Circumenatal arch

Cincunzenital arc is an optical phenomenon similar to a rainbow, but it forms from refraction of sunlight horizontally through properly oriented ice crystals, usually in Cirrus or Cirrostratus clouds, rather than raindrops. Their colors are purer than those of the rainbow because there is much less color overlap in their formation; blue is inside the arch and red is outside.

This type of bow gives the impression of an upside down rainbow. It is rarely noticed, as it occurs quite high in the sky, but is actually relatively common. If the sun is low and you see one or two Cirrus cloud parelels, you can also see a circumzenital arc if they are also in the zenith (the middle of the sky).

The cincunzenital arc only forms when the sun is below 32 ° (ie if the sun is less than half of the sky). The circumzenital arc is brighter when the sun is 22 ° above the horizon.