A 46° halo is a rare optical halo

Halo (optical phenomenon)

A halo from Greek ἅλως; also known as a nimbus, icebow or gloriole) is an optical phenomenon produced by ice crystals creating colored or white arcs and spots in the sky. Many are near the sun or moon but others are elsewhere and even in the opposite part of the sky...

 centred on the sun

Sun

The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

. At sun elevations between 15-27°, it is often confused with the more colourful and frequently observed supralateral

Supralateral arc

A supralateral arc is a rare halo, an optical phenomenon often confused with the indeed infrequently appearing 46° halo. Distinguishing between the two is furthermore difficult as supralateral arcs typically only appears in fragments while the 46° halo is very faint.In contrast to the static 46°...

 and infralateral arc

Infralateral arc

An infralateral arc is a rare halo, an optical phenomenon appearing similar to a rainbow under a white parhelic circle...

s. It is named for crossing the parhelic circle

Parhelic circle

A parhelic circle is a halo, an optical phenomenon appearing as a horizontal white line on the same altitude as the sun, or occasionally the Moon. If complete, it stretches all around the sky, but more commonly it only appears in sections....

 46° from the sun.

46° halos are similar to but much broader and much fainter than the more common 22° halo

22° halo

A 22° halo is a halo, one type of optical phenomenon, forming a circle 22° around the sun, or occasionally the moon. It forms as sunlight is refracted in hexagonal ice crystals suspended in the atmosphere...

s. They form when sunlight enters randomly oriented hexagonal ice crystals through a prism

Prism (optics)

In optics, a prism is a transparent optical element with flat, polished surfaces that refract light. The exact angles between the surfaces depend on the application. The traditional geometrical shape is that of a triangular prism with a triangular base and rectangular sides, and in colloquial use...

 face and exits through a hexagonal base.
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.

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

Circumzenithal arc

The circumzenithal arc or circumzenith arc , also called the Bravais' arc, is an optical phenomenon similar in appearance to a rainbow; but it arises from refraction of sunlight through horizontally-oriented ice crystals, generally in cirrus clouds, rather than from raindrops...

, 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

Zenith

The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction opposite to the apparent gravitational force at that location. The opposite direction, i.e...

, however, circumhorizontal

Circumhorizontal arc

A circumhorizontal arc is an optical phenomenon - an ice-halo formed by plate-shaped ice crystals in high level cirrus clouds.The current accepted names are circumhorizon arc or lower symmetric 46° plate arc The complete halo is a huge, multi-coloured band running parallel to the horizon with its...

 or infralateral arc

Infralateral arc

An infralateral arc is a rare halo, an optical phenomenon appearing similar to a rainbow under a white parhelic circle...

s are located 46° under the sun and can be confused with the 46° halo.

External links

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