The title is not a typo: this is about a sky line, not a skyline. The last time I saw a nice example of this was three years ago: sky lines. This posting is an adaptation of what I wrote then, but with the current picture.
A white line across the sky is easy to interpret: a contrail (a trail of condensation from aircraft).
What about a dark line across the sky, such as seen here? This line also results from a contrail, but indirectly. Here, the contrail is above the clouds and not visible, but the shadow it casts on a lower cloud traces a dark line across the sky.
Brilliant!
What an observation – cool!
That’s very interesting.