A circumzenithal arc is an optical effect which looks like an upside-down rainbow.
A circumzenithal arc (sometimes known as Bravais' arc) is a type of halo. They are formed when sunlight refracts through horizontal ice crystals at such an angle that the light enters the crystal through its flat top face and exits through a side prism face causing the distinctive upside-down rainbow effect. They are commonly associated with cirrus clouds where ice crystals readily form.
Circumzenithal arcs are actually quite common as these types of clouds occur throughout the year, however we only sometimes see them as they are usually obscured by clouds underneath
In order to be able to see a circumzenithal arc, a combination of atmospheric conditions must coincide just right. The height, depth and position of the ice clouds must be right as the cloud needs to be at a specific angle convex to the sun. The position of the observer is also important since its visibility can vary greatly over short distances.
Below are some examples of circumzenithal arcs:
Last updated: 23 October 2014