The fast-moving band of upper-atmosphere winds that generally blows from west to east is called a jet stream.
Jet streams are narrow corridors of especially strong wind near the tropopause, the boundary between the troposphere and stratosphere. They form mainly because of strong horizontal temperature contrasts between large air masses. Earth’s rotation deflects moving air, helping organize the flow into powerful west-to-east currents.
The polar jet stream and subtropical jet stream are especially important in weather forecasting. Their position and shape influence the paths of storms and the movement of warm and cold air. A jet stream can develop large meanders, called Rossby waves, that sometimes help weather patterns persist.
Jet streams are not solid tubes of air and do not remain at one fixed altitude or latitude. Commercial aircraft may use favorable jet-stream winds to shorten flights, while flying against them increases travel time and fuel use.