I believe this is the result of a very rapidly developing - HOT and moist - cumulonimbus reaching a much colder layer. I've seen lenticular clouds, but they are much more stable than this was. The entire event lasted just a few seconds, long enough to snap 3 cell pics, and then the "halo" was gone. I think the downflow of very cold air about the perimeter of the hot thunderhead actually generated snow at that altitude. The low angle of sunlight really lit up the halo. I've flown near the perimeter of thunderheads at 12,000 feet AGL and experienced incredible lift on the warm side, and enough downflow in the cold air drop to cause a prudent departure from the area. Lots of power in these babies.:shocked: