The Jet Streams
Jet streams are areas of high-speed winds that occur at about 25,000 to 50,000 feet (7,600 to 15,200 m) above the earth. Several distinct jet streams are known, and all seem to be associated with weather and climate at the earth's surface. The course, speed, and altitude of each stream varies from day to day. Jet streams are normally 100 miles (160 km) or more wide and sometimes extend completely around the earth. Wind speed is highest at the core of each stream, where it may reach 150 to 300 miles per hour (240 to 480 kmh).
Commercial and military airplanes sometimes use jet streams as powerful tailwinds in order to achieve higher than normal airspeeds or to conserve fuel.