Jet streams are fast flowing, confined air currents found in the atmosphere at around 12 km above the surface of the Earth, just under the tropopause. They form at the boundaries of adjacent air masses with significant differences in temperature, such as of the polar region and the warmer air to the south (the meridional temperature gradient). Because of the effect of the Earth's rotation the streams flow west to east, propagating in a serpentine or wave-like manner at lower speeds than that of the actual wind within the flow.

There are two main jet streams at polar latitudes, one in each hemisphere, and two minor subtropical streams closer to the equator. In the Northern Hemisphere the streams are most commonly found between latitudes 30°N and 70°N for the polar jet stream, and between latitude 20°N and 50°N for the subtropical stream. The wind speeds vary according to the temperature gradient, averaging 55km/h or 35 mph in summer and 120km/h or 75 mph in winter, although speeds of over 400km/h or 250 mph are known. Technically the wind speed has to be higher than 90km/h or 55 mph to be called a jet stream.

Airline great circle track at top, jet stream track at bottom

The location of the jet stream is an extremely important datum for airlines. In the United States and Canada, for example, 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 the same amount if it must fly west against it. On international flights, the difference is even greater, and it is often actually faster flying eastbound than taking the great circle route between two points.

The jet streams were first discovered during World War II by pilots flying bombers at high altitudes. The first practical use of jet streams was presumably the Japanese fire balloon attacks on the American mainland later during the war.

See also

Great circle

External links

• Current movies of the Jet Stream (http://virga.sfsu.edu/crws/jetstream.html)