San Luis Circumscribed Halo

July 19, 2007


Provided and copyright by: Mario Freitas, Universidade Tecnológica Federal do Paraná
Summary author: Mario Freitas

High clouds like cirrostratus can be composed of floating ice crystals even in the neighborhood of the Equator and at sea level. As white sunbeams reach such clouds and penetrate hexagonal ice-crystals, light is decomposed by refraction, providing for the possibility of observing different halo patterns. On the above picture, the elliptical shape to the 22-degree halo suggests it's circumscribed. Halos such as this are characterized by having somewhat more uniformly oriented column crystals (their "c" axes are horizontal but the "a" and "b" axes are randomly oriented) than is the case for the circular 22 degree halo. The solar disk is hidden behind a facade at the historical center of Sao Luis, in northeastern Brazil. This site is included in UNESCO’s World Heritage list. Photo taken on January 28, 2007.

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