Bright Infralateral Arc over Curitiba, Brazil August 29, 2008. Imaged by Mario Freitas (site) of Universidade Tecnológica Federal do Paraná at 10:12am with the sun 44° high. ©Mario Freitas, shown with permission.

The intense halo display was seen over a large area of Southern Brazil (more images).

22° and circumscribed halos dominate the image but the unusual feature is the bright arc with widely separated colours at lower left - an infralateral arc.

Infralateral arcs are produced by singly oriented column crystals - the same orientation that generated the circumscribed halo. The latter results from rays that pass through two prism side faces inclined 60° to one another. Rays forming the infralateral arc instead enter a near vertical hexagonal end (basal) face and exit through a side face. These faces are inclined at 90° rather than 60° and the greater angle disperses the halo colours more widely.

Infralateral arcs are fairly rare and some of that rarity arises from their ray path. The end faces of column crystals are often deformed and indented. Halos whose rays would pass through or be reflected from them (infralateral, supralateral, 46°, Wegener - called collectively 46° arcs) are therefore rarer than ones passing only through the usually more perfect crystal side faces (circumscribed, tangent arcs, 22° halo called collectively 22° arcs).

Another factor is that the infralateral arc is wider than 22° arcs and its light is hence spread over a greater sky area.

The lower left of Mario's image has a white streak crossing the infralateral arc. This is possiblya rare lower Wegener arc.

The arc shows on the accompanying HaloSim ray tracing simulation. A fisheye projection was used and a box marks the approximate extent of Mario's image. The upper Wegener arc was seen elsewhere in Brazil and intersecting lower Wegener, infralateral and a 46° halo were imaged at Águas de Santa Bárbara-SP by Juliana Medeiros.

Antarctic infralateral arc, Colorado infralateral and supralateral arcs. More halos.


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