Scattering by spherical particles in the atmosphere can result in three distinct displays: coronas, rainbows, and glories. If the particles depart somewhat from perfect spheres, the displays are not greatly changed. An entirely new and more varied set of displays arises when the particles are ice crystals, which are far from spherical and, because of their shape, can be oriented by aerodynamic forces. As with rainbows the gross features of ice-crystal displays can be described simply, but approximately, by following the various trajectories of rays incident on crystals. Colorless displays (e.g., the subsuns touched on in Sec. 4.1) are generally associated with reflected rays, colored displays (e.g., sun dogs and halos) with refracted rays. Because of the wealth of ice-crystal displays, we cannot treat all of them, but one example should point the way toward understanding many of them.
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