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Jet Stream

Jet stream

The jet stream is a narrow, fast, upper atmospheric wind current, flowing quasi-horizontally at high altitudes around Earth. By definition, the wind speed should be higher than 57 mph (92 kph) for jet streams, although the term is sometimes also erroneously used for all upper-level winds. The jet stream may extend for thousands of miles around the world, but it is only a few hundred miles wide and less than a mile thick. The wind speeds in the core sometimes can reach 200300 mph (322483 kph). These wind speeds within the jet stream that are faster than the surrounding regions are called jet streaks. On average, the jet stream flows from east to west, but it often meanders into northern or southern moving loops. Jet streams occur in both hemispheres, but the Southern Hemisphere jet streams show less daily variability. Jet streams can be

detected by drawing isothachs (the lines connecting points of equal wind speed) on a weather map.

Jet streams form in the upper troposphere , between 6.28.7 mi (1014 km) high, at breaks in the tropopause, where the tropopause changes height dramatically. Jet streams are located at the boundaries of warm and cold air, above areas with strong temperature gradients. For example, the polar front, which separates cold polar air from warmer subtropical air, has a great temperature contrast along the frontal zone, leading to a steep pressure gradient. The resulting wind is the polar jet stream at about 6.2 mi (10 km) high, reaching maximum wind speed in winter. Sometimes the polar jet can split into two jets, or merge with the subtropical jet, which is located at about 8 mi (13 km) high, around 30 degrees latitude . A low-level jet stream also exists above the Central Plains of the United States, causing nighttime thunderstorm formation in the summertime. Over the subtropics, there is the tropical easterly jet, at the base of the tropopause in summertime, about 15 degrees latitude over continental regions. Near the top of the stratosphere exists the stratospheric polar jet during the polar winter.

Jet streams are well known since World War II. Detailed knowledge about the jet stream's location, altitude, and strength is essential not only for safe and efficient routing of aircrafts, but also for weather forecasting .

See also Atmospheric circulation; Coriolis effect; Troposphere and tropopause

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jet stream

jet stream, narrow, swift currents or tubes of air found at heights ranging from 7 to 8 mi (11.3–12.9 km) above the surface of the earth. They are caused by great temperature differences between adjacent air masses. There are four major jet streams. Although discontinuous at some points, they circle the globe at middle and polar latitudes, both in each hemisphere. The mean position of the stream in the Northern Hemisphere is between lat. 20 and 50 degrees N; the polar stream is between lat 30 and 70 degrees N. Wind speeds average 35 mi (56.3 km) per hr in summer and 75 mi (120.7 km) per hr in winter, although speeds as high as 200 mi (321.9 km) per hr have been recorded. Instead of moving along a straight line, the jet stream flows in a wavelike fashion; the waves propagate eastward (in the Northern Hemisphere) at speeds considerably slower than the wind speed itself. Since the progress of an airplane is aided or impeded depending on whether tail winds or head winds are encountered, in the Northern Hemisphere the jet stream is sought by eastbound aircraft, in order to gain speed and save fuel, and avoided by westbound aircraft.

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jet stream

jet stream Concentrated, high-speed air flow, generally in a broadly westerly (i.e. west to east) direction. The principal global jets are the polar front and the subtropical jets, at heights of about 10–12 and 12–15 km respectively, and the polar-night or winter jet stream in the upper stratosphere or mesosphere at about 50–80 km. The intensity of the jets in narrow bands (the maximum velocity is commonly about 50–100 m/s, but greater speeds are sometimes observed) results from a large poleward increase in pressure gradient with altitude. This is a product of the pole-equator temperature gradient in the air beneath the jet. As pressure decreases more rapidly with height the lower the temperature, so pressure in the colder, polar air masses decreases more rapidly with height than over the regions with warmer air masses. Above the jet, wind speed diminishes as the pressure gradient declines with increasing height, owing to the effects of a different heating pattern in the stratosphere. See also POLAR JET STREAM; and SUBTROPICAL JET STREAM.

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jet stream

jet stream A concentrated, high-speed air flow, generally in a broadly westerly (i.e. west to east) direction. The principal global jets are the polar-front and the subtropical jet streams, at heights of about 10–12 and 12–15 km respectively, and the polar-night or winter jet stream in the upper stratosphere or mesosphere at about 50–80 km. The intensity of the jets in narrow bands (the maximum velocity is commonly about 50–100 m/s, but greater speeds are sometimes observed) results from a large poleward increase in pressure gradient with altitude. This is a product of the pole–equator temperature gradient in the air beneath the jet. As pressure decreases more rapidly with height the lower the temperature, so pressure in the colder, polar air masses decreases more rapidly with height than over the regions with warmer air masses. Above the jet, wind speed diminishes as the pressure gradient declines with increasing height, owing to the effects of a different heating pattern in the stratosphere.

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jet stream

jet stream • n. 1. a narrow, variable band of very strong, predominantly westerly air currents encircling the globe several miles above the earth. There are typically two or three jet streams in each of the northern and southern hemispheres. 2. a flow of exhaust gasses from a jet engine.

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jet stream

jet stream Narrow, swift winds between slower currents at altitudes of 10–16km (6–10mi) in the upper troposphere or lower stratosphere, principally the zone of prevailing westerlies.

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jet stream

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Jet Stream

Jet Stream

The jet stream is a narrow, fast, upper atmospheric wind current, flowing at high altitudes around the Earth. Although often erroneously applied to all upperlevel winds, by definition jet stream wind speeds are in excess of 57 mph (about 92 km/h). The jet stream may extend for thousands of miles around the world, but it is only a few hundred miles wide, and usually less than one mile thick.

Undulating jet stream movements often greatly influence storm formation and weather changes. Research sponsored by the National Aeronautics and Space Administration (NASA) culminated in a 2001 report that also correlated solar activity, jet stream migration, and precipitation patterns over North America. Further research performed by scientists from the University of Washington and the University of Utah, and published in the journal Science, showbased on data taken between 1979 and 2005that the jet stream has moved toward the Earths poles by about one degree of latitude (or about 70 miles in each direction). This result indicate that the tropical band of the Earth, centered about the equator, is expanding outward slightly (by about 140 miles).

The wind speeds in the core of the stream sometimes can reach 200 to 300 mph (332 to 483 km/h). These wind speeds within the jet stream that are faster than the surrounding regions are called jet streaks. On average, the jet stream flows from east to west, but it often meanders into northern or southern moving loops. Jet streams occur in both hemispheres, but the Southern Hemisphere jet streams show less daily variability. Jet streams can be detected by drawing isothachs (the lines connecting points of equal wind speed) on a weather map.

Jet streams form in the upper troposphere, between 6 and 9 mi (10 and 14 km) high, at breaks in the tropopause, where the tropopause changes height dramatically. Jet streams are located at the boundaries of warm and cold air, above areas with strong temperature gradients. For example, the polar front, which separates cold polar air from warmer subtropical air, has a great temperature contrast along the frontal zone, leading to a steep pressure gradient. The resulting wind is the polar jet stream at about 6 mi (10 km) high, reaching maximum wind speed in winter. Sometimes the polar jet can split into two jets, or merge with the subtropical jet, which is located at about 8 mi (13 km) high, around 30 degrees latitude. A lowlevel jet stream also exists above the Central Plains of the United States, causing nighttime thunderstorm formation in the summertime. Over the subtropics, there is the tropical easterly jet, at the base of the tropopause in summertime, about 15 degrees latitude over continental regions. Near the top of the stratosphere exists the stratospheric polar jet during the polar winter.

Detailed knowledge about the jet streams location, altitude, and strength is essential not only for safe and efficient routing of aircrafts, but also for weather forecasting.

See also Atmosphere observation; Atmosphere, composition and structure; Atmospheric circulation; Global climate; Solar activity cycle; Solar illumination: Seasonal and diurnal patterns.

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Jet Stream

Jet Stream

Introduction

Jet streams are relatively strong, fast-moving, but narrow (flat tube-shaped) currents of air located in the upper levels of the atmosphere. They are found both in the northern and southern hemispheres of planet Earth, but are less variable in the southern half. They range from 5 to 10 mi (8 to 16 km)—but average 7.5 mi (12 km)— above Earth's surface. This habitat is considered just under the boundary of the tropopause—the layer of atmosphere where weather occurs. Jet streams vary in altitude depending on the boundaries of warm and cold air. For instance, a frontal boundary is formed between cold polar air and warm tropical air, causing a strong polar jet stream.

The winds within a jet stream can top at 200 to 300 mi (320 to 480 km) per hour or higher but usually range around 50 to 60 mi (80 to 97 km) per hour. An average jet stream is thousands of miles long and hundreds of miles wide, but only a few miles thick. They generally travel from westerly to easterly directions, but sometimes detour in northern- and southern-directed loops.

Historical Background and Scientific Foundations

The swiftly flowing air currents, now called jet streams, that travel around Earth were first noticed in the 1800s by weather scientists experimenting with balloons and kites. They sometimes called these high winds “westerlies”: winds coming from the west. In the 1920s, Japanese meteorologist Wasaburo Ooishi used balloons to measure high winds over Mt. Fuji, just west of the country's capital of Tokyo. Other than inJapan, hisresearchonthe jetstreamfailedtobe recognized at that time by the world meteorological community.

American aviator Wiley Post (1898–1935) flew through some of these high winds in the late 1920s and early 1930s. On December 7, 1934, he discovered a strong wind that he used at around 20,000 ft (6,100m) to increase the speed of his airplane. Post is generally given credit for the discovery of the jet stream.

The jet stream was measured in great detail beginning in the early 1940s when radar was developed by engineers during World War II (1939–1945) and, concurrently, when weather radar first became available to scientists. The term RADAR was created in 1941 as an acronym for “radio detection and ranging.” Engineers found that when a transmitter emitted radio waves (a particular type of electromagnetic radiation), they could be reflected off a target and returned to a receiver, which was usually positioned with the transmitter. Objects could be detected and measured quite accurately with respect to their altitude, distance, and speed.

In the early twenty-first century, radar is used to detect precipitation (such as snow and rain) in meteorology, speeding motor vehicles (such as automobiles and motorcycles) in police traffic control, and enemy vessels (such as ships and aircraft) in the military.

Impacts and Issues

Differences in the jet stream can strongly affect the climate, especially in the Northern Hemisphere of Earth. A strong jet stream means that its winds are intense, regular, and compressed with its nominal path close to the pole. Because the jet stream, in this case, stays farther north, the warm climate at the equator is able to move farther northward, causing much warmer and temperate weather in these regions. Monsoon rains, for instance, can become more frequent and heavy, bringing flooding to affected areas such as those in southeastern Asia.

However, a weak jet stream contains highly variable and unpredictable winds that cause more irregularity in its path. The jet stream will oftentimes dip into the lower latitudes causing colder weather than normal. Snow in Houston, Texas, for instance, can result from a weak jet stream.

The jet stream also controls the movement of strong horizontal temperature and pressure gradients (contrasts, or differences, in temperature or pressure) such as weather fronts and high- and low-pressure weather systems. Jet streams often force large weather systems (such as hurricanes and rainstorms) to move in particular directions across the globe. For instance, a jet stream can move over the Gulf of Mexico and then turn north to drop large amounts of rainfall on the state of Texas and massive amounts of snowfall further northward on the Great Plains states of the United States.

WORDS TO KNOW

METEOROLOGY: The science that deals with Earth's atmosphere and its phenomena and with weather and weather forecasting.

MONSOON: An annual shift in the direction of the prevailing wind that brings on a rainy season and affects large parts of Asia and Africa.

RADAR: An acronym for RAdio Detection And Ranging—the use of electromagnetic waves at sub-optical frequencies (i.e., less than about 1012 Hz) to sense objects at a distance.

RADIO WAVES: Electromagneticwavesthatoscillateorvibratebetween 3 and 300 billion times per second. Radio waves are physically identical to light waves, except that they do not vibrate as rapidly.

TROPOPAUSE: The boundary between the troposphere and the stratosphere (about 5 mi (8 km) in polar regions and about 9.3 mi (15 km) in tropical regions), usually characterized by an abrupt change of lapse rate. The regions above the troposphere have increased atmospheric stability than those below. The tropopause marks the vertical limit of most clouds and storms.

Thus, information collected about jet streams is important for the prediction of weather around the world and in the study of meteorology in general. Such weather forecasting uses numerical data of jet streams—their strength, location, altitude, and other physical characteristics—to develop complex systems of mathematical equations that are fed into supercomputers to produce scientific models of the atmosphere, and thus of global climate.

See Also Africa: Climate Change Impacts; Global Warming; Meteorology; Ocean Circulation and Currents;Oceans and Seas; Sea Temperatures and Storm Intensity.

BIBLIOGRAPHY

Books

Pernetta, John. Guide to the Oceans. Richmond Hill,Ontario, Canada: Firefly Books, 2003.

Voituriez, Bruno. The Changing Ocean: Its Effects on Climate and Living Resources. Paris, France: UNESCO, 2003.

Voituriez, Bruno. The Gulf Stream. Paris, France: UNESCO, 2006.

Wang, Chunzia, Shang-Ping Xie, and James A. Carton, eds. Earth's Climate: The Ocean-Atmosphere Interaction. Washington, DC: American Geophysical Union, 2004.

Web Sites

“Abrupt Climate Change: Should We Be Worried?” Woods Hole Oceanographic Institution. <http://www.whoi.edu/page.do?pid=12455&tid=282&cid=9986> (accessed October 27, 2007).

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Jet Stream

Jet stream

The jet stream is a narrow, fast, upper atmospheric wind current, flowing at high altitudes around Earth . Although often erroneously applied to all upper-level winds, by definition jet stream wind speeds are in excess of 57 MPH (92 km/h). The jet stream may extend for thousands of miles around the world, but it is only a few hundred miles wide, and usually less than a mile thick.

Undulating jet stream movements often greatly influence storm formation and weather changes. Research sponsored by the National Aeronautics and Space Administration (NASA) culminated in a 2001 report that also correlated solar activity, jet stream migration , and precipitation patterns over North America .

The wind speeds in the core of the stream sometimes can reach 200–300 MPH (322-483 km/h). These wind speeds within the jet stream that are faster than the surrounding regions are called jet streaks. On average, the jet stream flows from east to west, but it often meanders into northern or southern moving loops. Jet streams occur in both hemispheres, but the Southern Hemisphere jet streams show less daily variability. Jet streams can be detected by drawing isothachs (the lines connecting points of equal wind speed) on a weather map .

Jet streams form in the upper troposphere, between 6 and 9 mi (10 and 14 km) high, at breaks in the tropopause, where the tropopause changes height dramatically. Jet streams are located at the boundaries of warm and cold air, above areas with strong temperature gradients. For example, the polar front, which separates cold polar air from warmer subtropical air, has a great temperature contrast along the frontal zone, leading to a steep pressure gradient. The resulting wind is the polar jet stream at about 6 mi (10 km) high, reaching maximum wind speed in winter. Sometimes the polar jet can split into two jets, or merge with the subtropical jet, which is located at about 8 mi (13 km) high, around 30 degrees latitude. A low-level jet stream also exists above the Central Plains of the United States, causing nighttime thunderstorm formation in the summertime. Over the subtropics, there is the tropical easterly jet, at the base of the tropopause in summertime, about 15 degrees latitude over continental regions. Near the top of the stratosphere exists the stratospheric polar jet during the polar winter.

Detailed knowledge about the jet stream's location, altitude, and strength is essential not only for safe and efficient routing of aircrafts, but also for weather forecasting .

See also Atmosphere observation; Atmosphere, composition and structure; Atmospheric circulation; Global climate; Solar activity cycle; Solar illumination: Seasonal and diurnal patterns.

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