A halo (from Ancient Greek ἅλως (hálōs) 'threshing floor, disk') is an optical phenomenon produced by light (typically from the Sun or Moon) 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 (properly called the 22° halo), light pillars, and sun dogs, but many others occur; some are fairly common while others are extremely rare. The ice crystals responsible for halos are typically suspended in cirrus or cirrostratus clouds in the upper troposphere (5–10 km (3.1–6.2 mi)), 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 is reflected and refracted by the ice crystals and may split into colors because of dispersion. The crystals behave like prisms and mirrors, refracting and reflecting light between their faces, sending shafts of light in particular directions. Atmospheric optical phenomena like halos were part of weather lore, which was an empirical means of weather forecasting before 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 glory and the rainbow.
History While Aristotle had mentioned halos and parhelia in antiquity, the first European descriptions of complex displays were those of Christoph Scheiner in Rome (c. 1630), Johannes Hevelius in Danzig (1661), and Tobias Lowitz in St Petersburg (1790). 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; "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, 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 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 halos can form at 46° to the Sun, or at the horizon, or around the zenith, and can appear as full halos 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 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 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.
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 omen of bad weather. The term is related to the Breton word kog-heol (sun cock) which has the same meaning. In Nepal, 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, 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.
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