<P> The polar - night jet stream forms mainly during the winter months when the nights are much longer, hence polar nights, in their respective hemispheres at around 60 ° latitude . The polar night jet moves at a greater height of about 80,000 feet (24,000 m) than it does during the summer . During these dark months the air high over the poles becomes much colder than the air over the equator . This difference in temperature gives rise to extreme air pressure differences in the stratosphere, which, when combined with the Coriolis effect, create the polar night jets, that race eastward at an altitude of about 30 miles (48 km). The polar vortex is circled by the polar night jet . The warmer air can only move along the edge of the polar vortex, but not enter it . Within the vortex, the cold polar air becomes increasingly cold with neither warmer air from lower latitudes nor energy from the Sun during the polar night . </P> <P> There are wind maxima at lower levels of the atmosphere that are also referred to as jets . </P> <P> A barrier jet in the low levels forms just upstream of mountain chains, with the mountains forcing the jet to be oriented parallel to the mountains . The mountain barrier increases the strength of the low level wind by 45 percent . In the North American Great Plains a southerly low - level jet helps fuel overnight thunderstorm activity during the warm season, normally in the form of mesoscale convective systems which form during the overnight hours . A similar phenomenon develops across Australia, which pulls moisture poleward from the Coral Sea towards cut - off lows which form mainly across southwestern portions of the continent . </P> <P> A valley exit jet is a strong, down - valley, elevated air current that emerges above the intersection of the valley and its adjacent plain . These winds frequently reach a maximum of 20 m / s (45 mph or 72 km / h) at a height of 40--200 m above the ground . Surface winds below the jet may sway vegetation, but are significantly weaker . </P>

Jet streams winds over the united states generally move from