The 'vortex ring', which is being hotly debated online, is similar to ones seen occasionally in countries around the world.
The unexplained appearance of a strange circle in the sky above a city in northern China has become a hot debate online.
Footage and pictures of the phenomenon taken from various angles appear to show a dark hazy ring hovering over the streets of Yulin in the Shaanxi region last week.
It vaguely resembles a smoke ring, or similar ‘vortex rings’ caused by puffs of floating particles, but fails to drift or dissipate quickly.
Similar sights have been recorded around the world, often going viral because of how odd they look and the frequent lack of an obvious explanation.
Experts have suggested a number of possible causes, and they generally fall into two categories.
The first is a large ring of smoke or condensed water caused by combustion from a nearby source, such as a factory or natural formation, which can hover in the air due to pressure differences.
Mount Etna notably produced pink rings of condensed floating water droplets (the same as what clouds are made of) in an unusual spectacle earlier this year.
A dark ring similar to the one in China was seen near Leamington Spa in 2014, and it was eventually revealed to have been caused by the testing of fireworks for a display in Warwick Castle.
The second is insects, such as midges, which have been known to congregate in huge swarms for mating – though rarely in neat rings or so high in the air.
Insect conservationists at BugLife said this was the most likely explanation for a dark moving ring which appeared around 100 feet in the sky above Legoland in 2019.
The Yulin ring has become the subject of intense speculation on Chinese social media.
One person who claimed to have witnessed it said the circle was ‘rising into the sky very slowly’ and that there were a number of other rings elsewhere in the sky at the time.
Commentators suggested explanations such as fireworks or factory emissions, though no one has claimed responsibility.
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