<P> On Earth, 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 . The term jet stream in these contexts thus usually implies the northern polar jet stream . </P> <P> The location of the jet stream is extremely important for aviation . Commercial use of the jet stream began on 18 November 1952, when Pan Am flew from Tokyo to Honolulu at an altitude of 7,600 metres (24,900 ft). It cut the trip time by over one - third, from 18 to 11.5 hours . Not only does it cut time off the flight, it also nets fuel savings for the airline industry . 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, or increased by more than that amount if it must fly west against it . </P> <P> 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 . </P> <P> 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 1 percent would be needed to meet the world's current energy needs . The required technology would reportedly take 10--20 years to develop . There are two major but divergent scientific articles about jet stream power . Archer & Caldeira claim that the Earth's jet streams could generate a total power of 1700 terawatts (TW) and that the climatic impact of harnessing this amount would be negligible . However, Miller, Gans, & Kleidon claim that the jet streams could generate a total power of only 7.5 TW and that the climatic impact would be catastrophic . </P>

Where is the jet stream located in the layer of the atmosphere