What is a circumhorizontal arc? A circumhorizontal arc is an optical phenomenon caused by sunlight/moonlight refracting off of ice crystals in the sky. They appear as a rainbow parallel to the horizon. The sun or moon has to be quite high in the sky for a circumhorizontal arc to form. We often see fragments of the arc because the arc forms only in cirrus clouds. Cirrus clouds are made of ice crystals.
Circumhorizontal arc’s form quite often over the united states during the summer months. That is because the sun’s angle is closer to 90 degrees. The circumhorizontal arc can not form in the higher latitudes because the sun angle is too low.