Have you ever seen an inverted rainbow with its curved back towards the ground and ends pointing up, like a smile in the sky? Some people call them “upside-down rainbows”, but they are not in fact, rainbows, because they aren’t formed by light refracting through raindrops or mist, as in rainbows. Instead they are caused by light refracting through ice crystals in high altitude clouds such as cirrus or cirrostratus. The correct term for this meteorological phenomenon is circumzenithal arc, and they belong to the family of halos.

The circumzenithal arc is one of the brightest and most colorful members of the halo family. Its colors ranges from violet on top to red at the bottom, but are purer than those of a rainbow because there is much less overlap in their formation.

Photo credit: Charlie Harvey/Flickr