The particularly colorful halo featured above was snapped at Yamdrok Tso Lake, Tibet on October 29, 2011. This is a 22-degree halo — the radius of the inner ring to the Sun. It’s the most frequently observed of the circular halos. Randomly oriented, hexagonal ice crystals in cirrus clouds are responsible for its formation. Photographer Alan Millar
After entering one of the ice crystal’s side faces, sunlight (or moonlight) is refracted by 22 degrees (the angle of minimum deviation) before exiting an alternate side face of the same crystal. Note that the darkened area within the halo is similar to the dark region between the primary and secondary rainbow (Alexander’s dark band). In this region of the sky, light can’t be scattered by means of refraction alone. The rainbow-like rings on the inside of the halo are lens artifacts. Shield your eyes whenever you look in the direction of the Sun.