An ice crystal "rainbow" hangs in the air behind skier Thomas Lambert of Switzerland on January 12 during practice for the 2007 Chevrolet Freestyle International World Cup in Park City, Utah.
The faint halo, also called a sun dog, isn't a real rainbow. By definition a rainbow is created only when sunlight is broken into its component colors by raindrops. Sun dogs are usually formed when light passes through flat, six-sided ice crystals high in Earth's atmosphere. (Related photo: "Rare 'Rainbow' Spotted Over Idaho" [June 19, 2006].)
But so-called diamond dust—free-floating ice crystals lower in the sky—can also create the visual treat.