I can see your halo – scientists systematise all halos discovered over thousands of years

No, I’m not talking about the halos you find surrounding the heads of holy people in art, or in the iconic 2008 song by Beyoncé.

Instead, these are atmospheric halos; incredible phenomena caused by the scattering, refraction, and focusing of light by ice crystals in a disordered, horizontal, or vertical orientation in the air. Now, for the first time, scientists have systematised information about every recorded event from thousands of years ago until the end of 2021.

There are 119 different ice crystal halo forms known today, with the oldest recorded mentions of halos are recorded four to five thousand years ago in the cuneiform tablets of the Sumerian-Babylonian culture.

By 1820 the number of registered halo types was about 20, a number which had increased to 60 by 1990 and has almost doubled since, due to the advent of mobile phones with advanced camera capabilities.

The study has been published in the Journal of Quantitative Spectroscopy and Radiative Transfer.

How are atmospheric halos formed?

Atmospheric halos are caused by light, usually from the Sun, Moon, and artificial sources, being reflected and refracted by water ice crystals – smaller than 10 micrometres – that have accumulated in the atmosphere.

Atmospheric halo
A photo of a halo of diamond dust was made on December 12, 2010 in Tampere (Finland). Credit: Marko Riikonen

A halo’s appearance is determined by a combination of the shape of the ice crystals, their orientation, and the path of light through them. This governs the its configuration, whether it is coloured, and whether it displays white rings, spots, or arcs.

This means that determining the type of atmospheric halo can give scientists information about the kinds of ice crystals present in the sky during the display.

Atmospheric halos are usually divided into two groups, according to how often they are observed. Common halos are observed at least once a year on average, whereas uncommon or exotic halos – which comprise ∼1% or less of halos observed – are seen less than this.

“As a rule, halos are formed as a result of the interaction of light with hexagonal crystals of water ice,” says co-author Jarmo Moilanen, a researcher at the Finnish Geospatial Research Institute (FGI) and PhD student at the University of Helsinki, Finland.

“However, some of the documented exotic halos cannot be explained in this way. For example, the mysteries of the origin of elliptical halos and Bottlinger rings have not been solved since their discovery at the beginning of the 20th century. Among the mysterious ones is the so-called Moilanen arc, which I first discovered in 1995.”

Hexagonal ice crystals
Most probable ray paths through hexagonal ice crystals. Halos created by these ray paths are the parhelic circle, the subsun (or the pillar), the 22° halo, and the 46° halo. Credit: Moilanen and Gritsevich (2022). DOI:10.1016/j.jqsrt.2022.108313

Some halos can also only be seen on various surfaces, including plain lake ice, snow, frosty grass, and even car windshields. But the phenomenon isn’t just isolated to Earth – atmospheric halos have even been observed on Mars.

“This observation proves that clouds of hexagonal crystals of water ice or other minerals exist in the atmosphere of Mars,” says Gritsevich. “There are suggestions that the halos could be formed by carbon dioxide crystals.

Monte Carlo modeling of the factors that can lead to the formation of a halo will provide valuable information about the state of the Martian atmosphere.”

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