There is a entire system of strong winds that constantly stream around the Earth at altitudes of between 8 and 16km. These winds are known as jet streams and they occur in the area between the upper troposphere and the stratosphere. Jet streams are driven by strong temperature differences in the upper atmosphere, together with the rotation of the Earth. These enormously strong winds can reach speeds of up to 540km/h, even stronger than the wind speeds measured inside tornadoes.
Because jet streams can always be relied on to blow in the same regions, aircraft on intercontinental flights make use of them. Although this means deviating from routes that are actually shorter, it reduces flying time and cuts fuel costs.
The regularity of jet streams was first recognized during high-altitude Second World War bombing raids.