For the first time in nearly 5,000 years of observations, researchers have fully cataloged optical illusions that appear in the sky when light shines through ice crystals known as atmospheric halos.
The Atmospheric Halo Inventory showcases commonly seen atmospheric optical illusions from known sources and sheds light on rarer halos, including those whose origins are currently a mystery.
Halos are formed by the accumulation of water ice crystals smaller than 10 microns in the atmosphere. Properties of these atmospheric illusions, such as their colors or whether they have arcs, spots, or white rings, are determined by the shape and orientation of the ice scattering and the light’s path to these crystals. Often the type of crystal behind the scatter can be identified by the shape of the halo they produce.
Related: What makes the earth’s atmosphere so special?
These atmospheric illusions have been used by mankind since at least the Babylonian era – which was around 1895 B.C. – documented when the phenomena were detailed on cuneiform tablets. However, thanks to the availability of cameras following the proliferation of cell phones, scientists have never had so much data on these phenomena at their fingertips.
In addition to an intensive catalog of halos, this new research, based on observations collected up to the end of 2021, reveals gaps in the understanding of the phenomena. The authors behind the inventory also present promising methods both for their further observation and for processing the data collected, and encourage the public to participate in recording these phenomena with their phones and digital photographs.
The complete catalog divides the phenomenon into 119 different types and also describes the conditions, such as temperature and humidity, that are required for their occurrence.
Halos seen at least once a year have been defined as “commonly observed” and usually result from the scattering, refraction, and focusing of light from the sun or moon reflected by hexagonal ice crystals in a disordered, horizontal, or vertical orientation.
Another category — “rare halos” — which includes atmospheric illusions, which account for only about one percent of all observations, is further subdivided into halos of known origin and those of unknown and exotic origin.
“Typically, halos are formed by the interaction of light with hexagonal water ice crystals,” says the scientist and Ph.D. Student at Helsinki University Jarmo Moilanen said in a statement (opens in new tab). “However, some of the documented exotic halos cannot be explained in this way.”
Moilanen, who developed the inventory of atmospheric halos with Maria Gritsevich, a professor at the University of Helsinki, gave some examples of such exotic halos. “The Mysteries of the Origin of Elliptical Halos and Bottlinger Rings (opens in new tab) have not been solved since their discovery in the early 20th century,” said the researcher. “One of the mysterious ones is the so-called Moilanen arc (opens in new tab)which I first discovered in 1995.”
The researchers suspect that ice crystals with anomalous shapes, or crystals of other minerals scattered in the air, may be key to the formation of mysterious atmospheric halos.
Gritsevich said the research suggests the unusual shape of exotic halos is due to human-caused factors such as atmospheric emissions or the powerful electromagnetic fields generated by high-voltage power lines. She added that these factors can disrupt the alignment of airborne ice crystals, potentially resulting in exotic halos.
“In order to unravel such mysteries, samples of ice crystals that form exotic halos were collected specifically in the atmosphere, but this experience also gave more questions than answers,” Gritsevich said in the statement, adding that the atmospheric halos found around other planets discovered around could be of use to unravel this mystery.
These include halos that have been observed and documented in the atmosphere of Mars, which could also be used to better understand the red planet’s thin, diffuse atmosphere.
“This observation proves that clouds of hexagonal crystals of water ice or other minerals exist in the atmosphere of Mars,” says Maria Gritsevich. “There is evidence that the halos could be formed by carbon dioxide crystals.”
She added that mathematically modeling the factors that can lead to the formation of a halo could provide valuable information about the state of the Martian atmosphere.
The duo’s research was published in Journal of Quantitative Spectroscopy and Radiative Transfer. (opens in new tab)
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