View source for Earth:Halo (optical phenomenon)

{{Short description|Optical phenomenon of the sky}}
[[File:Solar Halo Feb 13 2021.jpg|thumb|A 22° halo around the Sun, observed over Bretton Woods, New Hampshire, USA on February 13, 2021]]
[[File:Moon Halo Over Mayall (Moon-Halo-Over-Mayall-3-CC).jpg|thumb|A 22° halo and circumscribed halo around the Moon, observed near Kitt Peak National Observatory]]
[[File:Solar halos, Salem, MA, Oct 27, 2012.JPG|thumb|right|From top to bottom:<br>A circumzenithal arc, supralateral arc, Parry arc, upper tangent arc, and 22° halo]]
[[File:Sun halo optical phenomenon edit.jpg|thumb|A halo display observed over the [[Earth:South Pole|South Pole]]]]

A '''halo''' ({{etymology|grc|''{{wiktgrc|ἅλως}}'' ({{grc-transl|ἅλως}})|threshing floor, disk}})<ref>{{cite American Heritage Dictionary |halo}}</ref> is an [[Physics:Optical phenomenon|optical phenomenon]] produced by light (typically from the Sun or Moon) interacting with [[Earth:Ice crystal|ice crystal]]s 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 [[Astronomy:Sun|Sun]] or [[Astronomy:Moon|Moon]], but others occur elsewhere or even in the opposite part of the sky. Among the best known halo types are the '''circular halo''' (properly called the [[Earth:22° halo|22° halo]]), [[Physics:Light pillar|light pillar]]s, and [[Astronomy:Sun dog|sun dog]]s, but many others occur; some are fairly common while others are extremely rare.

The [[Earth:Ice crystal|ice crystal]]s responsible for halos are typically suspended in [[Earth:Cirrus cloud|cirrus]] or [[Earth:Cirrostratus cloud|cirrostratus cloud]]s in the upper [[Earth:Troposphere|troposphere]] ({{cvt|5–10|km|mi}}), but in cold weather they can also float near the ground, in which case they are referred to as [[Earth:Diamond dust|diamond dust]]. The particular shape and orientation of the crystals are responsible for the type of halo observed. [[Physics:Light|Light]] is [[Physics:Reflection|reflected]] and [[Physics:Refraction|refracted]] by the [[Earth:Ice crystal|ice crystal]]s and may split into [[Color|color]]s because of [[Physics:Dispersion (optics)|dispersion]]. The crystals behave like [[Physics:Prism (optics)|prisms]] and [[Physics:Mirror|mirror]]s, refracting and reflecting light between their faces, sending shafts of light in particular directions.

[[Physics:Atmospheric optics|Atmospheric optical phenomena]] like halos were part of weather lore, which was an [[Empirical evidence|empirical]] means of [[Earth:Weather forecasting|weather forecasting]] before [[Physics:Meteorology|meteorology]] was developed. They often do indicate that rain will fall within the next 24 hours, since the cirrostratus clouds that cause them can signify an approaching frontal system.

Other common types of optical phenomena involving water droplets rather than ice crystals include the [[Earth:Glory (optical phenomenon)|glory]] and the [[Earth:Rainbow|rainbow]].

==History==
While [[Biography:Aristotle|Aristotle]] had mentioned halos and parhelia, in antiquity, the first European descriptions of complex displays were those of [[Biography:Christoph Scheiner|Christoph Scheiner]] in Rome ({{circa|1630}}), [[Biography:Johannes Hevelius|Johannes Hevelius]] [[Astronomy:Sevenfold Sun miracle|in Danzig (1661)]], and Tobias Lowitz in St Petersburg ({{circa|1794}}).
Chinese observers had recorded these for centuries, the first reference being a section of the "Official History of the Chin Dynasty" (''Chin Shu'') in 637, on the "Ten Haloes", giving technical terms for 26 solar halo phenomena.<ref>{{cite journal |last1=Ping-Yü |first1=Ho |last2=Needham |first2=Joseph |title=Ancient Chinese Observations of Solar Haloes and Parhelia |journal=Weather |publisher=Wiley |volume=14 |issue=4 |year=1959 |issn=0043-1656 |pages=124–134 |bibcode=1959Wthr...14..124P |doi=10.1002/j.1477-8696.1959.tb02450.x}}</ref>

===Vädersolstavlan===
[[File:Vädersoltavlan cropped.JPG|thumb|The so-called "Sun Dog Painting" (''Vädersolstavlan'') depicting Stockholm in 1535 and the celestial phenomenon at the time interpreted as an ominous presage]]
While mostly known and often quoted for being the oldest color depiction of the city of [[Place:Stockholm|Stockholm]], ''Vädersolstavlan'' ([[Social:Swedish language|Swedish]]; "The Sundog Painting", literally "The Weather Sun Painting") is arguably also one of the oldest known depictions of a halo display, including a pair of [[Astronomy:Sun dog|sun dog]]s. For two hours in the morning of 20 April 1535, the skies over the city were filled with white circles and arcs crossing the sky, while additional suns (i.e., sun dogs) appeared around the Sun.

==Light pillar==
{{Main|Physics:Light pillar}}
A light pillar, or sun pillar, appears as a vertical pillar or column of light rising from the Sun near sunset or sunrise, though it can appear below the Sun, particularly if the observer is at a high elevation or altitude. Hexagonal plate- and column-shaped ice crystals cause the phenomenon. Plate crystals generally cause pillars only when the Sun is within 6 degrees of the horizon; column crystals can cause a pillar when the Sun is as high as 20 degrees above the horizon. The crystals tend to orient themselves near-horizontally as they fall or float through the air, and the width and visibility of a sun pillar depend on crystal alignment.

Light pillars can also form around the Moon, and around street lights or other bright lights. Pillars forming from ground-based light sources may appear much taller than those associated with the Sun or Moon. Since the observer is closer to the light source, crystal orientation matters less in the formation of these pillars.

==Circular halo==
{{Further|Earth:22° halo|Earth:46° halo}}
[[File:20200712 22 degree halo - diagram.svg|thumb|right|upright=1.75 |Ice crystals (only four represented above) form the 22° halo, with red and blue light being refracted at slightly different angles]]
Among the best-known halos is the [[Earth:22° halo|22° halo]], often just called "halo", which appears as a large ring around the Sun or Moon with a radius of about 22° (roughly the width of an outstretched hand at arm's length). The ice crystals that cause the 22° halo are oriented semi-randomly in the atmosphere, in contrast to the horizontal orientation required for some other halos such as [[Astronomy:Sun dog|sun dog]]s and light pillars. As a result of the optical properties of the ice crystals involved, no light is reflected towards the inside of the ring, leaving the sky noticeably darker than the sky around it, and giving it the impression of a "hole in the sky".<ref name="Atoptics">{{cite web |title=Disk with a hole in the sky |website=Atmospheric Optics |url=https://atoptics.co.uk/blog/disk-with-a-hole-in-the-sky/ |access-date=3 August 2016}}</ref> The 22° halo is not to be confused with the corona, which is a different optical phenomenon caused by water droplets rather than ice crystals, and which has the appearance of a multicolored disk rather than a ring.

Other haloes can form at [[Earth:46° halo|46° to the Sun]], or [[Earth:Parhelic circle|at the horizon]], or [[Physics:Circumzenithal arc|around the zenith]], and can appear as full haloes or incomplete arcs.

==Bottlinger's ring==
A ''Bottlinger's ring'' is a rare type of halo that is elliptical instead of circular. It has a small diameter, which makes it very difficult to see in the Sun's glare and more likely to be noticed around the dimmer [[Earth:Subsun|subsun]], often seen from mountain tops or airplanes. Bottlinger's rings are not well understood yet. It is suggested that they are formed by very flat [[Pyramid (geometry)|pyramidal]] ice crystals with faces at uncommonly low angles, suspended horizontally in the atmosphere. These precise and physically problematic requirements would explain why the halo is very rare.<ref name="atoptics-bottlinger's-ring">{{cite web |title=Bottlinger's Rings |publisher=Atmospheric Optics |author=Les Cowley |url=https://atoptics.co.uk/blog/bottlingers-rings/ |access-date=2017-06-26}}</ref>

==Other names==
In the Cornish dialect of English, a halo around the sun or the moon is called a ''cock's eye'' and is an [[Philosophy:Sign|omen]] of bad weather. The term is related to the [[Social:Breton language|Breton]] word ''kog-heol'' (sun cock) which has the same meaning.<ref name="Nance – Cornish Sea-Words">{{cite book |last1=Nance |first1=Robert Morton |last2=Pool |first2=P. A. S.|title=A Glossary of Cornish Sea-Words |date=1963 |publisher=Federation of Old Cornwall Societies |location=Cornwall |pages=61}}</ref> In Nepal, the halo round the sun is called ''Indrasabha'' with a connotation of the assembly court of [[Religion:Indra|Lord Indra]] – the Hindu god of lightning, thunder, and rain.<ref name="THT">{{cite news |title=Nepal skies graced with extraordinary 'circular rainbow' halo around sun |date=9 July 2015 |newspaper=The Himalayan Times |url=https://thehimalayantimes.com/science-technology/nepal-skies-graced-with-extraordinary-circular-rainbow-halo-around-sun/ |access-date=3 August 2016}}</ref>

==Artificial halos==
The natural phenomena may be reproduced artificially by several means. Firstly, by computer simulations,<ref>{{cite web | title= HaloSim3| first1= Les |last1= Cowley |first2= Michael | last2= Schroeder| url= https://atoptics.co.uk/blog/halosim3-software/ | website = atoptics.co.uk | publisher =| date =| accessdate =}}</ref><ref>{{cite web | title= HaloPoint 2.0| url= http://www.saunalahti.fi/~jukkruos/halopoint2.html | website= saunalahti.fi| publisher =| archiveurl= https://web.archive.org/web/20161007061251/http://www.saunalahti.fi/~jukkruos/halopoint2.html |archivedate=2016-10-07| accessdate =}}</ref> or secondly by experimental means. Regarding the latter, this occurs when a single crystal is rotated around the appropriate axis/axes, or a chemical approach. A still further and more indirect experimental approach is to find analogous refraction geometries.

===Analogous refraction approach===
[[File:Artificial CZA.png|thumb|Analogous refraction demonstration experiment for the circumzenithal arc.<ref name=ArtCZA/> Here, it is mistakenly labelled as an artificial rainbow in Gilberts book.<ref name=Gilbert1920/>]] This approach employs the fact that in some cases the average geometry of refraction through an ice crystal may be imitated / mimicked via the refraction through another geometrical object. In this way, the circumzenithal arc, the circumhorizontal arc, and the suncave Parry arcs may be recreated by refraction through rotationally symmetric (i.e. non-prismatic) static bodies.<ref name=ArtCZA>{{cite journal |last1=Selmke |first1=Markus |last2=Selmke |first2=Sarah |title= Artificial circumzenithal and circumhorizontal arcs |journal= American Journal of Physics |publisher=[[Organization:American Association of Physics Teachers|American Association of Physics Teachers]] |volume=85 |issue=8 |year=2017 |issn=0002-9505 |pages=575–581 |arxiv=1608.08664 |bibcode=2017AmJPh..85..575S |s2cid=118613833 |doi= 10.1119/1.4984802}}</ref> A particularly simple table-top experiment reproduces artificially the colorful circumzenithal and circumhorizontal arcs using a water glass only. The refraction through the cylinder of water turns out to be (almost) identical to the rotationally averaged refraction through an upright hexagonal ice crystal / plate-oriented crystals, thereby creating vividly colored circumzenithal and the circumhorizontal arcs. In fact, the water glass experiment is often confused as representing a rainbow and has been around at least since 1920.<ref name= Gilbert1920>{{cite book| title= Gilbert light experiments for boys | year= 1920| page = 98| chapter= Experiment No. 94| url= https://archive.org/stream/gilbertlightexpe00lynd#page/98/mode/2up | via= archive.org}}</ref>

Following Huygens' idea of the (false) mechanism of the 22° parhelia, one may also illuminate (from the side) a water-filled cylindrical glass with an inner central obstruction of half the glasses' diameter to achieve upon projection on a screen an appearance which closely resembles parhelia (cf. footnote [39] in Ref.,<ref name= ArtCZA/> or see here<ref name= DIYExperiments>{{cite  web | title = Experiments| url= https://photonicsdesign.jimdofree.com/experiments/ | website = photonicsdesign.jimdofree.com| publisher =| date =| accessdate =}}</ref>), an inner red edge transitioning into a white band at larger angles on both sides of the direct transmission direction. However, while the visual match is close, this particular experiment does not involve a fake caustic mechanism and is thus no real analogue.

===Chemical approaches===
The earliest chemical recipes to generate artificial halos has been put forward by Brewster and studied further by A. Cornu in 1889.<ref>{{cite journal | title = Sur la reproduction artificielle des halos et des cercles parhéliques| journal = Comtes Rendus Ac. Paris | volume =
108| pages= 429–433| first = A. | last=Cornu| year=1889| language= fr| accessdate =}}</ref> The idea was to generate crystals by precipitation of a salt solution. The innumerable small crystals hereby generated will then, upon illumination with light, cause halos corresponding to the particular crystal geometry and the orientation / alignment. Several recipes exist and continue to be discovered.<ref name= Tammer1998>{{cite journal |last1= Vollmer |first1=Michael |last2=Tammer |first2=Robert |title=Laboratory experiments in atmospheric optics |journal=Applied Optics |publisher=The Optical Society |volume=37 |issue=9 |date=1998-03-20 |issn=0003-6935 |page=1557-1568 |pmid=18268748 |bibcode=1998ApOpt..37.1557V |doi=10.1364/ao.37.001557}}</ref> Rings are a common outcome of such experiments.<ref>{{cite journal |last1= Gislén |first1=Lars |last2= Mattsson |first2=Jan O |title=Tabletop divergent-light halos |journal=Physics Education |publisher=IOP Publishing |volume=42 |issue=6 |date=2007-10-16 |issn= 0031-9120 |pages=579–584 |bibcode= 2007PhyEd..42..579G |s2cid= 122261149 |doi=10.1088/0031-9120/42/6/003}}</ref> But also Parry arcs have been artificially produced in this way.<ref>{{cite journal |last= Ulanowski |first=Zbigniew |title=Ice analog halos |journal=Applied Optics |publisher=The Optical Society |volume= 44 |issue=27 |date=2005-09-20 |pages=5754–5758 |issn=0003-6935 |pmid=16201438 |bibcode= 2005ApOpt..44.5754U |doi= 10.1364/ao.44.005754}}</ref>

===Mechanical approaches===

====Single axis====
The earliest experimental studies on halo phenomena have been attributed<ref>{{cite book | first=M. Élie | last= de Beaumont| title=Memoir of Auguste Bravais | publisher= Smithsonian Institution| place= Washington, DC| year = 1869}}</ref> to [[Biography:Auguste Bravais|Auguste Bravais]] in 1847.<ref name= Bravais>{{cite journal | title=Mémoire sur les halos et les phénomènes optiques qui les accompagnent| year = 1847| work= J. de l'École Royale Polytechnique | volume= 31| number= 18| pages= 1–270|  location= §XXIV – Reproduction artificielle des phénomènes optiques dus à des prismes à axe vertical, Figures: PL I: Fig. 48, PL II: Fig: 49–54| language = fr}}</ref> Bravais used an equilateral glass prism which he spun around its vertical axis. When illuminated by parallel white light, this produced an artificial [[Earth:Parhelic circle|parhelic circle]] and many of the embedded parhelia. Similarly, A. Wegener used hexagonal rotating crystals to produce artificial subparhelia.<ref>{{cite journal| title= Die Nebensonnen unter dem Horizont| journal= Meteorol. Z. | volume= 34–52| number= 8/9| pages= 295–298| first= A. | last= Wegner| year= 1917 | language = de}}</ref> In a more recent version of this experiment, many more embedded parhelia have been found using commercially available<ref>{{cite web | title= Homogenizing Light rods / Light pipes| url= https://www.edmundoptics.com/c/light-pipes-homogenizing-rods/697/# | website= edmundoptics.com| publisher = | date =| accessdate= }}</ref> hexagonal BK7 glass crystals.<ref>{{cite journal |last1=Borchardt |first1= Sarah |last2=Selmke |first2=Markus |title=Intensity distribution of the parhelic circle and embedded parhelia at zero solar elevation: theory and experiments |journal=Applied Optics |publisher=The Optical Society |volume=54 |issue=22 |date=2015-07-21 |pages=6608–6615 |issn=0003-6935 |pmid=26368071 |bibcode= 2015ApOpt..54.6608B |s2cid= 39382489 |doi= 10.1364/ao.54.006608}}</ref> Simple experiments like these can be used for educational purposes and demonstration experiments.<ref name= Tammer1998/><ref name= Selmke2015>{{cite journal |last=Selmke |first=Markus |title=Artificial halos |journal=American Journal of Physics |publisher=American Association of Physics Teachers |volume=83 |issue=9 |year=2015 |issn=0002-9505 |pages= 751–760 |bibcode= 2015AmJPh..83..751S |doi= 10.1119/1.4923458}}</ref> Unfortunately, using glass crystals one cannot reproduce the circumzenithal arc or the circumhorizontal arc due to total internal reflections preventing the required ray-paths when <math>n<\sqrt{2}</math>.

Even earlier than Bravais, the Italian scientist F. Venturi experimented with pointed water-filled prisms to demonstrate the circumzenithal arc.<ref>{{cite book| first= F.| last= Venturi| title= Commentarii sopra ottica| location = Tav VIII, Fig 17, p. 219; arc: PGQ, Fig 27, p. 213}}</ref><ref>{{cite book | first=Johann Samuel Traugott| last= Gehler |title=Physikalisches Wörterbuch: neu bearbeitet von Brandes, Gmelin, Horner, Muncke, Pfaff |year=1829 |publisher=E. B. Schwickert |page=[https://archive.org/details/bub_gb_euIPAAAAQAAJ/page/n501 494] |url= https://archive.org/details/bub_gb_euIPAAAAQAAJ| language = de| via= archive.org| accessdate =}}</ref> However, this explanation was replaced later by the CZA's correct explanation by Bravais.<ref name= Bravais/>

[[File:Artificial Halo projected on a spherical screen.jpg|thumb|Artificial Halo projected on a spherical screen.<ref name= BoredPanda/><ref name= SelmkeSelmke2016/> Visible are: Tangential arcs, Parry arcs, (sub)parhelia, parhelic circle, heliac arcs]] Artificial ice crystals have been employed to create halos which are otherwise unattainable in the mechanical approach via the use of glass crystals, e.g. circumzenithal and circumhorizontal arcs.<ref>{{cite  web | publisher = Arbeitskreis Meteore e.V. | title = 26. - 29.11.2015 - Boží Dar| url= https://www.meteoros.de/akm/halotreffen/2015/ | website = meteoros.de| date = 2015| accessdate = January 31, 2024}}</ref> The use of ice crystals ensures that the generated halos have the same angular coordinates as the natural phenomena. Other crystals such as [[Chemistry:Sodium fluoride|sodium fluoride]] (NaF) also have a refractive index close to ice and have been used in the past.<ref>{{cite journal | title =An Analog Light Scattering Experiment of Hexagonal Icelike Particles. Part II: Experimental and Theoretical Results| work = Journal of the Atmospheric Sciences| volume =56| first1= B.| last1=  Barkey | first2= K.N.| last2=  Liou | first3= Y.| last3=  Takano | first4= W. | last4= Gellerman | first5= P. | last5= Sokolkly| display-authors = 3| year= 1999| accessdate=}}</ref>

====Two axes====
In order to produce artificial halos such as the tangent arcs or the circumscribed halo one should rotate a single columnar hexagonal crystal about 2 axes. Similarly, the Lowitz arcs can be created by rotating a single plate crystal about two axes. This can be done by engineered halo machines. The first such machine was constructed in 2003;<ref>{{cite journal |last1=Vollmer |first1=Michael |last2=Greenler |first2= Robert |title=Halo and mirage demonstrations in atmospheric optics |journal=Applied Optics |publisher=The Optical Society |volume=42 |issue=3 |date=2003-01-20 |pages=394–398 |issn=0003-6935 |pmid=12570259 |bibcode=2003ApOpt..42..394V |doi=10.1364/ao.42.000394}}</ref> several more followed.<ref name= SelmkeSelmke2016>{{cite journal |last1=Selmke |first1=Markus |last2= Selmke |first2=Sarah |title=Complex artificial halos for the classroom |journal=American Journal of Physics |publisher=American Association of Physics Teachers |volume=84 |issue=7 |year=2016 |issn=0002-9505 |pages=561–564 |bibcode= 2016AmJPh..84..561S |doi= 10.1119/1.4953342}}</ref><ref name= "Großmann2015">{{cite journal |last1= Großmann |first1=Michael |last2= Möllmann |first2=Klaus-Peter |last3= Vollmer |first3=Michael |title=Artificially generated halos: rotating sample crystals around various axes |journal= Applied Optics |publisher=The Optical Society |volume=54 |issue=4 |date= 2014-12-15 |pages=B97-106 |issn= 1559-128X |pmid=25967845 |doi= 10.1364/ao.54.000b97}}</ref> Putting such machines inside spherical projection screens, and by the principle of the so-called sky transform,<ref>{{cite web | title= Sky Transform| website = atoptics.co.uk | url= http://www.atoptics.co.uk/fz983.htm| publisher =| date =| accessdate =}}</ref> the analogy is nearly perfect. A realization using micro-versions of the aforementioned machines produces authentic distortion-free projections of such complex artificial halos.<ref name= ArtCZA/><ref name= BoredPanda>{{cite web| website  =BoredPanda.com| url= https://www.boredpanda.com/we-recreate-beautiful-atmospheric-ice-halos-using-spinning-glass-crystals/ | title= Spherical projection screen for artificial halos| publisher =| date = | accessdate =}}</ref><ref name= SelmkeSelmke2016/> Finally, superposition of several images and projections produced by such halo machines may be combined to create a single image. The resulting superposition image is then a representation of complex natural halo displays containing many different orientation sets of ice prisms.<ref name= BoredPanda/><ref name= SelmkeSelmke2016/>

====Three axes====
The experimental reproduction of circular halos is the most difficult using a single crystal only, while it is the simplest and typically achieved one using chemical recipes. Using a single crystal, one needs to realize all possible 3D orientations of the crystal. This has recently been achieved by two approaches. The first one using pneumatics and a sophisticated rigging,<ref name="Großmann2015"/> and a second one using an Arduino-based random walk machine which stochastically reorients a crystal embedded in a transparent thin-walled sphere.<ref name=Selmke2015/>

==Gallery==
<gallery mode="packed">
File:HaloSolar.jpg|360 degree panorama of a [[Earth:Parhelic circle|parhelic circle]] and several other haloes in Madrid on March 25, 2017
File:Altaskiarea.jpg|Solar halo at the Alta Ski Area near the Snowpine lift on February 12, 2023
File:Лунное гало.jpg|Long exposure of a night-time lunar halo display, including an upper tangent arc, [[Earth:22° halo|22° halo]], and [[Earth:Parhelic circle|paraselenic circle]]
File:Halointhesky.jpg|A [[Earth:Circumscribed halo|circumscribed halo]] (outer ring, partially visible on the bottom left and top left/right) together with a 22° halo (inner ring)
File:Sun pillar and kitesurfers.jpg|[[Physics:Light pillar|Sun pillar]] in San Francisco
File:Halo padang.jpg|[[Earth:22° halo|22° halo]] around the [[Astronomy:Sun|Sun]], above PT Semen Padang building at Padang, Indonesia, October 2, 2009, at 11:09 am
File:Solar halo thermal.jpg|Atmospheric temperatures responsible for ice crystals around 22° halo, as viewed through a thermal camera (°C). The halo itself is not present in the thermal spectrum. The Sun is partially visible at the top of the image.
File:Halo phantom-sun.jpg|Complex halo display ([[Earth:22° halo|22° halo]], [[Astronomy:Sun dog|sun dog]]s, upper tangent arc, upper and lower [[Physics:Light pillar|Sun pillar]], [[Earth:Parhelic circle|parhelic circle]], [[Earth:Supralateral arc|supralateral arc]]) observed in Les Ménuires (elevation ≈2200 metres), Rhone-Alpes, France on January 23, 2015, during sunset at 16:30
File:Doppelsonne Halo Hoherodskopf Hessen 15-07-2017.jpg|[[Astronomy:Sun|Sun]] with [[Astronomy:Sun dog|Sun dog]]s at Hoherodskopf / {{wipe|Germany}} - July 15, 2017
File:9 degree pyramidal halo.png|A circumscribed halo (outer ring) together with a comparatively rare 9° halo (inner ring), caused by pyramidal ice crystals. Midsland, the Netherlands, 2019.
File:IMG-20210524-WA0004.jpg|22° halo around the Sun, above VBHC Serene Town at Bangalore, India, on May 24, 2021, at 12:15 pm
File:Sun halo, Nov 1, 2021.jpg|Solar halo, at Cedar Key, Florida, US on November 1, 2021, about 15:26 EST
File:Halo solar en Buenos Aires, Argentina (10 de diciembre de 2022).jpg|Halo around the Sun in Buenos Aires, Argentina (December 10, 2022)
File:Sun pillar and upper tangent arc.jpg|Sun pillar and upper tangent arc over the San Francisco bay. April 9, 2023.
File:Optical refraction in nightsky.jpg|22° lunar halo behind coconut tree in Chikmagaluru on May 24, 2021
File:Halo around the Sun in Coimbatore, Tamil Nadu, India on 25 September 2023 at 11 45 AM.jpg|Halo around the Sun in Coimbatore, Tamil Nadu, India (25 September 2023  11:45 AM)
File:22° Halo, Moon.jpg|Moon with 22° halo over New York City, USA on November 30, 2020, at 01:16:28 AM EST
</gallery>

==See also==
{{Div col}}
* {{annotated link|Earth:22° halo}}
* {{annotated link|Earth:46° halo}}
* {{annotated link|Earth:120° parhelion}}
* {{annotated link|Earth:Anthelion}}
* {{annotated link|Physics:Atmospheric optics}}
* {{annotated link|Earth:Circumhorizontal arc}}
* {{annotated link|Earth:Circumscribed halo}}
* {{annotated link|Physics:Circumzenithal arc}}
* {{annotated link|Earth:Corona (optical phenomenon)|Corona}}
* {{annotated link|Earth:Diamond dust}}
* {{annotated link|Earth:False sunrise}}
* {{annotated link|Earth:Glory (optical phenomenon)|Glory}}
* {{annotated link|Astronomy:Green flash}}
* {{annotated link|Earth:Heiligenschein}}
* {{annotated link|Earth:Infralateral arc}}
* {{annotated link|Earth:Kern arc}}
* {{annotated link|Physics:Light pillar}}
* {{annotated link|Earth:Liljequist parhelion}}
* {{annotated link|Earth:Parhelic circle}}
* {{annotated link|Earth:Parry arc}}
* {{annotated link|Earth:Rainbow}}
* {{annotated link|Earth:Subhelic arc}}
* {{annotated link|Earth:Subparhelic circle}}
* {{annotated link|Earth:Subsun}}
* {{annotated link|Astronomy:Sun dog}}
* {{annotated link|Earth:Supralateral arc}}
* {{annotated link|Earth:Tangent arc}}
{{Div col end}}

==References==
{{Reflist}}

==External links==
{{Commons|Halo}}
* [https://atoptics.co.uk/blog/atmospheric-halos/ Halo explanations and image galleries] at [https://atoptics.co.uk/ Atmospheric Optics]
* [https://www.meteoros.de/themen/halos Meteoros AKM – Halo explanations and image galleries] (in German)
* [https://haloreports.blogspot.com/ Halo reports of interesting halo observations around the World]
* [https://web.archive.org/web/20190119030529/http://astronomy.net.nz/ Southern Hemisphere Halo and other atmospheric phenomena]
* [https://valeriu.tihai.md/soarele-incercuit-280.html Halo in Chisinau Moldova (photo and video)]
* [https://scholarworks.alaska.edu/handle/11122/6400 Walter Tape & Jarmo Moilanen, ''Atmospheric Halos and the Search for Angle x'' (free e-book)]
* [http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/halo.html Halo Phenomena] – Hyperphysics

[[Category:Atmospheric optical phenomena]]

{{Sourceattribution|Halo (optical phenomenon)}}