Halo (light effect)

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Several halo phenomena: 22 ° ring, two adjacent suns, upper contact arc, Parry arc and horizontal circle with a person in the foreground
Halo

Halo ( singular of the halo ; plural halos or halons ) is a collective term for light effects in atmospheric optics that arise from the reflection and refraction of light on ice crystals .

Depending on the size and orientation of the ice crystals as well as the angle at which light hits the crystals, sometimes whitish, sometimes colored circles, arches, columns or spots of light appear in different parts of the sky.

etymology

The word halo originally comes from the Greek ( ἅλως ) halos , which denoted a ring of light around the sun or moon or the disk of the sun or moon itself; other meanings of the word were “ threshing floor ” or “threshing floor” and “disc”.

The Greek word was adopted as halos in Latin , and its dative / ablative halo found its way into German and other languages ​​such as English.

External requirements

Water preferentially crystallizes as thin hexagonal platelets and small hexagonal columns

In order for halos to arise, the ice crystals must have grown as regularly as possible and be transparent. Most often they form at a high altitude of 8 to 10 km and their presence is indicated by cirrus clouds . In winter, however, they can also form in polar snow ( diamond dust ), ice fog or near snow cannons . The regularity of the ice crystals is caused by the slowest possible growth of the crystals, which requires the slowest possible saturation of the air with water vapor.

Water crystallizes in the hexagonal crystal system . Thin hexagonal plates and small hexagonal pillars are the most common ice crystal shapes and are mainly responsible for the formation of halos. Small ice crystals of a few tenths of a millimeter can float in the air for a long time and do not adopt a preferred orientation in the air. However, if the crystals slowly get bigger, they have a correspondingly higher sink rate and assume a stable position, caused by symmetrical eddies on the side facing away from the direction of fall. As a rule, this is only possible with a vertical axis of symmetry, which is why the shape of the crystals means they have maximum air resistance when falling. When the air is calm, the hexagonal plates are horizontal, as are the longitudinal axis of the pillars.

The sunlight is refracted when it penetrates such ice crystals and, depending on the orientation of the crystals and the angle of incidence of the light, emerges again after (multiple) reflection inside the crystals. When it exits, it is broken one more time. The refraction of light is responsible for the visible splitting of the colors of the light. The direct reflection of the light on the outer crystal surfaces plays a subordinate role in halo phenomena.

Halo effects can also be observed around the moon. However, the human eye is hardly able to perceive colors in low light intensity, which is why the weaker moon halos appear white. Halos can be observed around almost any strong light source under the above conditions.

In addition, ice crystals can form on snow-covered surfaces, causing certain types of halos.

Types of halos

The main halos and their location in the sky

The graphic above shows the position of halos in the sky when the sun is at an altitude of 25 °. The illustration is not complete. The designations in the graphic can be found in the table below in the 2nd column "Halo phenomena" in brackets

Halo key Halo appearance description Emergence comment
EE01 22 ° ring (1) It is a ring which, from the observer's point of view, runs 22 ° from the sun or moon. Refraction on randomly oriented ice crystals This is the most common halo.
EE02 / 03/04 Side sun (2) Two bright spots of light to the left and right of the sun. Refraction on horizontally floating ice platelets Often occurs together with the 22 ° ring (see also side moon ).
EE05 / 06/07 Upper & lower touch arc (3/4) / circumscribed halo Usually only parts of the contact arcs can be seen as "horns", which then join together to form the circumscribed halo from a sun height of 32 °. Refraction at horizontally floating ice pillars
EE08 / 09/10 Pillar of light A pillar of light above or below the sun. Reflection on horizontally floating ice flakes
EE13 Horizontal circle (5) A white circle of light that runs parallel to the horizon. He cuts the sun. Reflection or refraction on ice platelets or pillars
EE11 Circumzenital Arch (6) A moon-shaped colorful arch that can be seen near the zenith. Refraction on horizontally floating ice platelets Often occurs in connection with the suns.
EE23 Circumhorizontal arc A colorful arc below the sun that can only be seen a few degrees above the horizon. Refraction on horizontally floating ice platelets Is only visible at sun heights of more than 58 °. Often attracts a lot of attention when observed by chance.
EE12 46 ° ring (7) A ring of light around the sun at a distance of 46 °. Refraction on randomly oriented pillars This halo phenomenon occurs very rarely and requires a very bright 22 ° ring.
EE44 Under sun (8) A white spot of light that lies below the horizon. Reflection on horizontally floating ice flakes The sun can only be seen when looking down from a mountain into the valley or looking out of the airplane window.
EE21 Supralateral Arch (9) It forms a parabolic arch above the 22 ° ring, the apex of which lies on the circumzenital arch. Refraction at simply oriented ice pillars The supralateral arch changes its shape with the height of the sun. It can be confused with the 46 ° halo.
EE22 Infralateral arch (10) The infralateral arch is a colored convex arch that can be found to the left or right of the sun. Refraction at simply oriented ice pillars The infralateral arch changes its shape with the height of the sun. The two arcs touch from about 60 ° of the sun.
EE27 Parry Arch (11) The Parry arch has four characteristics and is divided into: Upper / lower concave / convex Parry arch Refraction on doubly oriented ice pillars The Parry Arch is one of the rare species of halo.
EE61 Sunbow (12) The solar arc looks like a loop and crosses the sun like an "X". Reflection on doubly oriented ice pillars This type of halo is very rare, but can be seen more often in the ice fog.
EE56 Wegener's counter-solar arc (13) Wegener's counter-solar arc runs as a loop within the horizontal circle. In doing so, it crosses the opposing sun and its apex lies on the upper contact arc. Refraction and reflection on simply oriented ice pillars This type of halo is very rare.
EE45 / 46/47 Lower Sun (14) The sub-suns are the sub-suns of the sub-sun. Refraction and reflection on horizontally floating ice flakes Can only be seen below the horizon.
EE40 Below horizontal circle (15) Similar to the horizontal circle below the horizon. Refraction and reflection on horizontally floating ice flakes Can only be seen below the horizon. Very rare halo type.
EE60 Tapes sheet (16) Appears as 4 V-shaped short arches that touch the supralateral / infralateral arch. Refraction on doubly oriented ice pillars Very rare halo type.

If different forms of ice crystals are present at the same time, different halo effects can occur together. Halos are also quite common in Central Europe, even more common than rainbows . Unfortunately, they're not as colorful as these, and most face the sun, making them less noticeable and easily outshone by sunlight.

In addition to the above types, there are some less common types Halo, including the Trickers anthelic arc, the 9 ° ring , the Moilanenbogen and the counter sun .

Classification of the working group meteors e. V.

The working group meteors e. V. has developed a classification for determining the individual halo types, the so-called halo key. All known halo types, but also unexplained phenomena, are recorded in the halo key.

images

  • Circumscribed halo and circumhorizontal arc in Tulum, Mexico on May 21, 2008

  • Halo due to deposited ice crystals on frozen lake, Sigtuna (Sweden)

  • Halo above Lenzerheide (Switzerland) on December 19, 2012

  • Halo with suns near Echzell / Hessen on August 12, 2012

  • 22 ° moon halo 23 October 2010, as seen from Graz , Austria from

  • The upper contact arc and convex Parry arc form a “double V” shortly before sunset.

  • Right side sun, 22 ° ring, 46 ° ring, horizontal circle and right-hand tape arches near Altenberg on January 25, 2014

  • 9 ° ring around the sun, taken on March 27, 2013 in Wildau

  • Different halos around the sun on Madritsche-Nassfeld in Carinthia

  • 22 ° -Halo around the moon, as seen in Wallerfing on February 5, 2017

  • Halo with sundials at Hoherodskopf / Hessen on July 15, 2017

  • 22 ° -Halo in Großkrotzenburg / Hessen on September 6, 2020

Physical basics of the 22 ° ring

Beam path in a hexagonal prism

Ice crystals crystallize in the hexagonal crystal system . Light that passes through these crystals is accordingly refracted as if it were passing through a hexagonal prism . Rays of light that pass two surfaces of these ice crystals that are tilted by 60 ° to each other are refracted at an angle of about 22 ° to 46 °. The halo is perceptible at precisely this angle between the primary luminous object and the observer. Like the rainbow and other refraction effects, it is dependent on the position of the luminous object as well as that of the observer.

Visible light at the hexagonal prism has a minimum deflection between 21.7 ° ( red , 656 nm) and 22.5 ° ( violet , 400 nm). No visible light is refracted at smaller angles, so that the impression of an empty space is created between the luminous object and the halo. Most of the light rays that reach the viewer are refracted at angles close to the minimum of deflection, creating the perception of a bright inner edge. Entry and exit angles are not linked to one another linearly. With every degree that the entrance angle is away from the optimum, the light is refracted more strongly. Because of this, the halo fades outward.

Due to the different refraction of the spectral colors, the inner edge of a 22 ° ring often shimmers reddish. Sub-suns arise in the same way.

Physical basics of the 46 ° ring

This type of halo occurs when the rays of light are refracted along two surfaces of the hexagonal prism that are perpendicular to each other. This is the case when a beam of light passes through a side surface and the top or bottom of the crystal. The minimum of the deflection in this beam path is 46 °, which is why the ring is brightest right here.

The light rays must hit the crystals at a narrow angle so that they are refracted accordingly, otherwise they will be reflected in directions away from the observer. Because of this, they appear weaker. It also disperses the light more so that the halos are more colorful.

Circumzenital arcs are created in the same way.

Artificial halos

The natural atmospheric light phenomena can also be generated artificially or demonstrated experimentally. On the one hand, computer experiments, i.e. simulations of halos, can be created using ray tracing. On the other hand, chemical reactions and mechanical approaches can also be followed. In the latter case, a single crystal (typically made of acrylic glass, glass or ice) is rotated around the corresponding axis (s). Another variant consists in considering equivalent refraction geometries.

Approach 1: Analog Refractions

This approach is only suitable for the experimental demonstration of a few halo species. However, this includes the circumzenital arcs and circumhorizontal arcs, which are difficult to achieve in any other way, as well as parry arcs. The idea for this is based on the fact that the relevant refraction through a hexagonal prism corresponds on average (across all orientations with respect to the vertical axis) to that which parallel light experiences when it hits a cylinder of water. The refractive index of water is very close to that of ice. A Parry arch can be created by refraction of light through a cocktail glass (in the form of a martini glass). The water glass experiment has been known since at least 1920, but is often wrongly associated with the rainbow.

Approach 2: chemical reactions

The first artificial halos were studied by A. Cornu around 1889, according to a suggestion by Brewster. The idea here is to create regular crystal populations by precipitating salts from a solution. The countless crystals in the solution then generate corresponding halos under the incidence of parallel light. The exact appearance depends on the geometry of the crystals produced and is often ring-shaped in solution. Some videos can be found on YouTube. But Parry bows have also been produced in this way in the laboratory.

Approach 3: Mechanical orientation realizations

One axis

The first experiments by rotating a single crystal are ascribed to Auguste Bravais around 1847. Bravais used an equilateral triangular prism made of glass which he rotated around the vertical axis. When illuminated, this created what is probably the first artificially created horizontal arch with many of its embedded suns. Similarly, A. Wegener used rotating hexagonal crystals to create the lower suns. The use of hexagonal crystals allows the investigation of a large number of sub-suns (120 °, 22 °, 90 °, 90 ° (2nd order), a series of sharp maxima, cyan blue spots). Commercially available light guide rods with a hexagonal cross section can be used for such experiments. Simple experiments with rotating prisms can serve as demonstration experiments in the classroom and supplement / replace classic experiments on rainbows. Parry arches can also be created in this way.

Even before A. Bravais, the Italian scientist F. Venturi experimented with water-filled pointed prisms, in particular to explain the circumzenital arc. However, his explanation was later found to be false and was replaced by Bravais' explanation.

Artificial ice crystals can also be used to create halos that cannot be achieved with glass crystals. For example, the circumzenital arch was created with an artificially produced macroscopic ice crystal. A few other materials also have an index of refraction close to that of ice, for example sodium fluoride .

Two axes

To create artificial tangential halos or Lowitz halos you have to rotate a crystal around two axes at the same time. The mechanical effort for such experiments is therefore somewhat greater. The first such Halo machine was built in 2003, and several more followed. If such a halo machine is placed in a spherical projection screen, a distortion-free, almost perfect analogy to the natural phenomenon in the sky is created based on the sky transform principle . If you superimpose many such halo projections, you can artificially create complex halo appearances.

Three axes

The realization of mechanically generated circular (ring) halos requires special tricks, since a simultaneous rotation of a crystal around three spatial axes is necessary without blocking the beam path. In the approach using chemical reactions, however, the round halos are the simplest. The mechanical 3D reorientation to generate artificial ring halos was accomplished in two ways: On the one hand, through a sophisticated and complicated mechanical framework, and on the other hand, with the help of an Arduino technology-based random walk machine that rotates a crystal embedded in a thin-walled hollow sphere.

Mutable halos

Occasionally, rapidly changing halos are observed in the vicinity of thunderclouds, mostly directly in the ice shield of cumulonimbus clouds . These ice clouds are created by the rapid rise of warm, humid air up to the tropopause . Shortly before this, the air cools below freezing point and forms the classic ice umbrella of the cloud. The resulting ice particles can also be regularly arranged by the strong electromagnetic field of the thundercloud and generate halo effects through the incident sunlight. If the electromagnetic field of the cloud changes due to lightning , the particles spontaneously reorient themselves, which leads to fast and sometimes spectacular movement patterns of the halo phenomena.

The light phenomena are also called crown flashes or jumping sundogs due to their place of origin at the upper edge of the storm clouds .

See also

Web links

Commons : Halo  album with pictures, videos and audio files

Individual evidence

  1. ↑ a b Halo in Merriam-Webster (accessed March 6, 2010)
  2. ↑ a b c Halo in the Online Etymology Dictionary (accessed March 6, 2010)
  3. ↑ Halo in Duden (accessed March 6, 2010)
  4. ↑ Article on snow cover halos at meteoros.de (accessed October 16, 2015)
  5. ↑ Atmospheric-optical phenomenon, observed by Mr. Langberg on Wikisource - historical record of a snow cover halo
  6. ↑ 22 ° ring (accessed October 17, 2015)
  7. ↑ Nebensonne (accessed October 17, 2015)
  8. ↑ Upper contact sheet (accessed October 17, 2015)
  9. ↑ Lower contact sheet (accessed October 17, 2015)
  10. ↑ Rewritten Halo (accessed October 17, 2015)
  11. ↑ Upper light column (accessed October 17, 2015)
  12. ↑ Lower light column (accessed October 17, 2015)
  13. ↑ Horizontal circle (accessed October 17, 2015)
  14. ↑ Circumzenital arc (accessed October 17, 2015)
  15. ↑ Circumhorizontal arc (accessed October 17, 2015)
  16. ↑ 46 ° ring (accessed October 17, 2015)
  17. ↑ Under Sun (accessed October 17, 2015)
  18. ↑ Supralateral arch (accessed October 17, 2015)
  19. ↑ Differentiation between the 46 ° ring and the supralateral arch
  20. ↑ Infralateral arch (accessed October 16, 2015)
  21. ↑ Parry Arch (accessed October 17, 2015)
  22. ↑ Sonnenbogen (accessed October 17, 2015)
  23. ↑ Wegener's counter-solar arc (accessed October 17, 2015)
  24. ↑ Lower Sun (accessed October 17, 2015)
  25. ↑ Sub-horizon circle (accessed October 17, 2015)
  26. ↑ Tapes sheet (accessed October 17, 2015)
  27. ↑ meteoros.de 57
  28. ↑ meteoros.de 31
  29. ↑ atoptics.co.uk
  30. ↑ meteoros.de 17
  31. ↑ Halo key of the AKM e. V. (accessed October 16, 2015)
  32. ↑ HaloSim3 by Les Cowley and Michael Schroeder link
  33. ↑ HaloPoint 2.0 link ( Memento from October 7, 2016 in the Internet Archive )
  34. ↑ a b c M. Selmke, S. Selmke: Artificial circumzenithal and circumhorizontal arcs. In: American Journal of Physics. Volume 85 (8), 2017, pp. 575-581. doi : 10.1119 / 1.4984802 .
  35. ↑ Images of artificial circumhorizontal / circumzenital / Parry arches: [1]
  36. ↑ Gilbert light experiments for boys - (1920), p. 98, experiment no. 94 link
  37. ↑ “Sur la reproduction artificielle des halos et des cercles parh eliques”, Comtes Rendus Ac. Paris 108, 429-433, A. Cornu, 1889.
  38. ↑ a b “Laboratory experiments in atmospheric optics”, Opt. Express 37 (9), 1557–1568, M. Vollmer and R. Tammer, 1998. link
  39. ↑ “Tabletop divergent-light halos”, Physics Education 42 (6), L. Gisle and J. O Mattsson, 2007. link
  40. ↑ Z. Ulanowski, “Ice analog halos,” Appl. Optics 44 (27): 5754-5758, 2005. link
  41. ^ M. Élie de Beaumont, Memoir of Auguste Bravais (Smithsonian Institution, Washington, 1869)
  42. ↑ a b "Mémoire sur les halos et les phénomènes optiques qui les accompagnent", 1847, J. de l'École Royale Polytechnique 31 (18), p. 1-270, §XXIV - Reproduction artificielle des phénomènes optiques dus à des prismes à ax vertical, Figures: PL I: Fig. 48, PL II: Fig. 49-54.
  43. ↑ “The Suns Below the Horizon,” Meteorol. Z. 34-52 (8/9), 295-298, A. Wegner, 1917.
  44. ^ "Intensity distribution of the parhelic circle and embedded parhelia at zero solar elevation: theory and experiments", Applied Optics (Appl. Opt.), Vol. 54, Issue 22, 6608-6615, S. Borchardt and M. Selmke, 2015 . link
  45. ↑ Homogenizing Light rods / Light pipes link
  46. ↑ a b "Artificial Halos", American Journal of Physics (Am. J. Phys.), Vol. 83 (9), 751-760, M. Selmke, 2015. link
  47. ^ F. Venturi, "Commentarii sopra ottica", p. 219, Tav VIII, Fig 17, arc: PGQ, Fig 27, p. 213.
  48. ^ "Physical dictionary", revised by Brandes. Gmelin. Horns. Muncke. Pfaff, p. 494, [2]
  49. ↑ Homepage: Arbeitskreis Meteore eV link
  50. ↑ "An Analog Light Scattering Experiment of Hexagonal Icelike Particles. Part II: Experimental and Theoretical Results", JOURNAL OF THE ATMOSPHERIC SCIENCES, Vol. 56, B. Barkey, KN Liou, Y. Takano, W. Gellerman, P. Sokolkly, 1999.
  51. ↑ “Halo and mirage demonstrations in atmospheric optics,” Appl. Opt. 42 (3), 394-398, M. Vollmer and R. Greenler, 2003. link
  52. ↑ a b “Artificially generated halos: rotating sample crystals around various axes”, Applied Optics Vol. 54, Issue 4, pp. B97 – B106, Michael Großmann, Klaus-Peter Möllmann, and Michael Vollmer, 2015. link
  53. ↑ a b c "Complex artificial halos for the classroom", American Journal of Physics (Am. J. Phys.), Vol. 84 (7), 561-564, M. Selmke and S. Selmke, 2016. link
  54. ↑ Experiments by Michael Großmann on Haloblog.net: link
  55. ^ "Sky Transform" on atoptics.co.uk: link
  56. ↑ a b Article with images on BoredPanda: Spherical projection screen for artificial halos
  57. ↑ Crown Flash in the WetterOnline.de weather dictionary: link