From Event: SPIE Optical Engineering + Applications, 2017
A halo is one of the most frequent and impressive optical phenomena easily observable in the sky. It is also one of the natural optical phenomena most often visible through an airplane window. Halos and related phenomena vary from a single spot of light formed by reflection of the sun from the tops of plate-shaped ice crystals to large rings with splashes of colors, caused by a combination of reflection and refraction in ice crystals. Even with extreme heat at the ground, an airplane quickly rises through sufficient altitude to find ice crystals in the clouds, enabling an alert passenger (or pilot) to see ice-crystal optical phenomena. This paper briefly reviews these phenomena with photographs and diagrams. Photographs include commonly seen halos, as well as Bottlinger’s rings, a rare halo that is still not fully explained. Tips are given for enhancing your chances of seeing and understanding halos.
Joseph A. Shaw, "Observing halos through airplane windows," Proc. SPIE 10367, Light in Nature VI, 103670E (Presented at SPIE Optical Engineering + Applications: August 07, 2017; Published: 6 September 2017); https://doi.org/10.1117/12.2274545.
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