities as high as 300 mph (500 km/h) have 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 path followed by jet streams is variable. They may break apart into two separate streams and then

KEY TERMS

Convection —The transfer of heat by means of a moving fluid such as air in Earth’s atmosphere.

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.

rejoin or remain as a single stream. They also tend to meander north and south from a central west-east axis. The movement of the jet streams has a major effect on weather 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 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, 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.

See also Air masses and fronts; Global climate; Monsoon.

Resources

BOOKS

Ahrens, Donald C. Meteorology Today. Pacific Grove, Calif.: Brooks Cole, 2006.

Palmer, Tim, and Renate Hagedorn, editors. Predictability of Weather and Climate. New York: Cambridge University Press, 2006.

David E. Newton

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.

See also Air masses and fronts; Global climate; Monsoon.

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

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.

Atmospheric Circulation

Introduction

Atmospheric circulation includes the movement of air on a global scale. It is the manner in which that heat is distributed throughout the atmosphere, from equatorial regions that are warmer to polar regions that are colder. The circulation of air in the atmosphere varies somewhat from year to year, but overall, the basic mechanism of circulation remains the same. This helps produce a stable global climate.

Atmospheric circulation is linked to ocean temperature and winds. An example of this relationship are two naturally occurring variations in the temperature of the tropical Pacific Ocean known as El Niño and La Niña. The warming sea temperature in the case of El Niño and cooling temperature in the case of La Niña can persist for several years. These alterations affect wind patterns, which in turn alter weather. For example, the greater frequency and severity of tropical storms that occurred in the equatorial Atlantic Ocean in 2005 and 2006 (the best known example from the U.S. perspective was Hurricane Katrina) has been linked to the increased upper atmosphere easterly blowing winds that were stimulated by a La Niña. These winds reduce vertical wind shear— wind changes with altitude—which increases the likelihood of the formation of thunderstorms and tropical storms.

Whether global warming is altering atmospheric circulation is still debatable, as of 2007. However, measurements of circulation of air over the tropical Pacific have provided evidence of human-induced changes.

Historical Background and Scientific Foundations

Circulation of air in the atmosphere has likely occurred ever since the formation of the modern-day atmosphere following the appearance of life on Earth. The basis for

atmospheric circulation is the differential heating of Earth. Tropical regions are heated more so than the polar regions because the thickness of the atmosphere that the

sun's rays penetrate through is greater at the poles. As a result, the tropical atmosphere is warmer than the atmosphere over the poles, which causes the movement of warm air northward or southward from the equator.

Winds are essential to atmospheric circulation. Aside from winds that occur temporarily, the general circulation of the atmosphere involves surface winds that blow regularly. There are three so-called belts of wind in each hemisphere. Polar easterlies are found from 60 to 90 degrees latitude. In the Northern Hemisphere, these blow from the northeast to the southwest, and from the southeast to the northwest in the Southern Hemisphere. The prevailing winds from 30 to 60 degrees latitude are called prevailing westerlies. In the Northern Hemisphere, the prevailing westerlies blow from the southwest to the northeast, and from the northwest to the southeast in the Southern Hemisphere. Finally, the region from the equator to 30 degrees latitude north and south is the area where tropical easterlies are found. In the Northern Hemisphere, they blow from the northeast to the southwest, and from the southeast to the northwest in the Southern Hemisphere.

Tropical easterlies are also called trade winds. The northern and southern trade winds converge near the equator in a zone; this intertropical convergence zone is an area of cloud and thunderstorms that encircles Earth. It is also an area where winds can be light and variable. Centuries ago, mariners could be calmed for days in the doldrums.

The directions of the winds in these belts are influenced by what are known as cells. The Northern and Southern Hemisphere polar winds are guided by northern and southern polar cells, which are created because of Earth's rotation. The hemispheric trade winds are guided by a cell called the Ferrel cell, while the tropical easterlies are guided by the Hadley cell.

The polar and Hadley cells are closed loops; warm, more southerly air rises and moves northward. As the air cools, it sinks and moves southward where it heats and rises again, completing the loop. The reason that the winds in the belts do not move north-south and south-north is because of the Coriolis effect—the influence that Earth's rotation exerts.

In addition to these three air circulation cells, other cells operate horizontally, from west-to-east to east-to-west. The horizontal circulation occurs because Earth's surface is composed of land and water. Water absorbs heat more slowly than does the land, and loses heat to the atmosphere more slowly than land. On a small scale, the result is clear along the ocean coast, where the winds blow onto shore during the day, as cooler air over the sea migrates toward land, and blows out to sea at night, when the air over the ground is cooler than the sea air. On a much larger scale, this back and forth flow of air occurs over a period of months or even years.

WORDS TO KNOW

CORIOLIS EFFECT: A pseudoforce describing the deflection of winds due to the rotation of Earth, which produce a clockwise or counterclockwise rotation of storm systems in the Southern and Northern Hemispheres, respectively.

GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.

ICE AGE: Period of glacial advance.

INTERTROPICAL: Literally, between the tropics: usually refers to a narrow belt along the equator where convergence of air masses of the Northern and Southern Hemispheres produces a low-pressure atmospheric condition.

POLAR CELLS: Air circulation patterns near the poles: relatively warm, moist air approaches the pole at a high altitude, cools, sinks at the pole, and flows southward at a lower altitude. Because of Earth's rotation, air approaching or receding from the poles flows eastward, producing the polar easterlies.

TRADE WINDS: Surface air from the horse latitudes (subtropical regions) that moves back toward the equator and is deflected by the Coriolis Force, causing the winds to blow from the Northeast in the Northern Hemisphere and from the Southeast in the Southern Hemisphere. These steady winds are called trade winds because they provided trade ships with an ocean route to the New World.

WIND CELLS: More commonly called convective or convection cells; vertical structures of moving air formed by warm (less-dense) air welling up in the center and cooler (more-dense) air sinking around the perimeter. Thunderstorms are shaped by convective cells.

Atmospheric circulation is crucial for the global climate and the global pattern of precipitation. The movement of air from regions of low pressure, which tends to encourage precipitation, to regions of high pressure, which do not favor precipitation, helps to distribute moisture through the atmosphere.

Impacts and Issues

Climate changes due to altered atmospheric circulation are not new. Scientists have evidence that altered air circulation in the tropical Pacific Ocean, similar to that which occurs during El Niño, triggered a global climate change about one million years ago. Then, the changed circulation of air caused the polar ice sheets to grow in area, which lengthened the periods of glaciation (the ice ages).

This research has relevance in modern times, for it indicates that tropical regions are very influential to global climate. Thus, conditions that alter atmospheric circulation can change the global climate.

It is known that Earth's atmosphere is warming, due to the increased retention of heat. One reason for this has been suggested to be the gradual accumulation of greenhouse gases—gases produced by human activities. The link between human activity and atmospheric change used to be very contentious. In 2007, however, only a small minority of scientists still argued that atmospheric warming is free from human influence.

The question of whether human activities are influencing atmospheric circulation, however, remains contentious. As one example, a paper published in Nature in 2006 reported on data gathering from 1861 to the early years of the 21st century, which revealed that the difference in pressure between the higher pressure of the western Pacific to the lower pressure of the eastern Pacific has declined over the past 150 years. The data were used in several computer models of climate; some models factored in the influence to pressure change of only natural conditions, and others had the added influence of human activities. The model that incorporated human-influenced atmospheric change most closely matched the actual data.

Other scientists are skeptical of the concluded link between human activities and atmospheric change, because data collected on sea surface temperatures for a much longer time do not support the air pressure data. Whether or not human activities have influenced the changed environment over the Pacific Ocean, however, it is clear that change has occurred, and that such changes in atmospheric circulation do affect global climate.

See Also Abrupt Climate Change; Atmospheric Chemistry; Atmospheric Pollution; Atmospheric Structure; El Niño and La Niña; Global Warming; Greenhouse Effect.

BIBLIOGRAPHY

Books

Barry, Roger G. Atmosphere, Weather and Climate. Oxford, United Kingdom: Routledge, 2003.

Lutgens, Frederick K., Edward J. Tarbuck, and Dennis Tasa. The Atmosphere: An Introduction to Meteorology. New York: Prentice Hall, 2006.

Trefil, Calvo. Earth's Atmosphere. Geneva, IL: McDougal Littell, 2005.

Periodical

Vecchi, Gabriel A., Brian J. Soden, Andrew T. Wittenberg, et al. “Weakening of Tropical Pacific Atmospheric Circulation Due to Anthropogenic Forcing.” Nature 441, no. 7089 (May 4, 2006): 73–76.

Atmospheric Circulation

Introduction

Atmospheric circulation includes the movement of air on a global scale. It is the mechanism by which heat is distributed from the warmer equatorial regions to the cooler temperate and polar regions. The detailed circulation of air in the atmosphere varies somewhat from year to year, but the basic mechanism of circulation remains the same. This helps produce a stable global climate.

Atmospheric circulation is linked to ocean temperature, ocean circulation, and winds. An example of this relationship are two naturally occurring variations in the temperature of the tropical Pacific Ocean known as El Niño and La Niña. The warming sea temperature in the case of El Niño and cooling temperature in the case of La Niña can persist for several years. These alterations affect wind patterns, which in turn alter weather. For example, the greater frequency and severity of tropical storms that occurred in the equatorial Atlantic Ocean in 2005 and 2006 (the best known example of which, from the U.S. perspective, being Hurricane Katrina) has been linked to the increased upper atmosphere easterly blowing winds that were stimulated by a La Niña. These winds reduce vertical wind shear—wind changes with altitude—which increases the likelihood of the formation of thunderstorms and tropical storms.

To what extent global warming is altering atmospheric circulations was still debatable as of 2008. However, measurements of circulation of air over the tropical Pacific have provided evidence of human-induced changes.

Historical Background and Scientific Foundations

The basis for atmospheric circulation is the differential (uneven) heating of Earth. Tropical regions are heated more than polar regions because the sun’s rays strike the poles at a glancing angle. As a result, the tropical atmosphere is warmer than the atmosphere over the poles. Because warm air is less dense than cool air, warm air tends to float upward and cool air to sink downward wherever the two coexist. In Earth’s atmosphere, this density difference—caused by regional heating differences—causes the movement of warm air northward and southward from the equator. Near the poles, warm equatorial air radiates its heat away to space and becomes cool air, which makes its way back toward the equator to be heated again. As a whole, the system can be thought of as a rotating fluid cycle driven by uneven heating from the sun. Its details, of course, are more complex.

Atmospheric circulation produces winds: winds are simply air in motion, driven ultimately by regional differences in air temperature (and thus in density). Aside from winds that occur temporarily and local, the general circulation of the atmosphere consists of winds that tend to blow steadily. There are three belts of wind in each hemisphere. Polar easterlies are found from 60 to 90 degrees latitude. In the Northern Hemisphere, these blow from the northeast to the southwest, and from the southeast to the northwest in the Southern Hemisphere. The prevailing winds from 30 to 60 degrees latitude are called prevailing westerlies. In the Northern Hemisphere, the prevailing westerlies blow from the southwest to the northeast, and from the northwest to the southeast in the Southern Hemisphere. Finally, the region from the equator to 30 degrees latitude north and south is the area where tropical easterlies are found. In the Northern Hemisphere, they blow from the northeast to the southwest, and from the southeast to the northwest in the Southern Hemisphere.

Tropical easterlies are also called trade winds. The northern and southern trade winds converge near the equator in a zone; this intertropical convergence zone is an area of cloud and thunderstorms that encircles Earth. It is also an area where winds can be light and variable. Centuries ago, mariners could be calmed for days in the doldrums.

The directions of the winds in these belts are influenced by what are known as cells. The Northern and Southern Hemisphere polar winds are guided by northern and southern polar cells, which are created because of Earth’s rotation. The hemispheric trade winds are guided by a cell called the Ferrel cell, while the tropical easterlies are guided by the Hadley cell.

The polar and Hadley cells are closed loops; warm, more southerly air rises and moves northward. As the air cools, it sinks and moves southward where it heats and rises again, completing the loop. The reason that the winds in the belts do not move north-south and south-north is the Coriolis effect—a result of Earth’s rotation.

In addition to these three air circulation cells, other cells operate horizontally, from west-to-east to east-to-west. The horizontal circulation occurs because Earth’s surface is composed of land and water. Water absorbs heat more slowly than does the land, and loses heat to the atmosphere more slowly than land. On a small scale,

WORDS TO KNOW

CORIOLIS EFFECT: A force exemplified by a moving object appearing to travel in a curved path over the surface of a spinning body.

GREENHOUSE GASES: Gases whose accumulation in the atmosphere increase heat retention.

ICE AGE: Period of glacial advance.

INTERTROPICAL: Pertaining to a narrow belt along the equator where convergence of air masses of the Northern and Southern Hemispheres produces a low-pressure atmospheric condition.

POLAR CELLS: Part of a group of atmospheric air circulation patterns occurring at Earth’s poles.

TRADE WINDS: Surface air from the horse latitudes that moves back toward the equator and is deflected by the Coriolis Force, causing the winds to blow from the Northeast in the Northern Hemisphere and from the Southeast in the Southern Hemisphere.

WIND CELLS: Vertical structures of moving air formed by warm (less-dense) air welling up in the center and cooler (more-dense) air sinking around the perimeter; also called convective or convection cells.

the result is clear along the ocean coast, where the winds blow onto shore during the day, as cooler air over the sea migrates toward land, and blows out to sea at night, when the air over the ground is cooler than the sea air. On a much larger scale, this back and forth flow of air occurs over a period of months or even years.

Atmospheric circulation—which is itself shaped by the distribution of landmasses over Earth’s surface, by the circulations of the oceans, and by the ever-changing orientation of Earth with respect to the sun, the source of all energy for Earth’s large-scale air and water movements—creates global climate, including the global pattern of precipitation. The movement of air from regions of low pressure, which tends to encourage precipitation, to regions of high pressure, which do not favor precipitation, helps to distribute moisture through the atmosphere.

Impacts and Issues

Scientists have evidence that altered air circulation in the tropical Pacific Ocean, similar to that which occurs during El Nino, triggered a global climate change about one million years ago. At that time, the changed circulation of air caused the polar ice sheets to grow in area, which lengthened the periods of glaciation (the ice ages).

This research has relevance in modern times, for it indicates that tropical regions are particularly influential to global climate. In general, any phenomenon that alters atmospheric circulation changes global climate.

Earth’s surface and atmosphere are warming, due to the increased retention of heat caused by the rapid accumulation of greenhouse gases—gases produced by human activities. The link between human activity and atmospheric change used to be very contentious, but by the early 2000s, only a small minority of scientists still argued that atmospheric warming is free from significant human influence. Rather, the global consensus or generally agreed-upon view, as expressed with unprecedented authority and definiteness by the United Nations’ Intergovernmental Panel on Climate Change (IPCC) in 2007, was that the fact of global warming was “unequivocal” (definite) and that human activities were more than 90% likely to be the major cause of the recent warming.

The question of whether, how, and to what extent human activities are influencing atmospheric circulation, however, remained contentious. As one example, a paper published in Nature in 2006 reported on data gathering from 1861 to the early years of the twenty-first century that revealed that the difference in pressure between the higher pressure of the western Pacific to the lower pressure of the eastern Pacific has declined over the past 150 years. The data were used in several computer models of climate; some models factored in the influence to pressure change of only natural conditions, and others had the added influence of human activities. The model that incorporated human-influenced atmospheric change most closely matched the actual data. This supported the hypothesis that human activities had contributed to the observed change in circulation.

Whether or not human activities have influenced the changed environment over the Pacific Ocean, however, it is clear that climate change is occurring on a global scale, with likely effects—present or future—on large-scale atmospheric circulations.

See Also Global Warming; Weather and Climate; Wind and Wind Power

BIBLIOGRAPHY

Books

Barry, Roger G. Atmosphere, Weather and Climate. Oxford, United Kingdom: Routledge, 2003.

Lutgens, Frederick K., Edward J. Tarbuck, and Dennis Tasa. The Atmosphere: An Introduction to Meteorology. New York: Prentice Hall, 2006.

Trefil, Calvo. Earth’s Atmosphere. Geneva, IL: McDougal Littell, 2005.

Periodicals

Vecchi, Gabriel A., et al. “Weakening of Tropical Pacific Atmospheric Circulation Due to Anthropogenic Forcing.” Nature 441 (2006): 73-76.

Brian D. Hoyle

Atmospheric circulation

The troposphere , the lowest 9 mi (15 km) of Earth's atmosphere, is the layer in which nearly all weather activity takes place. Weather is the result of complex air circulation patterns that can best be described by going from the general to more localized phenomena.

The prime mover of air above Earth's surface is the unequal heating and cooling of Earth by the Sun . Air rises as it is heated and descends as it is cooled. The differences in air pressure cause air to circulate, which results in the creation of wind , precipitation , and other weather related features.

Earth's rotation also plays a role in air circulation. Centrifugal force, friction and the apparent Coriolis force are responsible for the circular nature of its flow, as well as for erratic eddies and surges.

On a global scale, there are three circulation belts between the equator and each pole. From 0° to 30° latitude , the trade winds, or tropical easterlies, flow toward the equator and are deflected to the west by the earth's rotation as they move across the earth's surface. The winds then rise at the equator, then flow poleward at the tropopause, the boundary between the troposphere and the stratosphere . The trade winds descend back to the surface at 30° latitude. At the equator, where air from both trade wind belts rises, the lack of cross-surface winds results in the doldrums, an area of calm, which historically has been a bane to sailing vessels.

Between 30° and 60° are the mid-latitude, or prevailing westerlies. The circulation pattern of these wind belts is opposite that of the trades. They flow poleward at the earth's surface, deflecting eastward. They rise at 60°, flow back to the equator, then descend at 30°.

As with the equatorial calm, the earth's surface at 30° North and South has little lateral wind movement since the circulation of the tropical and mid-latitude belts is downward, then outward at this latitude. These calm regions are referred to as the horse latitudes because sailors who were stranded for lack of wind either had to eat their horses or throw them over-board to lighten the load.

The third set of circulation belts, the polar easterlies, range from 60° to 90° latitude at both ends of the earth and flow in the same pattern as the tropical easterlies.

This global circulation scheme is only the typical model. Other forces complicate the actual flow. Differences in the type and elevation of surface features have widespread effects.

The jet streams, high-speed winds blowing from the west near the tropopause, play a significant role in determining the weather. The northern and southern hemispheres each have two jet stream wind belts. The polar front jet stream is the stronger of the two. It flows eastward to speeds of 250 mph (400 kph) at the center and receives its energy from an accumulation of solar radiation. The subtropical jet stream is weaker and receives its force from an accumulation of westerly momentum.

The monsoons of Asia are a result of a combination of influences from the large Asian land mass and the movements of the inter-tropical front, which straddles the equator. From June to September, when the front runs north of the equator, warm moist winds are drawn northward, bringing heavy rains to India and Southeast Asia. From December to February, the front runs slightly south of the equator, drawing dry cooler air off of the Himalayas and out to sea.

On a more local level, air movements occur in the form of interacting air masses and frontal systems. Low-pressure cyclones and high-pressure anticyclones travel from the west to east. Low-pressure cells are responsible for instability in the weather, with cold and warm fronts radiating from the center of the cell. These fronts represent the interface of cold and warm air masses, which develop into storms.

Cold fronts are more active than warm fronts. The upward angle of the cold front line opposes the direction in which it moves, creating friction between the surface and the air, and causing a steeper pressure gradient. The rain band is narrower, but the cumulonimbus clouds that form hold a greater amount of energy and a greater potential for violent weather than the altostratus clouds associated with warm front activity.

Within each cyclonic system are even smaller cyclones. Each storm cell along a front is a cyclone in its own right. In addition to producing heavy rain, hail, high winds and electrical activity, these cells occasionally can produce tornadoes—destructive, whirling funnel-shaped clouds that stretch from the base of a storm cell to the ground. Tornadoes are the most powerful cyclones known on Earth.

Independent of air mass and frontal systems are hurricanes, also known as typhoons or cyclones. These tropical cyclones generate over warm moist ocean surfaces. The rising heat and moisture builds into a massive storm that can extend 1000 mi (1,600 km).

A hurricane tracks westward and will decay when the creative factors are eliminated. This occurs rapidly as the storm travels over land or more gradually as it encounters lower ocean surface temperatures. A lower tropopause in higher latitudes can also reduce the storm's mass.

An accurate understanding of atmospheric circulation began to emerge during the 1830s when Gustave de Coriolis put forth the theory that as Earth rotates, an object will appear to move in a deflected path. About twenty years later, American William Ferrel mathematically proved the Coriolis theory, establishing what became known as Ferrel's law .

The ability to make regular unmanned balloon soundings of the atmosphere in the late 1890s and early 1900s made it possible for new details to emerge. A group of Scandinavian meteorologists under the guidance of Vilhelm Bjerknes took full advantage of this new knowledge to develop mathematical and laboratory models of air mass properties.

Bjerknes first proposed the existence of air masses. His son Jacob went on to demonstrate the frontal systems that separate the air masses. Carl-Gustaf Rossby discovered the jet streams and hypothesized detailed movements and countermovements in the circulation complex.

Atmospheric circulation is a simple process with complex results. It is a system of cells within cells. When we observe leaves swirling in the shadow of a building or a bird soaring on an updraft of warm air, the same principles are at work as with larger global units of the same circulation system. It is a system that is worldwide, that reacts to everything it encounters, and that is even interactive with itself.

See also Air masses and fronts; Atmospheric composition and structure; Atmospheric inversion layers; Atmospheric lapse rate; Atmospheric pollution; Atmospheric pressure