Why do some flights take longer in one direction than the other – sometimes an hour or two more? The simple answer is wind, and an upper atmosphere effect known – appropriately – as the jet stream.

How does it work? Essentially, the Earth’s rotation causes winds in the northern hemisphere to blow from west to east. It also causes air and water currents to circulate from warmer to colder regions and back. Take the Pacific Ocean: the warm air and water circulates up the coast of Asia and Russia, drops down from the poles along the west coast of the American landmass, and crosses west back above the equator. For centuries, sailors have known these flows as trade winds.

A similar phenomenon takes place far above the surface of the earth. As cold and warm air masses meet, they create jet streams: tunnels of faster-moving westerly air that move further north or south, depending on the changing seasons.