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. The rotation of Earth on its axis and the unequal arrangement of land and water masses on the planet also contribute to various features of atmospheric circulation.
There are three wind cells or circulation belts between the equator and each pole: the trade winds (Hadley cells), prevailing westerlies (Ferrell cells), and polar easterlies (polar Hadley cells). The trade winds or Hadley cells are named after the English scientist George Hadley (1685–1768), who first described them in 1753. As air is heated at the equator, it rises in the troposphere, the lowest 10 miles (16 kilometers) of Earth's atmosphere. In the wake of the warm rising air, low pressure develops at the equator. When the air reaches the top of the troposphere, called the tropopause, it can rise no farther and begins to move toward the poles, cooling in the process.
At about 30 degrees latitude north and south, the cooled air descends back to the surface, pushing the air below it toward the equator, since air flows always move toward areas of low pressure. When the north and south trade winds meet at the equator and rise again, an area of calm develops because of the lack of cross-surface winds. Early mariners called this area the doldrums (from an Old English word meaning dull) because they feared their sailing ships would be stranded by the lack of wind.
While most of the trade-wind air that sinks at 30 degrees latitude returns to the equator, some of it flows poleward. At about 60 degrees latitude north and south, this air mass meets much colder polar air (the areas where this occurs are known as polar fronts). The warmer air is forced upward by the colder air to the tropopause, where most of it moves back toward the equator, sinking at about 30 degrees latitude to continue the cycle again. These second circulation belts over the middle latitudes between 30 degrees and 60 degrees are the prevailing westerlies or Ferrell cells, named after the American meteorologist William Ferrell (1817–1891), who discovered them in 1856.
Calm regions also occur at 30 degrees latitude where Hadley cells and Ferrell cells meet because of the lack of lateral wind movement. These regions were given the name horse latitudes by sailors bringing horses to the Americas. Stranded by the lack of winds, sailors often ate their horses as supplies ran low.
The air at the top of polar fronts that does not return toward the equator moves, instead, poleward. At the poles, this air cools, sinks, and flows back to 60 degrees latitude north and south. These third circulation belts over the poles are known as polar easterlies or polar Hadley cells because they flow in the same direction as the Hadley cells near the equator. However, they are not as powerful since they lack the solar energy present at the equator.
Words to Know
Coriolis effect: Moving object appearing to travel in a curved path over the surface of a spinning body.
Doldrums: Region of the equatorial ocean where winds are light and unpredictable.
Horse latitudes: Region of the oceans around 30 degrees latitude where winds are light and unpredictable.
Jet stream: Rapidly moving band of air in the upper atmosphere.
Polar front: Relatively permanent front formed at the junction of the Ferrell and polar Hadley cells.
Trade winds: Relatively constant wind patterns that blow toward the equator at about 30 degrees latitude.
The Coriolis effect
The air flows in these three circulation belts or cells do not move in a straight north to south or south to north route. Instead, the air flows seem to move east to west or west to east. This effect was first identified by the French mathematician Gaspard-Gustave de Coriolis (1792–1843) in 1835. Coriolis observed that, because of the spinning of the planet, any moving object above Earth's surface tends to drift sideways from its course of motion. In the Northern Hemisphere, this movement is to the right of the course of motion. In the Southern Hemisphere, it is to the left. As a result, surface winds in Hadley cells—both in the equatorial and polar regions—blow from the northeast to the southwest in the Northern Hemisphere and from the southeast to the northwest in the Southern Hemisphere. Surface winds in Ferrell cells tend to blow in the opposite direction: from the southwest to the northeast in the Northern Hemisphere and from the northwest to the southeast in the Southern Hemisphere.
Variations and wind patterns
The conditions of the wind cells described above are for general models. In the real world, actual wind patterns are far more complex. Many elements play a part in disrupting these patterns from their normal course, as described by Hadley and Ferrell. Since the Sun does not always shine directly over the equator, air masses in that area are not heated equally. While some masses in a cell may be heated quickly, creating a strong flow upward, others may not receive as much solar energy, resulting in a much weaker flow. Unevenness in the surface of the planet also affects the movement of air masses in a cell. A mass moving across a uniform region, such as an ocean, may be undisturbed. Once it moves over a region with many variations, such as a mountainous area, it may become highly disturbed.
The jet streams
In 1944, an especially dramatic type of atmospheric air movement was discovered: the jet streams. These permanent air currents are located at altitudes of 30,000 to 45,000 feet (11 to 13 kilometers) and generally move with speeds ranging from about 35 to 75 miles (55 to 120 kilometers) per hour. It is not uncommon, however, for the speed of jet streams to be as high as 200 miles (320 kilometers) per hour.
These narrow tubes of air, which usually travel west to east, are created by the great temperature and pressure differences between air masses. There are four major jet streams, two in each hemisphere. Polar jet streams, formed along the polar front between the Ferrell and polar Hadley cells, move between 30 degrees and 70 degrees latitude. The other jet streams move between 20 degrees and 50 degrees latitude.
Jet streams do not move in straight lines, but in a wavelike manner. They may break apart into two separate streams and then rejoin, or not. In winter, because of greater temperature differences, jet streams are stronger and move toward the equator. In summer, with more uniform temperatures, they weaken and move poleward. The movement of the jet streams is an important factor in determining weather conditions in mid-latitude regions since they can strengthen and move low-pressure systems.
[See also Air masses and fronts; Global climate; Monsoon; Wind ]
"Atmospheric Circulation." UXL Encyclopedia of Science. . Encyclopedia.com. (December 13, 2017). http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/atmospheric-circulation-2
"Atmospheric Circulation." UXL Encyclopedia of Science. . Retrieved December 13, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/atmospheric-circulation-2
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
"Atmospheric Circulation." World of Earth Science. . Encyclopedia.com. (December 13, 2017). http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/atmospheric-circulation
"Atmospheric Circulation." World of Earth Science. . Retrieved December 13, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/atmospheric-circulation
"circulation, atmospheric." World Encyclopedia. . Encyclopedia.com. (December 13, 2017). http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/circulation-atmospheric
"circulation, atmospheric." World Encyclopedia. . Retrieved December 13, 2017 from Encyclopedia.com: http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/circulation-atmospheric