Jeff took this photo of a circumhorizontal arc about 2:30 p.m. on June 12, 2009 while fishing at Logan, Utah's First Dam. A circumhorizontal arc (also called a fire rainbow) is a rare phenomenon. In order to be observed, the sun must be high in the sky, and cirrus clouds must be present. If cloud conditions are right, it can be seen along the horizon on the same side of the sky as the sun. The arc is formed as light rays enter the horizontally-oriented flat hexagon crystals through a vertical side face and exit through the horizontal bottom face. It is the 90° inclination that produces the well-separated rainbow-like colors and, if the crystal alignment is just right, it can make the entire cirrus cloud shine like an undulating rainbow. (Click on photo to enlarge)
Jeff witnessed a similar phenomenon in Colorado three years ago, and described it in an earlier blog entry, dated August 11, 2006
