How wind changes block time, and why eastbound is quicker
The jet stream makes some flights an hour shorter one way than the other. How much, in which months, and what an average can and cannot tell you.
Published by Overfly ·
Fly from New York to London and back and the return takes longer. Not by minutes: in January, by well over an hour. The reason is the westerly wind at cruising height in the middle latitudes, concentrated in the jet streams, which blows from west to east around both hemispheres and is strongest in winter.
What the average says
Overfly's route pages show block time for every month, both ways, for a business jet at Mach 0.85 and FL410 in that month's average upper wind. On New York JFK to London Heathrow the average along-track component in January is about 54 kt behind the aircraft eastbound and 55 kt on the nose westbound. Block time is 6h 00m eastbound against 7h 25m westbound, where still air would give 6h 40m each way.
The gap closes in early summer, when the jet weakens and moves north: in May the same route is 6h 20m eastbound and 7h 00m westbound. On Sydney to Los Angeles, the wind is a steady tailwind of 20 to 30 kt along the route all year, and the return to Sydney is between 1h 10m and 1h 45m longer, depending on the month.
The averages come from NOAA's NCEP/NCAR Reanalysis, monthly means for 1991 to 2020 on a 2.5° grid, at the pressure level nearest the cruise. They are sampled along the great circle and integrated in short segments, solving the wind triangle in each so that crosswind is accounted for, not just the head or tail part.
A headwind costs more than a tailwind gives back
If wind only shifted time from one direction to the other, a round trip would take as long as in still air. It does not. Time is distance divided by ground speed, and a headwind both lowers the ground speed and makes you spend longer in it. On New York and London in January's average, the round trip is 13h 25m against 13h 20m in still air. The difference grows quickly with wind speed, which is why the mapper adds time up segment by segment rather than averaging the wind first.
What an average cannot tell you
An average is smoother than any day. The jet stream on a given day is a narrow, meandering ribbon that can exceed 150 kt at its core, and its position moves by hundreds of miles from one day to the next. Averaged over thirty Januaries it becomes a broad, weaker band. A flight on a strong day can see far more: in February 2020, with an unusually strong jet behind it, a British Airways 747 crossed from New York to London in 4h 56m.
Airlines also chase the wind. The North Atlantic tracks are redrawn twice a day to follow the jet eastbound and avoid it westbound, so a real flight's time is better than an average along the great circle would suggest in one direction and less bad in the other. Overfly's figure is the honest middle: the same assumptions on every route, with the source named, labelled as an average.
For range, wind matters even more than for time: see usable range for why air distance, not ground distance, is what a fuel load has to cover.