A rare rainbow known as a circumhorizontal arc was spotted over Idaho on June 3rd. The arc isn't a traditional rainbow, but rather caused by light passing through wispy, high-altitude cirrus clouds, and only occurs when the sun is very high in the sky - more than 58 degrees above the horizon. Additionally, the hexagonal ice crystals that make up the cirrus clouds must be shaped like thick plates with their faces parallel to the ground, so that light enters through the vertical face of the ice crystal and leaves from the bottom face; refracting the way light passing through a prism does.
Rare rainbow spotted over Idaho
Published: July 26, 2006, 7:46 pm