Double Halos above Black Forest
Large halo around the Moon or the sun occurs when high thin clouds containing millions of tiny ice crystals cover much of the sky. Each ice crystal acts like a miniature lens. Because most of the crystals have a similar elongated hexagonal shape, light entering one crystal face and exiting through the opposing face refracts 22 degrees, which corresponds to the radius of the Sun or the Moon Halo. But in this view from the Black Forest (Schwarzwald) in Germany there are more aspects of the atmospheric optics. Both natural and contrail are visible. On the sides of the halo are sundogs (two bright colorful spots with a ray coming out of them). Above the halo shines an upper “tangent arc” and further away from the sun is the giant rare 46° halo (92° in diameter) with more obvious colors. For more information about halos around the moon and sun visit the Atmospheric Optics.