Cores of fast-moving air, with speeds of 100 to 200 knots, which occur near the tropopause in temperate latitudes. They are a few thousand metres in depth and some tens of kms. wide and move horizontally, but often in an irregularly wavy pattern, from west to east in both hemispheres. A sharp difference in temperature occurs across them, the warmer air lying to the right of the jet stream int he northern hemisphere and to the left in the southern.