1 July 2015
This must be the season for them because I saw one myself last weekend (here in Maryland) for a few seconds. I was unsure of what it was, but assumed that cirrus clouds were reflecting the sunlight in a strange way, much like a sundog. The one I saw did not move as much as this one though, and it is rather amazing. Language warning on the video, but I’d have been freaked as well!.
The Cloud Appreciation Society has more on what is happening here, but in general the ice crystals in the thin cirrus clouds are suddenly aligning with the electric field generated by a rising cumulus tower below. The ice crystals reflect the light but the changing field is turning the ice crystals. Phil Plait at Bad Astronomy found a letter to NATURE about it in 1971. I’ll be honest with you, most meteorologists have not likely even heard of this before!