Relatively narrow currents of very strong winds which move large air masses from west to east, affecting weather systems around the globe.
Jet streams are found 9-16 km above the surface of the Earth, just below the tropopause. They form at the boundaries of adjacent air masses with significant differences in temperature, such as the polar region and the warmer air to the south.
Climate change affects jet stream, causing so called “wandering jet streams”; when the jet stream follows a ‘wavy’ irregular path there are more cold weather fronts plunging south from the Arctic into mid-latitudes, bringing freezing conditions that persist for weeks at a time (e.g. record cold, snowy winters).