|Halo Pair imaged by Charley Hale in Lafayette, Colorado, USA.ęCharley Hale, shown with permission.|
|The bright arc is the familiar upper tangent arc, perhaps the third most common halo after sundogs and the 22° halo. Its appearance here is unusual because it is unaccompanied by a conspicuous 22° halo.
Whenever a bright and well defined upper tangent arc is seen, look above it for the rarer Parry arc. One is just visible in the picture.
Both arcs are made by hexagonal column crystals. Rays pass between two prism side faces inclined 60° to each other.
The crystals making the upper tangent arc drift with their long axis almost horizontal AND the column can take all rotational positions about the long axis. Individual crystals do not necessarily spin or rotate.
Parry arcs come from crystals that drift with the long axis horizontal AND a prism side face horizontal. that's improbable but the orientation is very efficient at making halos and so we see Parry arcs more often than we might otherwise. Unlike the orientation that makes upper tangent arcs, rays passing through Parry side faces inclined at 60° can do so in three ways giving separate Parry arcs.
Traces of only one 'sunvex Parry arc' is visible in the image but the approximately matching HaloSim ray tracing shows that two were potentially visible for a solar altitude of 11°.
Both Parry and upper tangent arcs alter in appearance as the sun climbs.