The jet stream is a narrow band of very strong wind high in the atmosphere that generally blows from west to east. Jet streams flow near the tropopause, the boundary between the troposphere and the stratosphere, at altitudes of about 8 to 15 kilometers (5 to 9 miles). They form where large temperature differences exist between neighboring air masses, which is why they are strongest in winter.
Jet stream winds usually blow at 129 to 225 km/h (80 to 140 mph) and can exceed 443 km/h (275 mph). Each hemisphere usually has two main jet streams: a subtropical jet near 30 degrees latitude and a polar-front jet whose position shifts with the boundary between polar and temperate air. Airlines take advantage of them, flying with a jet stream or above it rather than against it.