Supralateral arc

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Complex halo display. Top to bottom: Supralateral arc, Parry arc, Upper tangent arc, 22deg halo. Sun dog to the right of the Sun. Complex halo display 01-03-09.jpg
Complex halo display. Top to bottom: Supralateral arc, Parry arc, Upper tangent arc, 22° halo. Sun dog to the right of the Sun.
Circumzenithal arc, Supralateral arc, Parry arc, and Upper tangent arc, in Salem, Massachusetts, Oct 27, 2012. Solar halos with foreground, Salem, MA, Oct 27, 2012.JPG
Circumzenithal arc, Supralateral arc, Parry arc, and Upper tangent arc, in Salem, Massachusetts, Oct 27, 2012.

A supralateral arc is a comparatively rare member of the halo family which in its complete form appears as a large, faintly rainbow-colored band in a wide arc above the sun and appearing to encircle it, at about twice the distance as the familiar 22° halo. In reality, however, the supralateral arc does not form a circle and never reaches below the sun. When present, the supralateral arc touches the (much more common) circumzenithal arc from below. As in all colored halos, the arc has its red side directed towards the sun, its blue part away from it.

Halo (optical phenomenon) optical phenomenon

Halo is the name for a family of optical phenomena produced by sunlight interacting with ice crystals suspended in the atmosphere. Halos can have many forms, ranging from colored or white rings to arcs and spots in the sky. Many of these appear near the Sun or Moon, but others occur elsewhere or even in the opposite part of the sky. Among the best known halo types are the circular halo, light pillars, and sun dogs, but many others occur; some are fairly common while others are (extremely) rare.

22° halo

A 22° halo is an optical phenomenon that belongs to the family of ice crystal halos, in the form of a ring with a radius of approximately 22° around the Sun or Moon. When visible around the moon, it is called a moon ring or winter halo. It forms as the sun- or moonlight is refracted in millions of hexagonal ice crystals suspended in the atmosphere. The halo is large; the radius is roughly the size of an outstretched hand at arm's length. A 22° halo may be visible on as many as 100 days per year—much more frequently than rainbows.

Circumzenithal arc optical effect

The circumzenithal arc, also called the circumzenith arc (CZA), upside-down rainbow, and the Bravais arc, is an optical phenomenon similar in appearance to a rainbow, but belonging to the family of halos arising from refraction of sunlight through ice crystals, generally in cirrus or cirrostratus clouds, rather than from raindrops. The arc is located at a considerable distance above the sun and at most forms a quarter of a circle centered on the zenith. It has been called "a smile in the sky", its first impression being that of an upside-down rainbow. The CZA is one of the brightest and most colorful members of the halo family. Its colors, ranging from violet on top to red at the bottom, are purer than those of a rainbow because there is much less overlap in their formation.

Contents

Formation

Supralateral arcs form when sun light enters horizontally oriented, rod-shaped hexagonal ice crystals through a hexagonal base and exits through one of the prism sides. Supralateral arcs occur about once a year. [1]

Hexagon shape with six sides

In geometry, a hexagon is a six-sided polygon or 6-gon. The total of the internal angles of any simple (non-self-intersecting) hexagon is 720°.

Confusion with the 46° halo

Due to its apparent circular shape and nearly identical location in the sky, the supralateral arc is often mistaken for the 46° halo, which does form a complete circle around the sun at approximately the same distance, but which is much rarer and fainter. Distinguishing between the two phenomena can be difficult, requiring the combination of several subtle indicators for proper identification. [2]

46° halo

A 46° halo is a rare member of the family of ice crystal halos, appearing as a large ring centred on the Sun at roughly twice the distance as the much more common 22° halo. At solar elevations between 15–27°, the 46° halo is often confused with the less rare and more colourful supralateral and infralateral arcs, which cross the parhelic circle at about 46° to the left and right of the sun.

In contrast to the static 46° halo, the shape of a supralateral arc varies with the elevation of the sun. Before the sun reaches 15°, the bases of the arc touch the lateral (oriented sidewise) sides of the 46° halo. As the sun rises from 15° to 27°, the supralateral arc almost overlaps the upper half of the 46° halo, which is why many reported observations of the latter most likely are observations of the former. As the sun goes from 27° to 32°, the apex of the arc touches the circumzenithal arc centered on zenith (as does the 46° halo when the sun is located between 15° and 27°). In addition, the supralateral arc is always located above the parhelic circle (the arc located below it is the infralateral arc), and is never perfectly circular. [1] [3]

Sun Star at the centre of the Solar System

The Sun is the star at the center of the Solar System. It is a nearly perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process. It is by far the most important source of energy for life on Earth. Its diameter is about 1.39 million kilometers, or 109 times that of Earth, and its mass is about 330,000 times that of Earth. It accounts for about 99.86% of the total mass of the Solar System. Roughly three quarters of the Sun's mass consists of hydrogen (~73%); the rest is mostly helium (~25%), with much smaller quantities of heavier elements, including oxygen, carbon, neon, and iron.

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. the direction in which gravity pulls, is toward the nadir. The zenith is the "highest" point on the celestial sphere.

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.

Arguably the best way of distinguishing the halo from the arc is to carefully study the difference in colour and brightness. The 46° halo is six times fainter than the 22° halo and generally white with a possible red inner edge. The supralateral arc, in contrast, can even be confused with the rainbow with clear blue and green strokes. [1]

Rainbow meteorological phenomenon

A rainbow is a meteorological phenomenon that is caused by reflection, refraction and dispersion of light in water droplets resulting in a spectrum of light appearing in the sky. It takes the form of a multicoloured circular arc. Rainbows caused by sunlight always appear in the section of sky directly opposite the sun.

See also

Related Research Articles

Sun dog atmospheric phenomenon

A sun dog or mock sun, formally called a parhelion in meteorology, is an atmospheric optical phenomenon that consists of a bright spot to one or both sides of the Sun. Two sun dogs often flank the Sun within a 22° halo.

Anthelion a rare optical phenomenon appearing on the parhelic circle opposite to the sun as a faint white halo

An anthelion is a rare optical phenomenon of the halo family. It appears on the parhelic circle opposite to the sun as a faint white spot, not unlike a sundog, and may be crossed by an X-shaped pair of diffuse arcs.

Fog bow

A fog bow, sometimes called a white rainbow, is a similar phenomenon to a rainbow; however, as its name suggests, it appears as a bow in fog rather than rain. Because of the very small size of water droplets that cause fog—smaller than 0.05 millimeters (0.0020 in)—the fog bow has only very weak colors, with a red outer edge and bluish inner edge.

Circumhorizontal arc optical phenomenon that belongs to the family of ice halos

A circumhorizontal arc is an optical phenomenon that belongs to the family of ice halos formed by the refraction of sun- or moonlight in plate-shaped ice crystals suspended in the atmosphere, typically in cirrus or cirrostratus clouds. In its full form, the arc has the appearance of a large, brightly spectrum-coloured band running parallel to the horizon, located far below the Sun or Moon. The distance between the arc and the Sun or Moon is twice as far as the common 22-degree halo. Often, when the halo-forming cloud is small or patchy, only fragments of the arc are seen. As with all halos, it can be caused by the Sun as well as the Moon.

120° parhelion

A 120° parhelion is a relatively rare halo, an optical phenomenon occasionally appearing along with very bright sun dogs when ice crystal-saturated cirrus clouds fill the atmosphere. The 120° parhelia are named for appearing in pair on the parhelic circle ±120° from the sun.

Upper and lower tangent arcs

Tangent arcs are a type of halos, an atmospheric optical phenomenon, which appear above and below the Sun or Moon, tangent to the 22° halo. To produce these arcs, rod-shaped hexagonal ice crystals need to have their long axis aligned horizontally.

Circumscribed halo

A circumscribed halo is a type of halo, an optical phenomenon typically in the form of a more or less oval ring that circumscribes the circular 22° halo centred on the sun or moon. As the sun rises above 70° it essentially covers the 22° halo. Like many other halos, it is slightly reddish on the inner edge, facing the sun or moon, and bluish on the outer edge.

An infralateral arc is a rare halo, an optical phenomenon appearing similar to a rainbow under a white parhelic circle. Together with the supralateral arc they are always located outside the seldom observable 46° halo, but in contrast to supralateral arcs, infralateral arcs are always located below the parhelic circle.

Parry arc

A Parry arc is a rare halo, an optical phenomenon which occasionally appears over a 22° halo together with an upper tangent arc.

<i>Vädersolstavlan</i> painting by Jacob Heinrich Elbfas

Vädersolstavlan (help·info) is an oil-on-panel painting depicting a halo display, an atmospheric optical phenomenon, observed over Stockholm on 20 April 1535. It is named after the sun dogs appearing on the upper right part of the painting. While chiefly noted for being the oldest depiction of Stockholm in colour, it is arguably also the oldest Swedish landscape painting and the oldest depiction of sun dogs.

The Kern arc is an extremely rare atmospheric optical phenomenon belonging to the family of ice crystal halos. It is a complete and faint circle around the zenith, in contrast to the related and much more common circumzenithal arc, which is only ever a partial circle.

A subhelic arc is a rare halo, formed by internal reflection through ice crystals, that curves upwards from the horizon and touches the tricker arc above the anthelic point. Subhelic arcs are a result of ray entrance and exit through prism end faces with two intermediate internal reflections.

A Lowitz arc is an optical phenomenon that occurs in the atmosphere; specifically, it is a rare type of ice crystal halo that forms a luminous arc which extends inwards from a sun dog (parhelion) and may continue above or below the sun.

Atmospheric optics ray tracing codes - this article list codes for light scattering using ray-tracing technique to study atmospheric optics phenomena such as rainbows and halos. Such particles can be large raindrops or hexagonal ice crystals. Such codes are one of many approaches to calculations of light scattering by particles.

References

  1. 1 2 3 "Supralateral arc". Arbeitskreis Meteore e.V. Retrieved 2007-04-16. (Including a photo from January 1996, a 1980 computer simulation of a supralateral arc relative to a 46° halo, and a table pinning down differences between 46° halos and supralateral arcs.)
  2. ↑ "Is it a 46° halo or a supra/infralateral arc?". www.atoptics.co.uk. Retrieved 8 December 2015.
  3. ↑ "Supralateral Arcs". www.paraselene.de. Retrieved 2009-03-08.
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