Atmospheric circulation

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Atmospheric circulation

An idealized model of atmospheric circulation

Observed patterns of circulation

Patterns of surface pressure

The jet streams

Other violent wind systems

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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.