JET STREAM

any of several long, narrow high-speed air currents that flow eastward in a generally horizontal zone in the stratosphere or upper troposphere. Jet streams are characterized by wind motions that generate strong vertical shearing action, which is thought to be largely responsible for the clear air turbulence so hazardous to aircraft. They also are associated with certain types of atmospheric disturbances and so have an effect on weather. First discovered by airmen during World War II, the jet streams have speeds of up to 500 km per hour (310 miles per hour) along thousands of kilometres at their centres, but velocities fall off very sharply both laterally and vertically, so that the high speeds are limited to very narrow bands at heights between 10 and 50 km (32,800 and 164,100 feet), generally lying at breaks in the tropopause, where it changes suddenly in height. The kinetic energy contained in jet streams is the result of heat transport from the equator to the poles being deflected by the Coriolis force. Jet streams encircle the Earth in meandering paths, shifting position as well as speed with the seasons. During the winter their positions are nearer the equator and their speeds higher than during the summer. There are often two, sometimes three jet-stream systems in each hemisphere. One is related to the discontinuous Polar Front, lying in mid-latitudes where the air-mass contrasts (the fronts) promote the formation of squalls, storms, and cyclones. The other distinct system, the Subtropical Jet Stream, lies above the subtropical high-pressure belt and is usually associated with fair weather. During summer a third system occurs over Southeast Asia, India, the Arabian Sea, and tropical Africa. This tropical jet stream affects the formation and duration of Indian and African summer monsoons.