Vanishing suddenly in Seine-et-Marne, France

On September 7, 2014 at around 7 p.m., a motorist saw in the clear sky, just above a few white trails left by planes, a very distant horizontal black line, then a white luminous flash. The witness looked back at the road and lost sight of the phenomenon. Only one testimony was collected. The conjunction in the sky of a black line and airplane condensation trails is characteristic of the natural phenomenon of volumetric shadow seen edge-on ("edge shadow" in English). This phenomenon becomes visible when a straight airplane trail passes in front of the sun and an alignment of sun/trail/observer occurs. A cast shadow then forms over the entire volume of atmosphere contained in the planar slice containing the trail, the observer, and the sun. This slice of atmosphere behaves like a slightly tinted pane of glass, which one does not notice when looking at it head-on but which makes a dark streak for someone viewing it from the side (or edge-on), a streak that is darker the longer the pane is, that is, here, the longer the straight part of the trail is. The dark streak created is present across the entire height of the witness's line of sight, including therefore across the sky. This phenomenon requires precise geometric conditions (witness, airplane trail, sun in the same plane). It can therefore appear and disappear suddenly, especially for a witness in a car as here. The luminous flash in the sky is probably a reflection of the sun on the airplane, an intense and brief reflection (the airplane acts as a mirror to the sun) as often happens and which is rarely noticed except when looking at the sky and airplanes (the case here because the witness's attention was drawn by the black line). There is not necessarily a link between the two phenomena, just a temporal coincidence; there were several airplanes at that moment (several trails) and the reflecting airplane could have been the one of the trail causing the black line or another one. The strangeness at the origin of the testimony is the black line. We have no photo here. The analysis of another testimony (HYERES (83) 27.05.2016) with photos allows a more precise presentation of the phenomenon with a detailed report. In this case, it is not possible to provide proof of the explanation, but the probability is very high. Consequently, GEIPAN classifies this case as B: airplane trail (volumetric shadow).