Sighting in Provins, September 7, 2014

At around 7 p.m., a motorist noticed a very distant, horizontal black line in a clear sky, just above a few white aircraft contrails. A bright white flash followed. The witness had to keep their eyes on the road and lost sight of the phenomenon. Only one account was collected, and no photo was available.

GEIPAN linked the black line, seen together with condensation trails, to a natural phenomenon known as a volumetric shadow seen edge-on (an "edge shadow"). When a straight contrail passes in front of the sun and lines up with the observer, it casts a shadow across the whole slice of atmosphere lying in that plane. That slice behaves like a slightly tinted pane of glass: unnoticeable when viewed face-on, but a dark streak when seen from the side, and the darker the longer the straight part of the contrail. Because the geometry has to be very precise, the effect can appear and vanish suddenly, especially for someone in a moving car.

The flash was most likely a brief, intense reflection of the sun off an aircraft. It was not necessarily connected to the black line: several planes were in the sky, and the one reflecting the sun could have been the one that made the contrail or another. Without a photo, the explanation cannot be proven, but it is very probable. GEIPAN classified the case as B: aircraft contrail (volumetric shadow).