Horizontal distance between the Frankfurt airport and the aircraft depending on the aircraft altitude. The thick line indicates the mean distance, the green area delimits the range between 5^th and 95^th percentiles, over the period 2002–2012. Note that the offset distance (at the altitude of 0 m ASL) of a few kilometres roughly corresponds to the distance of the extremity of the airport runway. The Figure shows that the mean horizontal displacement of IAGOS aircraft in the lower troposphere ranges from 20 km at 1 km ASL to 90 km at 5 km ASL. DOI: https://doi.org/10.1525/elementa.280.f1