Fly from New York to London and it takes about 7 hours; fly back and it's closer to 8. Same distance, same aircraft — so why the difference? The answer is up in the atmosphere.
The Jet Stream
High in the atmosphere, fast ribbons of air called jet streams flow generally west to east, sometimes exceeding 100 mph. Aircraft flying eastward ride these tailwinds and arrive faster; those flying west fight them as headwinds and take longer.
Tailwinds vs Headwinds
A plane's speed through the air stays similar in both directions, but its speed over the ground changes with the wind. A strong tailwind adds to ground speed (shorter flight); a headwind subtracts from it (longer flight). On long east-west routes this can mean an hour's difference or more.
Why It Varies Day to Day
Jet streams shift in strength and position with the seasons and weather, so the exact time difference changes from flight to flight. Airlines plan routes to use favourable winds and avoid the worst headwinds, which is why flight paths aren't always a simple straight line.
Frequently Asked Questions
Because of the jet stream — fast high-altitude winds that flow generally west to east. Flying eastward, planes get a tailwind and arrive faster; flying westward, they face a headwind and take longer. On long routes this can add an hour or more to the westbound leg.