Yesterday morning while driving to work, I looked up from the road and saw two parallel lines in the sky: One white, one dark blue. They were, of course, a contrail and its shadow on a thin cloud layer below. Because it was a thin layer, I started looking (when I had the chance) for fragments of halos.
When I finally stopped the car, I took a picture of the contrail and its shadow (now nearly aligned with the sun), looked up and saw a faint edge of a halo. I moved a bit to the left, putting a tree in front of the sun, and there it was: a clear 22-degree circular halo centered on the sun..