A jet stream is a narrow, fast-moving band of strong winds in the upper troposphere.
Jet streams form near sharp temperature contrasts between large air masses. The strongest examples are usually found several kilometres above Earth’s surface, where pressure differences and Earth’s rotation help organize winds into long, meandering currents. The polar front jet stream is especially important for weather in the middle latitudes.
Jet streams generally move from west to east, but their paths can develop large waves called Rossby waves. These waves help steer low-pressure systems, storms and areas of warm or cold air. Aircraft can use a tailwind from a jet stream to reduce travel time and fuel use, while headwinds can have the opposite effect.
Jet streams are not perfectly continuous tubes. Their speed and position change with season, temperature patterns and the distribution of land and ocean.