Viewers in western Wisconsin today observed a unique atmospheric phenomenon: a circumhorizontal arc! Casually called a ‘fire rainbow,’ it occurs in summer-time as the sun must be high in the sky, at an angle of 58 degrees or higher.
The circumhorizontal arc forms similarly to a halo in that sunlight is refracted or bent through hexagonal shaped ice crystals that comprise high-level cirrus clouds. This process splits the white light into individual colors, like a prism.
The sunlight enters the vertical side face of the hexagonal ice crystal and exits through the lower horizontal face.
Its cousin, the majestic circumzenithal arc, appears as an upside-down rainbow, and forms when sunlight enters the top horizontal face and exits a vertical side face. This arc occurs when the sun is fairly low in the sky.
Both the circumhorizontal and the circumzenithal arc display colors more pure than those of a traditional rainbow.