How to work out where the plane overhead is going

Look up on a clear day near any large city and you will see aircraft. Most people see undifferentiated silver dots. With a little practice you can narrow almost any of them down to a plausible route, without an app.

Here is the order to think in.

Start with how high it is

Altitude is the single most useful thing you can judge from the ground, because it separates aircraft that are going somewhere far away from aircraft that are about to land.

An airliner at cruise sits between 30,000 and 42,000 feet. From directly underneath it looks small, moves slowly across the sky, and leaves a contrail in the right conditions. It will cross the whole visible sky in perhaps three or four minutes. That aircraft is in the middle of its journey, and both its origin and destination are probably hundreds or thousands of miles away.

An aircraft below about 10,000 feet is a different animal. You can make out the wings, sometimes the engines, and the shape of the tail. It is climbing out of or descending into an airport near you — which means one end of its journey is a place you can name.

The rule of thumb: the lower it is, the more likely one end of the route is local. That single observation does most of the work.

Then the direction

A cruising aircraft is flying a great circle — the shortest path across a sphere — and on a flat map that looks like a curve, not a straight line. This is why flights from Europe to the west coast of North America pass over Greenland rather than heading due west.

So the bearing tells you more than it appears to. An aircraft over southern England heading north-west at cruise is very likely transatlantic. One heading south-east is bound for the Gulf or Asia. Wind matters too: eastbound transatlantic flights ride the jet stream and mostly cross at night, while westbound ones fight it and cross during the day.

Time of day narrows it hard

Long-haul schedules are remarkably rigid, because they are shaped by airport curfews and by the need to arrive at a civilised hour.

Early morning, arriving: overnight long-haul from the east — Asia, the Gulf.
Mid-morning, departing: long-haul westbound, aiming for an afternoon arrival in North America.
Late evening, departing: eastbound overnight flights.
All day, low and frequent: short-haul, and almost certainly one end is the nearest large airport.

If you are watching an aircraft descending over a European city at eight in the morning, the odds are heavily on it having flown through the night from a long way east.

Finally, the tail

The tail is the most identifiable part of an aircraft from the ground, which is why airlines paint their branding there rather than on the fuselage. It is often readable long after the registration has become a blur.

You do not need to memorise many. A dozen carriers account for a large share of the traffic over most cities, and the colours are deliberately distinct — that is their entire purpose. Once you can pick out the four or five airlines that fly over your house regularly, you have effectively narrowed the route to a handful of possibilities before you have consciously reasoned about anything.

Putting it together

A widebody, high, over southern England, heading south-east at nine in the evening, with a red tail. That is not a guess any more — it is an inference, and you can get within a few hundred miles of the right answer from the ground.

This is exactly the reasoning Where To asks for. In the daily puzzle you get one aircraft, its position and a fragment of its course, and the same question: where did it take off, and where is it going?

If you would rather do it with a real aircraft above your own head, try Spotting.