It wouldn’t be surprising at all to hear that if a cloud moves in front of the sun, a shadow appears on the ground. But I recently observed pretty much the same thing and found it quite fascinating.

I was looking out of a plane window and saw long dark lines in the clouds below me. For quite some time I tried to come up with processes that might cause that. The line didn’t look like a lee wave of some very high tower. It also didn’t look like it was mirroring a channel or autobahn below. So what could it be?

That became quite obvious when I looked up (which I had tried to avoid before, because the spinning blades of the propeller were a bit unnerving): the dark line was the shadow of a plane’s condensation trail!

Not that surprising after all, but it took me surprisingly long to figure it out.

And it does look pretty cool, doesn’t it?