Halo (optical Phenomenon)
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A halo is an optical phenomenon produced by light (typically from the Sun or Moon) interacting with
ice crystals Ice crystals are solid ice exhibiting atomic ordering on various length scales and include hexagonal columns, hexagonal plates, dendritic crystals, and diamond dust. Formation The hugely symmetric shapes are due to depositional growth ...
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 The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width ...
, 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
22° halo A 22° halo is an atmospheric optical phenomenon that consists of a halo with an apparent radius of approximately 22° around the Sun or Moon. When visible around the Moon, it is also known as a moon ring or winter halo. It forms as sunligh ...
), light pillars, and sun dogs, but many others occur; some are fairly common while others are extremely rare. The
ice crystals Ice crystals are solid ice exhibiting atomic ordering on various length scales and include hexagonal columns, hexagonal plates, dendritic crystals, and diamond dust. Formation The hugely symmetric shapes are due to depositional growth ...
responsible for halos are typically suspended in cirrus or cirrostratus clouds in the upper
troposphere The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...
(), but in cold weather they can also float near the ground, in which case they are referred to as diamond dust. The particular shape and orientation of the crystals are responsible for the type of halo observed.
Light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...
is
reflected Reflection or reflexion may refer to: Science and technology * Reflection (physics), a common wave phenomenon ** Specular reflection, reflection from a smooth surface *** Mirror image, a reflection in a mirror or in water ** Signal reflection, in ...
and refracted by the
ice crystals Ice crystals are solid ice exhibiting atomic ordering on various length scales and include hexagonal columns, hexagonal plates, dendritic crystals, and diamond dust. Formation The hugely symmetric shapes are due to depositional growth ...
and may split into
color Color (American English) or colour (British English) is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physical specifications of color are assoc ...
s because of dispersion. The crystals behave like prisms and
mirror A mirror or looking glass is an object that Reflection (physics), reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the ...
s, refracting and reflecting light between their faces, sending shafts of light in particular directions. Atmospheric optical phenomena like halos were used as part of weather lore, which was an empirical means of
weather forecasting Weather forecasting is the application of science and technology to predict the conditions of the atmosphere for a given location and time. People have attempted to predict the weather informally for millennia and formally since the 19th centu ...
before
meteorology Meteorology is a branch of the atmospheric sciences (which include atmospheric chemistry and physics) with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did no ...
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 glory and
rainbow 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 ...
.


History

While
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical Greece, Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatet ...
had mentioned halos and parhelia, in antiquity, the first European descriptions of complex displays were those of Christoph Scheiner in Rome (circa 1630),
Johannes Hevelius Johannes Hevelius Some sources refer to Hevelius as Polish: * * * * * * * Some sources refer to Hevelius as German: * * * * *of the Royal Society * (in German also known as ''Hevel''; pl, Jan Heweliusz; – 28 January 1687) was a councillo ...
in Danzig (1661), and Tobias Lowitz in St Petersburg (c. 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.


Vädersolstavlan

While mostly known and often quoted for being the oldest color depiction of the city of Stockholm, '' Vädersolstavlan'' (
Swedish Swedish or ' may refer to: Anything from or related to Sweden, a country in Northern Europe. Or, specifically: * Swedish language, a North Germanic language spoken primarily in Sweden and Finland ** Swedish alphabet, the official alphabet used by ...
; "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 sun dogs. 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

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

Among the best-known halos is the
22° halo A 22° halo is an atmospheric optical phenomenon that consists of a halo with an apparent radius of approximately 22° around the Sun or Moon. When visible around the Moon, it is also known as a moon ring or winter halo. It forms as sunligh ...
, 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
sun dogs A sun dog (or sundog) or mock sun, also called a parhelion (plural parhelia) 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° h ...
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". 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 46° to the sun, or at the horizon, or 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 spotted around the dimmer
Subsun A subsun (also spelled sub-sun) is an optical phenomenon that appears as a glowing spot visible within clouds or mist when observed from above. The subsun appears directly below the actual Sun, and is caused by sunlight reflecting off of numer ...
, 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 A pyramid (from el, πυραμίς ') is a structure whose outer surfaces are triangular and converge to a single step at the top, making the shape roughly a pyramid in the geometric sense. The base of a pyramid can be trilateral, quadrila ...
al 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.


Other names

In the
Anglo-Cornish The Cornish dialect (also known as Cornish English, Cornu-English, kw, Sowsnek Kernowek) is a dialect of English spoken in Cornwall by Cornish people. Dialectal English spoken in Cornwall is to some extent influenced by Cornish grammar, an ...
dialect of English, a halo round the sun or the moon is called a ''cock's eye'' and is a
token Token may refer to: Arts, entertainment, and media * Token, a game piece or counter, used in some games * The Tokens, a vocal music group * Tolkien Black, a recurring character on the animated television series ''South Park,'' formerly known as ...
of bad weather. The term is related to the Breton word ''kog-heol'' (sun cock) which has the same meaning. In
Nepal Nepal (; ne, नेपाल ), formerly the Federal Democratic Republic of Nepal ( ne, सङ्घीय लोकतान्त्रिक गणतन्त्र नेपाल ), is a landlocked country in South Asia. It is ma ...
, the halo round the sun is called ''Indrasabha'' with a connotation of the assembly court of Lord Indra – the Hindu god of lightning, thunder and rain.


Artificial halos

The natural phenomena may be reproduced artificially by several means. Firstly, by computer simulations, or secondly by experimental means. Regarding the latter, one may either take a single crystal and rotate it around the appropriate axis/axes, or take a chemical approach. A still further and more indirect experimental approach is to find analogous refraction geometries.


Analogous refraction approach

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. 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.Gilbert light experiments for boys – (1920), p. 98, Experiment No. 9
link
/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 9in Ref., or see hereWebpage detailing several DIY experiment
link
/ref>), i.e. 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. 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. Rings are a common outcome of such experiments. But also Parry arcs have been artificially produced in this way.


Mechanical approaches


Single axis

The earliest experimental studies on halo phenomena have been attributed to Auguste Bravais in 1847."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 axe vertical, Figures: PL I: Fig. 48, PL II: Fig: 49-54. Bravais used an equilateral glass prism which he spun around its vertical axis. When illuminated by parallel white light, this produced an artificial Parhelic circle and many of the embedded parhelia. Similarly, A. Wegener used hexagonal rotating crystals to produce artificial subparhelia. In a more recent version of this experiment, many more embedded parhelia have been found using commercially available hexagonal BK7 glass crystals. Simple experiments like these can be used for educational purposes and demonstration experiments. 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 n<\sqrt. Even earlier than Bravais, the Italian scientist F. Venturi experimented with pointed water-filled prisms to demonstrate the circumzenithal arc. However, this explanation was replaced later by the CZA's correct explanation by Bravais. 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. The use of ice crystals ensures that the generated halos have the same angular coordinates as the natural phenomena. Other crystals such as NaF also have a refractive index close to ice and have been used in the past.


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; several more followed. Putting such machines inside spherical projection screens, and by the principle of the so-called sky transform,"Sky Transform" on atoptics.co.uk
link
/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.Article with images on BoredPanda
Spherical projection screen for artificial halos
/ref> 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.


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, and a second one using an Arduino-based random walk machine which stochastically reorients a crystal embedded in a transparent thin-walled sphere.


Gallery

File:Optical refraction in nightsky.jpg, 22° lunar halo behind coconut tree in Chikmagaluru on May 24th 2021 File:HaloSolar.jpg, 360 degree panorama of a parhelic circle and several other haloes in Madrid on March 25, 2017 File:Лунное гало.jpg, Long exposure of a night-time lunar halo display, including an upper tangent arc,
22° halo A 22° halo is an atmospheric optical phenomenon that consists of a halo with an apparent radius of approximately 22° around the Sun or Moon. When visible around the Moon, it is also known as a moon ring or winter halo. It forms as sunligh ...
, and paraselenic circle File:Halointhesky.jpg, A 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,
Sun pillar A light pillar is an atmospheric optical phenomenon in which a vertical beam of light appears to extend above and/or below a light source. The effect is created by the reflection of light from tiny ice crystals that are suspended in the atmosphe ...
in San Francisco File:Halo padang.jpg,
22° halo A 22° halo is an atmospheric optical phenomenon that consists of a halo with an apparent radius of approximately 22° around the Sun or Moon. When visible around the Moon, it is also known as a moon ring or winter halo. It forms as sunligh ...
around the Sun, above PT Semen Padang building at Padang,
Indonesia Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania between the Indian and Pacific oceans. It consists of over 17,000 islands, including Sumatra, Java, Sulawesi, and parts of Borneo and New Guine ...
, 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 (
22° halo A 22° halo is an atmospheric optical phenomenon that consists of a halo with an apparent radius of approximately 22° around the Sun or Moon. When visible around the Moon, it is also known as a moon ring or winter halo. It forms as sunligh ...
, sun dogs, upper tangent arc, upper and lower
Sun pillar A light pillar is an atmospheric optical phenomenon in which a vertical beam of light appears to extend above and/or below a light source. The effect is created by the reflection of light from tiny ice crystals that are suspended in the atmosphe ...
, parhelic circle,
supralateral arc 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 familia ...
) 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, Sun with Sun dogs at Hoherodskopf /
Germany Germany, officially the Federal Republic of Germany (FRG),, is a country in Central Europe. It is the most populous member state of the European Union. Germany lies between the Baltic and North Sea to the north and the Alps to the sou ...
- 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)


See also

*
22° halo A 22° halo is an atmospheric optical phenomenon that consists of a halo with an apparent radius of approximately 22° around the Sun or Moon. When visible around the Moon, it is also known as a moon ring or winter halo. It forms as sunligh ...
*
46° halo A 46° halo is a rare atmospheric optical phenomenon that consists of a halo with an apparent radius of approximately 46° around the Sun. At solar elevations of 15–27°, 46° halos are often confused with the less rare and more colourful ...
* 120° parhelion * Anthelion * Atmospheric optics * Circumhorizontal arc * Circumscribed halo * Circumzenithal arc * Corona * Diamond dust * False sunrise * Glory * Green flash * Heiligenschein * Infralateral arc * Kern arc * Light pillar *
Liljequist parhelion A Liljequist parhelion is a rare halo, an optical phenomenon in the form of a brightened spot on the parhelic circle approximately 150–160° from the sun; i.e., between the position of the 120° parhelion and the anthelion. While the sun touches ...
* 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. Discovery The halo was first described by Sir William Edward Parry (1790–1855) in 1820 during one of his Arct ...
*
Rainbow 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 ...
*
Subhelic arc 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 result from ray entrance and exit through prism end faces ...
* Subparhelic circle *
Subsun A subsun (also spelled sub-sun) is an optical phenomenon that appears as a glowing spot visible within clouds or mist when observed from above. The subsun appears directly below the actual Sun, and is caused by sunlight reflecting off of numer ...
* Sun dog *
Supralateral arc 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 familia ...
* Tangent arc


References


External links


Halo explanations and image galleries
a
Atmospheric Optics



Halo reports of interesting halo observations around the World

Southern Hemisphere Halo and other atmospheric phenomena

Halo in Chisinau Moldova (photo and video)

Walter Tape & Jarmo Moilanen, ''Atmospheric Halos and the Search for Angle x'' (free e-book)


– Hyperphysics {{authority control Atmospheric optical phenomena