halo arcs, the huge
circumhorizon arc (right) hugs the horizon and is seen only in summer
when the sun is high. The smaller circumzenithal arc
(top) is almost overhead and seen only when the sun is low.
Both are formed by plate shaped ice crystals drifting with their large faces almost horizontal in high cirrus clouds.
Circumzenithal arc rays enter the crystal top and leave through a side face. The refraction through the 90° prism disperses the sunlight widely and purely into its spectral colours.
Summer's circumhorizon arc rays are the reverse. Sunlight enters a side face and leaves through the lower horizontal face. The colours are equally pure and bright. Both ice halos have purer colours than the watery rainbow.