Circumzenithal Arcs Or Upside Down Rainbows

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

For a circumzenithal arc to form, the ice crystals must be at a specific orientation relative to the sun. Light must enter the ice crystal through its flat top face and exits through a side prism face. For this to happen, the sun must be at an altitude lower than 32° with the brightest formation occurring when the sun is at 22° above the horizon, which causes sunlight to enter and exit the crystals at the minimum deviation angle. The refraction of almost parallel sunlight through what is essentially a 90-degree prism accounts for the wide color separation and the purity of color.

The air also needs to be relatively still, so that the ice particles can have a common orientation, which can only occur in the absence of turbulence and when there is no significant updraft or downdraft.

Circumzenithal arcs are more common in cold climates.

Another related phenomenon is Circumhorizontal Arc or Fire Rainbows.

Photo credit: Roger Sanderson/Flickr

Photo credit: Matthew Bednarik/Flickr

Photo credit: Matthew Bednarik/Flickr

Sources: BBC / Wikipedia / Atoptics