Rare 'fire rainbow' appears over Vandenberg Published June 2, 2009 By 050509 VANDENBERG AIR FORCE BASE, Calif. -- Team V received a glimpse of a rare optical phenomenon here May 18. The circumhorizontal arc, more popularly known as a "fire rainbow," appeared in the skies above Vandenberg at approximately 11 a.m. to 1 p.m. The arc is formed when sunlight rays interact with the ice crystals of cirrus clouds, producing well-separated rainbow-like colors. If the crystal alignment is just right, this interaction makes the entire cirrus cloud appear to shine like a fiery rainbow.