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Jet stream
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==Cause== [[File:Earth Global Circulation - en.svg|thumb|250x250px|Highly idealised depiction of the global circulation. The upper-level jets tend to flow latitudinally along the cell boundaries.]] {{See also|Extratropical cyclone|Thermal wind}} In general, winds are strongest immediately under the [[tropopause]] (except locally, during [[tornado]]es, [[tropical cyclone]]s 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.<ref name="Stimac">John P. Stimac. [http://www.ux1.eiu.edu/~cfjps/1400/pressure_wind.html Air pressure and wind.] {{Webarchive|url=https://web.archive.org/web/20070927210111/http://www.ux1.eiu.edu/~cfjps/1400/pressure_wind.html |date=27 September 2007 }} Retrieved on 8 May 2008.</ref> 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.<ref name="Stimac"/> ===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.<ref>{{cite journal |last1=Messori |first1=Gabriele |last2=Caballero |first2=Rodrigo |title=On double Rossby wave breaking in the North Atlantic |journal=Journal of Geophysical Research: Atmospheres |date=2015 |volume=120 |issue=21 |pages=11,129–11,150 |doi=10.1002/2015JD023854|bibcode=2015JGRD..12011129M |doi-access=free }}</ref> 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.<ref name="GOMdef"/> ===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<ref>[[Lyndon State College]] Meteorology. [http://apollo.lsc.vsc.edu/classes/met130/notes/chapter10/subt_jet_form.html Jet Stream Formation – Subtropical Jet.] {{Webarchive|url=https://web.archive.org/web/20110927115127/http://apollo.lsc.vsc.edu/classes/met130/notes/chapter10/subt_jet_form.html |date=27 September 2011 }} Retrieved on 8 May 2008.</ref>
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