When I looked up this morning, I noticed a dark black line running across the bright blue sky. It took me a moment to realize it was a shadow created by the contrails of a jet. The cool part was that everything lined up in such a way that the plane was flying directly into its own shadow. It was a fun illusion to watch as this dark line slowly turned white as the jet traveled across sky.
I did a little more research into this phenomenon and found this great site about contrails and shadow effects. The type of shadow I saw was an Edge Shadow.
Update: Here is how this phenomena looks from above. It’s subtle, but look for the shadows appearing under the contrails on the left hand side of the screen.