[D231] DE PROVINS (77) VERS CHENOISE (77) 07.09.2014

On 7 September 2014 at about 19:00, a motorist on the D231 from Provins toward Chenoise saw a very distant horizontal black line in a clear sky, just above several white aircraft contrails, followed by a white flash. The witness looked back at the road and lost sight of it. Only 1 account was collected. GEIPAN considers the combination of a black line and contrails characteristic of a volumetric shadow seen edge-on, called an “edge shadow.” It becomes visible when a straight contrail passes in front of the Sun and the Sun, contrail and observer align. The contrail casts a shadow through the volume of atmosphere within the plane containing all three. That thin region of atmosphere behaves like a slightly tinted pane of glass: it is barely noticeable face-on but looks like a dark line when seen edge-on. The greater the length of tinted glass viewed along its edge, the darker the line; here this corresponds to a longer straight section of contrail. The dark line extends through the observer's entire vertical viewing field, including the sky. The phenomenon requires precise geometry, with the observer, contrail and Sun in the same plane, so it can appear and disappear abruptly, especially for an observer in a moving car. The white flash was probably a brief, intense reflection of sunlight from an aircraft acting like a mirror. Such reflections occur often but are seldom noticed unless someone is watching the sky and aircraft, as this witness was after noticing the black line. The 2 phenomena need not be related beyond occurring at the same time. Several contrails indicated multiple aircraft; the reflecting aircraft could have produced the shadow-making contrail or been another aircraft. The black line was what prompted the report. No photograph is available here, but the photographed case HYERES (83) 27.05.2016 provides a more detailed explanation in its investigation report. GEIPAN cannot prove the explanation in this case but considers its probability very high. It classifies the case B: an aircraft contrail's volumetric shadow.