What is a jet stream?
High above us there is a river of fast wind called the jet stream. Planes flying east ride it like a moving walkway, which is why flights from America to Europe are quicker than the other way.
A jet stream is a fast river of wind high in the sky. Flying with it saves time and fuel, which is why Atlantic flights are quicker going east.
What it is
Jet streams are narrow bands of very strong wind about 30,000 to 40,000 feet up. They usually blow from west to east and can reach more than 100 knots.
Why flights are faster eastbound
Flying east with the jet stream gives a tailwind, so the plane covers the ground faster. London to New York usually takes about an hour longer than New York to London.
Planning
Airlines choose routes that use the jet stream when flying east and avoid it when flying west, so the track changes from day to day.