this topic even showed up at spaceweather today, sort of...

Earlier today, a tnetwork ofhin layer of icy clouds passed in front of the sun over Vienna, Austria. Immediately, a bright ice halo surrounded the sun--a result of refraction by the cloud's inner ice crystals. As Jennifer Werner was photographing the display, a plane flew by, bisecting the halo. Look at the picture and answer this question: Is the contrail above or below the clouds?

Visually, the dense white contrail appears to be underneath the clouds. But the contrail's shadow tells a different story. In fact, the contrail is above, casting its shadow on the clouds below. Where the shadow steals sunlight from the ice crystals in the cloud, the circular halo vanishes.

Not everything in the sky is as it seems. More examples may be found in the realtime photo gallery:

www.spaceweather.com