12/21/15, 13:16Halo (optical phenomenon) - Wikipedia, the free encyclopedia

Page 1 of 4https://en.wikipedia.org/wiki/Halo_(optical_phenomenon)

A 22° halo around the sun in NepalHimalayas.

Halo (optical phenomenon)From Wikipedia, the free encyclopedia

A halo (from Greek ἅλως, halōs;[1] also known as a nimbus, icebow orgloriole) is an optical phenomenon produced by light interacting withice crystals suspended in the atmosphere, resulting in a wide variety ofcolored or white rings, arcs and spots in the sky. Many halos are near theSun or Moon, but others occur elsewhere or even in the opposite part ofthe sky. Among the most well known halo types are the circular halo(properly called the 22° halo), light pillars and sun dogs, but there aremany more; some of them fairly common, others (extremely) rare.

The ice crystals responsible for halos are typically suspended in cirrus orcirrostratus clouds high (5–10 km, or 3–6 miles) in the uppertroposphere, but in cold weather they can also float near the ground, inwhich case they are referred to as diamond dust. The particular shapeand orientation of the crystals is responsible for the type of haloobserved. Light is reflected and refracted by the ice crystals and maysplit up into colors because of dispersion. The crystals behave likeprisms and mirrors, refracting and reflecting light between their faces,sending shafts of light in particular directions.

Atmospheric phenomena such as halos were used as part of weather loreas an empirical means of weather forecasting before meteorology wasdeveloped. They often do mean that rain is going to fall within the next24 hours as the cirrostratus clouds that cause them can signify an approaching frontal system.

Other common optical phenomena involving water droplets rather than ice crystals include the glory and therainbow.

Contents

1 Light pillar

2 Circular halo

3 Other names

4 Gallery

5 See also

12/21/15, 13:16Halo (optical phenomenon) - Wikipedia, the free encyclopedia

Page 2 of 4https://en.wikipedia.org/wiki/Halo_(optical_phenomenon)

From top to bottom:A circumzenithal arc, supralateral arc,Parry arc, upper tangent arc, and 22°halo.

6 References

7 External links

Light pillarA light pillar, or sun pillar, appears as a vertical pillar or column of lightrising from the sun near sunset or sunrise, though it can appear belowthe sun, particularly if the observer is at a high elevation or altitude.Hexagonal plate- and column-shaped ice crystals cause thephenomenon. Plate crystals generally cause pillars only when the sun iswithin 6 degrees of the horizon, or below it; column crystals can cause apillar when the sun is as high as 20 degrees above the horizon. Thecrystals tend to orient themselves near-horizontally as they fall or floatthrough the air, and the width and visibility of a sun pillar depends oncrystal alignment.

Light pillars can also form around the moon, and around street lights orother bright lights. Pillars forming from ground-based light sources mayappear much taller than those associated with the sun or moon. Since theobserver is closer to the light source, crystal orientation matters less in the formation of these pillars.

Circular haloAmong the most well known halos is the 22° halo, often just called "halo", which appears as a large ring aroundthe Sun or Moon with a radius of about 22° (roughly the width of an outstretched hand at arm's length). The icecrystals that cause the 22° halo are oriented semi-randomly in the atmosphere, in contrast to the horizontalorientation required for some other halos such as sun dogs and light pillars. As a result of the optical propertiesof the ice crystals involved, no light is reflected towards the inside of the ring, leaving the sky noticeably darkerthan the sky around it, and giving it the impression of a "hole in the sky".[2] The 22° halo is not to be confusedwith the corona, which is a different optical phenomenon caused by water droplets rather than ice crystals, andwhich has the appearance of a multicolored disk rather than a ring.

Other namesIn the Anglo-Cornish dialect of English a halo round the sun or the moon is called a cock's eye and is a token ofbad weather. The term is related to the Breton word kog-heol (sun cock) which has the same meaning.[3]

Gallery

12/21/15, 13:16Halo (optical phenomenon) - Wikipedia, the free encyclopedia

Page 3 of 4https://en.wikipedia.org/wiki/Halo_(optical_phenomenon)

Circumscribed halo aroundthe sun, taken

on PuntaAlta,

Argentina,captured onJanuary 28,

2013 at 11:58am.

Sun pillar in San Francisco.

22° halo around the Moon,Graz, Austria, October 23,

2010.

Sun dogs and possibleSun pillar in Hesse,August 12, 2012.

Atmospherictemperatures responsible

for ice crystals around22° halo, as viewed

through a thermal camera(°C). The halo itself is

not present in the thermalspectrum. The sun is

partially visible at the topof the image.

Complex halo display (22° halo,sun dogs, upper tangent arc,upper and lower Sun pillar,

parhelic circle, supralateral arc)observed in Les Ménuires

(elevation ~2200 metres), Rhone-Alpes, France on January 23,2015 during sunset at 16:30.

See also

120° parhelion22° halo46° haloAnthelionAtmospheric optics

Diamond dustFalse sunriseGloryGreen flashHeiligenschein

Parhelic circleParry arcRainbowSubhelic arcSubparhelic circle

12/21/15, 13:16Halo (optical phenomenon) - Wikipedia, the free encyclopedia

Page 4 of 4https://en.wikipedia.org/wiki/Halo_(optical_phenomenon)

Wikimedia Commons hasmedia related to Halo.

Circumhorizontal arcCircumscribed haloCircumzenithal arcCorona

Infralateral arcKern arcLight pillarLiljequist parhelionLower tangent arc

SubsunSun dogSupralateral arcUpper tangent arc

References1. Harper, Douglas. "halo". Online Etymology Dictionary. ἅλως (http://www.perseus.tufts.edu/hopper/text?

doc=Perseus:text:1999.04.0057:entry=a(/lws). Liddell, Henry George; Scott, Robert; A Greek–English Lexicon at thePerseus Project.

2. http://www.atoptics.co.uk/halo/circ2.htm3. Nance, Robert Morton; Pool, P. A. S. (1963). A Glossary of Cornish Sea-Words. Cornwall: Federation of Old Cornwall

Societies. p. 61.

External linksPhotos and explanation of 22° halo phenomenon(http://www.abcactionnews.com/dpp/news/science_tech/whats-that-rainbow-ring-around-the-sun-its-a-22-degree-halo-formed-by-ice-crystals)Halo explanations and image galleries (http://www.atoptics.co.uk/halosim.htm) at Atmospheric Optics(http://www.atoptics.co.uk/)Meteoros AKM - Halo explanations and image galleries (http://www.meteoros.de/haloe.htm)Halo reports of interesting halo observations around the World (http://haloreports.blogspot.com)Southern Hemisphere Halo and other atmospheric phenomena (http://www.astronomy.net.nz)Moon Halo Gallery (http://www.lumis.com/tag/Moon_Ring/page1/)Halo in Chisinau Moldova (photo and video) (http://valeriu.tihai.md/?p=280)Sun Halo appeared in Padang after the earthquake(http://www.detiknews.com/read/2009/10/02/121219/1213668/10/fenomena-halo-muncul-di-kota-padang)

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Categories: Atmospheric optical phenomena

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