46° halo

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A 46° halo is a rare optical halo centred on the sun. At sun elevations between 15-27°, it is often confused with the more colourful and frequently observed supralateral and infralateral arcs. It is named for crossing the parhelic circle 46° from the sun.[1]

46° halos are similar to but much broader and much fainter than the more common 22° halos. They form when sunlight enters randomly oriented hexagonal ice crystals through a prism face and exits through a hexagonal base.[2] The 90° inclination between the two faces of the crystals causes the colours of the 46° halo to be more widely dispersed than those of the 22° halo. In addition, as a lot of rays are deflected at larger angles than the angle of minimum deviation, the outer edge of the halo is more diffuse.[3]

To tell the difference between a 46° halo and the infra-/supralateral arcs, one should carefully observe sun elevation and the fluctuating shapes and orientations of the arcs. The supralateral arc always touches the circumzenithal arc, while the 46° halo only achieves this when the sun is located 15-27° over the horizon, leaving a gap between the two at other elevations. In contrast, supralateral arcs cannot form when the sun is over 32°, so a halo in the 46°-region is always a 46° halo at higher elevations. If the sun is near zenith, however, circumhorizontal or infralateral arcs are located 46° under the sun and can be confused with the 46° halo. [4][5]

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  1. ^ The 46° halo was first explained as being caused by refractions through ice crystals in 1679 by the French physicist Edmé Mariotte (1620–1684). See: Mariotte, Quatrieme Essay. De la Nature des Couleur (Paris, France: Estienne Michallet, 1681). Sun dogs as well as the 22° and 46° halos are explained in terms of refractions from ice crystals on pages 466 - 524.
  2. ^ "46°-halo". Arbeitskreis Meteore e.V. Archived from the original on 31 March 2007. Retrieved 2007-04-16. 
  3. ^ Les Cowley (?). "46° Halo Formation". Atmospheric Optics. Retrieved 2007-04-16.  (including an illustration and an animation)
  4. ^ Les Cowley (?). "Is it a 46° halo or a supra/infralateral arc?". Atmospheric Optics. Retrieved 2007-04-16. 
  5. ^ "Supralateral arc". Arbeitskreis Meteore e.V. Retrieved 2007-04-16. 

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