Fast-moving dark bands cross halo
December 15, 1976. Chelmsford, Essex, England. At 1505 GMT, two groups of closely spaced grey bands were seen crossing the upper arc of contact of a 22° halo from left to right. The first group lasted 10 seconds, with a 15-second quiescent period before the second group. The second group lasted about 5 seconds. About 30 straight parallel, regularly spaced bands appeared during the first observation. Moving steadily, they took 2 seconds each to cross the arc of contact.
The most likely cause of the phenomenon was thought to be changes in the orientation of the ice crystals that created the upper arc of contact. However, the author could suggest no physical mechanism for producing such unusual motion in the ice crystals.
(Burton, B.J.; "Fast-Moving Dark Bands Crossing the Arc of Contact," Journal of Meteorology, (U.K.), 2:233 1977.)