As sunset approached - and after sunset - on December 7th. '15, Dutch skies had ice halos forming a striking double 'V' of upper Parry and tangent arcs.
The upper arc is the sunvex Parry, the lower the tangent arc.
Both arcs are formed by hexagonal columns drifting in high cirrus with their long axes horizontal. But there the similarity ends.
Tangent arc forming columns have random rotational positions about their long horizontal axis. However, the individual crystals do not necessarily rotate.
Parry arcs are from columns that, for whatever reason, do not have random rotations and keep their uppermost prism side face horizontal. This is a rare orientation and Parry arcs would be even rarer than they are except that very few Parry oriented crystals are needed to produce a bright halo.
Images ©The Photographer and shown with permission
An unusual sighting several minutes after the sun had set at ground level. The high cloud containing the halo forming ice crystals was still sunlit.
Under these conditions the Parry and tangent arcs nearly coincide at the bottom of the 'V' but separate higher up.