I took this photo with my mobile phone out of my window of my flat in Hannover-Limmer on 23th May, 2015 at about 10 o'clock in the morning. The photo features a prominent dark stripe on the cloud. What could it be?
The cloud ceiling was quite thin. You can see a few pale structures on the photo indicating, that the sun could weakly shine through the cloud coverage causing light shadows. The sun was located at the top of the right and is not visible on this photo. A few minutes later this optical phenomenon disappeared.
The reason for this appearance was a jet trail, that was located several kilometers above this cloud ceiling and lying between the sun and my line of vision. In this way the solar radiation under the jet trail was weakened and let the cloud ceiling appear darker at this point.