46° halo

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A 46° halo is a rare and huge halo, together with the smaller 22° halo a circular optical phenomenon 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.

46° halos are similar to but much broader and much fainter than 22° halos. They form when sunlight enters randomly oriented hexagonal ice crystals through a prism face and exits through a hexagonal base.^[1] 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.^[2]

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, circumhorisontal or infralateral arcs are located 46° under the sun and can be confused with the 46° halo. ^[3][4]

[edit] See also

• Circumzenithal arc
• Tangent arc

[edit] References

[edit] External links

• Atmospheric Optics - 46° Radius Halo - including a HaloSim computer simulation and an Antarctica fish eye photo.