Ice crystals produce a wonderful variety of optical phenomena in the atmosphere. These effects can arise from the refraction (bending) of light rays passing through crystals, reflections from ice crystal faces, or a combination of both. This chapter opens with a discussion of the optical effects produced by refraction in ice crystals. Figure 8.1 is a simulation from the HaloSim program that summarizes the main halos discussed in this chapter. Refer to it as you read about parhelia or sundogs (Section 8.1.1), 22° halos (Section 8.1.2), upper and lower tangent arcs (Section 8.1.3), subsuns and pillars (Section 8.2), and 46° halos and related arcs (Section 8.3). The figure also shows several halo arcs not discussed here.
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