Fast-flowing atmospheric air current
Jet streams are fast flowing, narrow air currents in the Earth's atmosphere.
The main jet streams are located near the altitude of the tropopause and are westerly winds, flowing west to east around the globe. The northern hemisphere and the southern hemisphere each have a polar jet around their respective polar vortex at around above sea level and typically travelling at around although often considerably faster. Closer to the equator and somewhat higher and somewhat weaker is a subtropical jet. 
The northern polar jet flows over the middle to northern latitudes of North America, Europe, and Asia and their intervening oceans, while the southern hemisphere polar jet mostly circles Antarctica. Jet streams may start, stop, split into two or more parts, combine into one stream, or flow in various directions including opposite to the direction of the remainder of the jet. 
The El Niño–Southern Oscillation affects the location of the jet streams, which in turn affects the weather over the tropical Pacific Ocean and affects the climate of much of the tropics and subtropics, and can affect weather in higher-latitude regions. The term "jet stream" is also applied to some other winds at varying levels in the atmosphere, some global (such as the higher-level polar-night jet), some local (such as the African easterly jet). Meteorologists use the location of some of the jet streams as an aid in weather forecasting. Airlines use them to reduce some flight times and fuel consumption. Scientists have considered whether the jet streams might be harnessed for power generation. In World War II, the Japanese used the jet stream to carry Fu-Go balloon bombs across the Pacific Ocean to launch small attacks on North America.
Jet streams have been detected in the atmospheres of Venus, Jupiter, Saturn, Uranus, and Neptune.
Discovery.
The first indications of this phenomenon came from American professor Elias Loomis (1811–1889), when he proposed the hypothesis of a powerful air current in the upper air blowing west to east across the United States as an explanation for the behaviour of major storms. After the 1883 eruption of the Krakatoa volcano, weather watchers tracked and mapped the effects on the sky over several years. They labelled the phenomenon the "equatorial smoke stream". In the 1920s Japanese meteorologist Wasaburo Oishi detected the jet stream from a site near Mount Fuji. He tracked pilot balloons ("pibals"), used to measure wind speed and direction, as they rose in the air. Oishi's work largely went unnoticed outside Japan because it was published in Esperanto, though chronologically he has to be credited for the scientific discovery of jet streams. American pilot Wiley Post (1898–1935), the first man to fly around the world solo in 1933, is often given some credit for discovery of jet streams. Post invented a pressurized suit that let him fly above . In the year before his death, Post made several attempts at a high-altitude transcontinental flight, and noticed that at times his ground speed greatly exceeded his air speed.
German meteorologist Heinrich Seilkopf is credited with coining a special term, "Strahlströmung" (literally "jet current"), for the phenomenon in 1939. Many sources credit real understanding of the nature of jet streams to regular and repeated flight-path traversals during World War II. Flyers consistently noticed westerly tailwinds in excess of in flights, for example, from the US to the UK. Similarly in 1944 a team of American meteorologists in Guam, including Reid Bryson, had enough observations to forecast very high west winds that would slow bombers raiding Japan.
Description.
The polar and subtropical jet streams are the product of two factors: the atmospheric heating by solar radiation that produces the large-scale polar, Ferrel, and Hadley circulation cells, and the action of the Coriolis force acting on those moving masses. The Coriolis force is caused by the planet's rotation on its axis. The polar jet stream forms near the interface of the polar and Ferrel circulation cells; the subtropical jet forms near the boundary of the Ferrel and Hadley circulation cells.
Polar jet streams are typically located near the 250 hPa (about 1/4 atmosphere) pressure level, or above sea level while the weaker subtropical jet streams are somewhat higher.
The polar jets, at lower altitude, and often intruding into mid-latitudes, strongly affect weather and aviation. The polar jet stream is most commonly found between latitudes 30° and 60° (closer to 60°), while the subtropical jet streams are located close to latitude 30°. These two jets merge at some locations and times, while at other times they are well separated. The northern polar jet stream is said to "follow the sun" as it slowly migrates northward as that hemisphere warms, and southward again as it cools.
The width of a jet stream is typically a few hundred kilometres or miles and its vertical thickness often less than .
Jet streams are typically continuous over long distances, but discontinuities are also common. The path of the jet typically has a meandering shape, and these meanders themselves propagate eastward, at lower speeds than that of the actual wind within the flow. Further, the meanders can split or form eddies.
Each large meander, or wave, within the jet stream is known as a Rossby wave (planetary wave). Rossby waves are caused by changes in the Coriolis effect with latitude. Shortwave troughs, are smaller scale waves superimposed on the Rossby waves, with a scale of long, that move along through the flow pattern around large scale, or longwave, "ridges" and "troughs" within Rossby waves.
The wind speeds are greatest where temperature differences between air masses are greatest, and often exceed . Speeds of have been measured.
The jet stream moves from west to east bringing changes of weather. The path of jet streams affects cyclonic storm systems at lower levels in the atmosphere, and so knowledge of their course has become an important part of weather forecasting. For example, in 2007 and 2012, Britain experienced severe flooding as a result of the polar jet staying south for the summer.
Cause.
In general, winds are strongest immediately under the tropopause (except locally, during tornadoes, tropical cyclones or other anomalous situations). If two air masses of different temperatures or densities meet, the resulting pressure difference caused by the density difference (which ultimately causes wind) is highest within the transition zone. The wind does not flow directly from the hot to the cold area, but is deflected by the Coriolis effect and flows along the boundary of the two air masses.
All these facts are consequences of the thermal wind relation. The balance of forces acting on an atmospheric air parcel in the vertical direction is primarily between the gravitational force acting on the mass of the parcel and the buoyancy force, or the difference in pressure between the top and bottom surfaces of the parcel. Any imbalance between these forces results in the acceleration of the parcel in the imbalance direction: upward if the buoyant force exceeds the weight, and downward if the weight exceeds the buoyancy force. The balance in the vertical direction is referred to as hydrostatic. Beyond the tropics, the dominant forces act in the horizontal direction, and the primary struggle is between the Coriolis force and the pressure gradient force. Balance between these two forces is referred to as geostrophic. Given both hydrostatic and geostrophic balance, one can derive the thermal wind relation: the vertical gradient of the horizontal wind is proportional to the horizontal temperature gradient. If two air masses in the northern hemisphere, one cold and dense to the north and the other hot and less dense to the south, are separated by a vertical boundary and that boundary should be removed, the difference in densities will result in the cold air mass slipping under the hotter and less dense air mass. The Coriolis effect will then cause poleward-moving mass to deviate to the East, while equatorward-moving mass will deviate toward the west. The general trend in the atmosphere is for temperatures to decrease in the poleward direction. As a result, winds develop an eastward component and that component grows with altitude. Therefore, the strong eastward moving jet streams are in part a simple consequence of the fact that the Equator is warmer than the north and south poles.
Polar jet stream.
The thermal wind relation does not explain why the winds are organized into tight jets, rather than distributed more broadly over the hemisphere. One factor that contributes to the creation of a concentrated polar jet is the undercutting of sub-tropical air masses by the more dense polar air masses at the polar front. This causes a sharp north–south pressure (south–north potential vorticity) gradient in the horizontal plane, an effect which is most significant during double Rossby wave breaking events. At high altitudes, lack of friction allows air to respond freely to the steep pressure gradient with low pressure at high altitude over the pole. This results in the formation of planetary wind circulations that experience a strong Coriolis deflection and thus can be considered 'quasi-geostrophic'. The polar front jet stream is closely linked to the frontogenesis process in midlatitudes, as the acceleration/deceleration of the air flow induces areas of low/high pressure respectively, which link to the formation of cyclones and anticyclones along the polar front in a relatively narrow region.
Subtropical jet.
A second factor which contributes to a concentrated jet is more applicable to the subtropical jet which forms at the poleward limit of the tropical Hadley cell, and to first order this circulation is symmetric with respect to longitude. Tropical air rises to the tropopause, and moves poleward before sinking; this is the Hadley cell circulation. As it does so it tends to conserve angular momentum, since friction with the ground is slight. Air masses that begin moving poleward are deflected eastward by the Coriolis force (true for either hemisphere), which for poleward moving air implies an increased westerly component of the winds
Effects.
Hurricane protection.
The subtropical jet stream rounding the base of the mid-oceanic upper trough is thought to be one of the causes most of the Hawaiian Islands have been resistant to the long list of Hawaii hurricanes that have approached. For example, when Hurricane Flossie (2007) approached and dissipated just before reaching landfall, the U.S. National Oceanic and Atmospheric Administration (NOAA) cited vertical wind shear as evidenced in the photo.
Uses.
The northern polar jet stream is the most important one for aviation and weather forecasting, as it is much stronger and at a much lower altitude than the subtropical jet streams and also covers many countries in the northern hemisphere, while the southern polar jet stream mostly circles Antarctica and sometimes the southern tip of South America.
Aviation.
The location of the jet stream is important for aviation. Aircraft flight time can be dramatically affected by either flying with the flow or against it. Often, airlines work to fly with the jet stream to obtain significant fuel cost and time savings.
Commercial use of the jet stream began on 18 November 1952, when Pan Am flew from Tokyo to Honolulu at an altitude of . It cut the trip time by over one-third, from 18 to 11.5 hours.
Within North America, the time needed to fly east across the continent can be decreased by about 30 minutes if an airplane can fly with the jet stream. Across the Atlantic Ocean the North Atlantic Tracks service allows airlines and air traffic control to accommodate the jet stream for the benefit for airlines and other users.
Associated with jet streams is a phenomenon known as clear-air turbulence (CAT), caused by vertical and horizontal wind shear caused by jet streams. The CAT is strongest on the cold air side of the jet, next to and just under the axis of the jet. Clear-air turbulence can cause aircraft to plunge and so present a passenger safety hazard that has caused fatal accidents, such as the death of one passenger on United Airlines Flight 826 in 1997. Unusual wind speed in the jet stream in late February 2024 pushed commercial jets to excess of relative to the ground.
Possible future power generation.
Scientists are investigating ways to harness the wind energy within the jet stream. According to one estimate of the potential wind energy in the jet stream, only one percent would be needed to meet the world's current energy needs. In the late 2000s it was estimated that the required technology would reportedly take 10–20 years to develop.
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