Book a round trip and you will often notice something odd. The flight out and the flight home cover the exact same distance, but one is longer, sometimes by more than an hour. New York to London might be scheduled at 7 hours, while London back to New York is closer to 8. The distance did not change. The wind did.
High above the ground, around the altitude where airliners cruise, there are fast rivers of air called jet streams. In the northern hemisphere the strongest of them generally flows from west to east, and it can move at 100 miles per hour or more. An aircraft flying east rides that current like a boat going downstream. Flying west, it fights upstream. Same plane, same route, very different ground speed.
This is why eastbound flights are usually the quick ones. A jet cruising at 500 knots through the air that also has a 100 knot tailwind is actually covering the ground at 600 knots. Turn around and fly into that same wind and your ground speed drops to 400 knots. Over a long ocean crossing, that difference adds up fast. It is the single biggest reason a return leg can run an hour longer than the outbound.
It depends on the route and the season, but on the busy North Atlantic and North Pacific corridors, the jet stream is strong enough that airlines plan for it. Winter jet streams are faster than summer ones, so the gap between eastbound and westbound times is widest in the colder months. Pilots even shift their exact track north or south each day to find the best tailwind or dodge the worst headwind.
Most simple distance calculators ignore wind entirely and give you the same number in both directions. FlightBeat does not. Our flight time calculator applies a jet-stream style wind adjustment based on the direction and latitude of your route, so an eastbound trip comes back shorter than the westbound one, the way it works in real life. Try it on a long east-west route and swap the airports to see the difference.
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