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 lesser streams closer to the equator. In the northern hemisphere the streams are most commonly found between latitude 30° - 70° for the polar jet stream and between latitude 20° - 50° for the other. The wind speeds vary according to the temperature gradient, they average 55 km/h in summer and 120 km/h in winter, although speeds of over 400 km/h are known. Technically the wind speed has to be higher than 90 km/h to be called a jet stream.
The location of the jet stream is an extremely important datum for airlines. In the United States, for example, the time needed to fly east/west across the US can be increased or decreased by about 30 minutes if the airplane can fly with or against the jet stream.