Atmospheric
Optics
Circumzenithal & Supralateral Arcs Seen by Carol Bramley over Kington, Herefordshire, England. Whenever the queen of halos, a circumzenithal arc, is seen look for a rarer supralateral arc touching it.
Images ©Carol Bramley, shown with permission
90° Prisms
Both arcs have widely separated colours, the result of refraction through ice crystal facets inclined 90° to each other.
Circumzenithal arc colours are purer. The plate crystals generating it have their top faces almost exactly horizontal. The millions of icy 90° prisms are therefore all well aligned.
The horizontal columns producing the supralateral arc take all rotational positions around their long axes. Their 90° prisms are therefore tilted over a whole range of angles and the halo colours overlap.
