The incredible phenomenon of atmospheric halos

No, I’m not talking about the halos you find around 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 systematized information about every recorded event from thousands of years ago to the end of 2021.

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

In 1820, the number of recorded halotypes was about 20, a number that had increased to 60 by 1990 and has almost doubled since then, due to the use of mobile phones with advanced camera functions.

The study is published in 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 – less than 10 micrometres – that have accumulated in the atmosphere.

Atmospheric halo
An image of a halo of diamond dust was taken on December 12, 2010 in Tampere (Finland). Credit: Marko Riikonen

The appearance of a halo is determined by a combination of the shape of the ice crystals, their orientation and the path of light through them. This controls the configuration, whether it is colored and whether it shows white rings, spots or arcs.

This means that determining the type of atmospheric halo can provide scientists with information about the types of ice crystals present in the sky during the view.

Atmospheric halos are usually divided into two groups, according to how often they are observed. Common haloes are observed at least once a year on average, while unusual or exotic haloes – which make up ~1% or less of observed haloes – are seen less often than this.

“As a rule, halos are formed as a result of the interaction between light and hexagonal crystals of water ice,” says co-author Jarmo Moilanen, 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 surrounding the origin of elliptical halos and Bottlinger rings have not been solved since their discovery in the early 20th century. Among the mysterious is the so-called Moilanen arch, which I first discovered in 1995.”

Hexagonal ice crystals
Most likely ray paths through hexagonal ice crystals. Halos created by these ray paths are the parhelical circle, the subsun (or 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 regular lake ice, snow, icy grass, and even car windshields. But the phenomenon is not just isolated to Earth – atmospheric halos have even been observed on Mars.

“This observation proves that there are clouds of hexagonal crystals of water ice or other minerals in the atmosphere of Mars,” says Gritsevich. “There are suggestions that the halos may 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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