Asymmetric flight times

Why does the same flight take noticeably longer in one direction than the other?

London to New York is scheduled at around eight hours; New York to London at around seven. Same aircraft type, same distance. The gap is routinely an hour or so, and on a good winter night it is larger than that.

The explanation people reach for first is that the Earth is turning underneath the aeroplane, so flying against the spin ought to take longer. It is a tempting picture and it is wrong — the atmosphere turns with the planet, and an aircraft flying in it shares that motion, which is why a helicopter hovering over Nairobi does not find itself over the Atlantic an hour later. So the asymmetry must come from something moving relative to the air's general rotation. What could produce a persistent one-way wind, five miles up, strong enough to cost an hour?

Begin with the one fact the whole atmosphere is organised around: the tropics receive far more solar energy per square metre than the poles do. That leaves a standing temperature difference across latitude, and a temperature difference is never just a temperature difference — warm air is less dense, so a column of warm tropical air is taller than an equally massive column of cold polar air.

Now think about what that does with height. At the surface the pressures can be nearly equal, but climb, and you are higher in the tall warm column than in the short cold one, so there is more air still above you on the tropical side. The pressure difference between tropics and pole therefore grows as you go up. That is the crucial step, and it is why this wind lives near cruise altitude rather than at ground level.

A pressure difference makes air move from high toward low — poleward, in this case. Ask what happens next. On a rotating planet, air that moves is deflected: to the right of its motion in the northern hemisphere. Air setting off northward gets turned east. It keeps being turned until the deflection balances the pressure force, at which point the air is no longer flowing poleward at all — it is running west to east, along the temperature contrast rather than across it. Which is exactly what we were looking for: a persistent one-way wind, strongest where the temperature gradient is sharpest and concentrated near the top of the troposphere. That is the jet stream.

Two consequences follow without any extra assumption. First, the jet must be stronger in winter, because that is when the pole-to-tropics contrast is steepest — and it is. Core speeds commonly run in the region of 100 to 200 km/h, with the strongest winter cores exceeding 300 km/h. Second, it will not be a tidy ring. A fast, thin river of air is unstable to wobbles, and the jet meanders in great north-south loops that shift day to day, which is why the useful wind is in a different place each morning.

That is enough to explain the schedule. Eastbound over the North Atlantic, a dispatcher plans a route that intersects the jet core and rides it; the aircraft's speed through the air is unchanged, but its speed over the ground is the sum. Westbound, the same wind is a headwind, so the route is planned to dodge it — typically further north or south — accepting extra distance to avoid worse air, which is why the North Atlantic tracks are redrawn daily around where the jet actually is. When the jet is exceptional the effect is dramatic: in February 2020 a British Airways 747 flew New York to London in 4 hours 56 minutes, the fastest subsonic crossing on record, in a jet stream well over 200 knots.

The wind does not make the aircraft fly faster. Airspeed — speed through the surrounding air — is set by the engines and the flight plan and barely changes. What changes is groundspeed, which is airspeed plus whatever the air itself is doing. The aircraft is not being pushed; it is embedded in a medium that is moving.

Nor is the eastbound route the shortest one. Ground distance is often deliberately increased to gain wind, because the airline is minimising time and fuel, not kilometres. So the asymmetry you experience is partly the wind and partly the routing decision made in response to it, and the two are not separable from a passenger seat.

There is also more than one jet. The polar-front jet described here dominates the North Atlantic, but a subtropical jet sits near 30 degrees latitude, and both shift equatorward in winter. Scheduled block times fold in a seasonal average of all this, which is why the timetable itself, not just the flight, is asymmetric.

Finally, an honest limit: winds dominate on long high-latitude routes but are not the only source of a directional difference. Runway assignment, taxi times, and terminal routings contribute minutes. They do not contribute hours.

• Why the jet's meanders, when they lock into place, also govern cold snaps and heatwaves at the surface.
• How the North Atlantic Track structure is rebuilt every day around the forecast jet.
• Whether a warming Arctic, by weakening the pole-to-tropics gradient, changes the jet — an active and genuinely unresolved argument.

Every term the collection defines is gathered in the glossary.