Editing Jet stream (section)

===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"/>