Contrails formed by aeroplanes on a busy air space in the skies over Paris.
<a rel="nofollow" class="external text" href="https://www.flickr.com/people/63488671@N00">Edgar Jiménez</a> from Porto, Portugal
CC BY-SA 2.0
<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/chilangoco/6850975255/">Shall we put a traffic light?</a>