Notes and correspondence: Origin of the Parry arc

Laboratory experiments to determine the preferred orientation of free-falling hexagonal prisms were performed at Reynolds numbers appropriate to falling ice crystals in the atmosphere. Hexagonal plates orient with their c axis vertical for aspect ratios < 0.9, whilst hexagonal columns fall with their c axis horizontal. A secondary alignment is also observed: regular hexagonal columns fall preferentially with two prism facets aligned vertically, and not horizontally the latter scenario was previously assumed responsible for the rare Parry arc. However, if the column is made scalene in its cross section, it can orient such that a pair of prism facets are horizontal. This finding indicates that the development of scalene crystals may be key to the production of certain ice crystal optical phenomena. Copyright c © 2010 Royal Meteorological Society