Why can the same flight vary in duration?
Flying east to west commonly takes longer than flying west to east because a high-altitude wind called the jet stream blows from the west, slowing the aircraft down. The jet stream always blows in the same direction because of the Earth’s rotation. Warm air from the equator rises and moves north or south towards colder regions. Areas nearer the poles rotate slower than the equator. Air from the equator has the same ‘fast’ rotation speed around the planet as the equator, and as this air moves to regions where the ground beneath it rotates slower, it turns into a wind moving from west to east.
Answered by Rik Sargent