Origin of Kern's arc

With the aid of computer-simulated halo patterns, we show that Kern's arc, as seen on the latitude of the circumzenithal arc and on the other side of the zenith, is produced by double-plate ice crystals with a vertical principal axis. Light rays that contribute to Kern's arc are demonstrated by geometric ray tracing. We also discuss the condition under which an arc that is opposite a circumhorizontal arc can appear.