Jet streams are strong westerly winds that blow in a narrow band in Earth's upper atmosphere at the same altitudes at which airplanes fly. They form because of temperature variations between the poles and equator, and they exist in both hemispheres, although those in the Northern Hemisphere are...
Johan, there is no one “cruising altitude” that ALL planes fly at. The plane’s altitude is set by air traffic control (although the pilot can request a new altitude). Individual jets have a “cruise altitude” which gives an optimum fuel efficiency, and obviously they would prefer to ...