Is there a way to estimate the height of a plane? © Getty Images

Is there a way to estimate the height of a plane?

It's difficult & inaccurate, but if you really want to know the dizzying height, you first have to be able to identify which high flyer you're looking at.

Asked by: Tony Carter, by E-mail


It’s tricky. If the aircraft is directly overhead, you can hold your thumb outstretched and compare it to the apparent size of the plane. Your thumb subtends an angle of roughly 1° (in other words, covers 1° of your field of view). A Boeing 747 has a wingspan of 64m so it’ll be exactly covered by your thumb at around 3700m, and half the size of your thumb at roughly 7400m, and so on.
But this is very inaccurate, relies on you identifying the plane and knowing its wingspan, and doesn’t work if the plane is at an oblique angle to you. Rain clouds form below 2000m and cirrus clouds between 5000 and 12,000m so you can use this to provide an upper limit if you see a plane flying under clouds. Also, contrails normally only form behind the plane at altitudes above 7900m. Commercial jetliners normally cruise at about 11,000m.


Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.