THE JET STREAMS are fast-flowing eastward currents of air in the mid-latitudes of both hemispheres, with their cores at altitudes above 30,000 ft. (9,144 m.). Although they flow eastward, they are driven by the temperature contrast between the equator and the poles. Near the equator where surface temperatures are at a maximum, the air rises and, in the upper atmosphere, flows poleward. As the air moves away from the equator, the eastward winds increase in speed relative to the Earth's surface, becoming the jet streams in mid-latitudes. (This is a consequence of the conservation of angular momentum.)
The jet streams are most intense during the winter season when the temperature difference between the equator and pole is at a maximum. When too intense, they become unstable, start to meander, and create weather phenomena. The meanders, which are associated with the southward flow of cold air and the northward flow of warm air, in due course become turbulent, creating a variety of weather phenomena including cyclones, anti-cyclones, warm fronts and cold fronts. This variability is strongest in winter, when the jets are most intense, and is weakest in summer when weather tends to be associated with local, convective phenomena such as thunderstorms.
Regarding global warming, if it warms primarily high northern latitudes, it will weaken the northern Jet Stream so that summer (rather than winter) weather will become more common.
see also: Coriolis Force; Doldrums; El Niño and La Niña; Winds, Easterlies.
BIBLioGRAPHY. Phil McKenna, "Flying Windmills Could Harness the Jet Stream," NewScientist (July 2007); National Weather Service Southern Region Headquarters, www.srh.noaa.gov (cited October 2007); "The Tropics May Be Expanding" University of Utah News and Views (June 2006).
S. George Philander Princeton University
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