The atmosphere is filled with constantly moving air streams, determined by the temperature differences in various areas of the earth. One of these is the jet stream, which runs east to west around the world.
The jet stream is a very fast current of air. It always moves faster than 57 miles per hour, and can reach speeds of around 200 miles per hour. It is located 6 to 9 miles above the surface of the Earth and moves quickest during the winter, where there is a greater temperature difference at the Earth's surface.
The jet stream is both used and avoided by aircraft. Because it so fast moving, flights that are heading east to west navigate around the jet stream in order to avoid it as a headwind, which can greatly reduce the speed of an aircraft and the flight time. However, flights heading east can use the jet stream to cut down their total flight time.
By flying with the jet stream, aircraft can also use less fuel by allowing the fast-moving air to push them along. This also allows them to carry less fuel than usual, making them lighter. This will also help the aircraft move faster, reducing flight time even more.