The title is not a typo: this is about sky lines, not skylines.
White lines across the sky are easy to interpret: contrails (trails of condensation from aircraft).
What about dark lines across the sky, such as seen in these pictures? These lines also result from contrails, but indirectly. Here, the contrails are above the clouds and not visible. But the shadows they cast on the lower clouds trace dark lines across the sky.
Well I’ll be jiggered ! ! ! Thank you, Alistair. I guess I never even thought about it, but it’s still very good to know.