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Chile, 1997 - the spectators of a show of the Air Force of Chile in Pucón were attending the stunts of the group "Los Halcones" in the afternoon of day 28 of February.
A ring appeared in the sky and has been filmed and witnessed by many people.
It has been claimed to be a UFO. However, Carlos Ruminot Núñez, air traffic controller and private pilot,
clarified the case to the Chilean investigator Diego Zúñiga: "I can assure that the sighted object corresponds to one ` vortex ring ', produced for the compression of a smoke column due to traction of the helix of one of Extra 300 in the acrobatic presentation. This smoke column, to the compressed being, blows up until the edges, with small "mushrooms", which if unfasten of it. One of them, to the being crossed for one "air arrow", forms a ring (Vortex Ring), whose substance (smoke), it turns around its proper axle. While it keeps its rotation, it will keep the circular form. The displacement and the time that this circular form keeps will depend on the direction and intensity of the wind."
Sighted Sunday from the Windsor Great Park near Heathrow (where, inevitably, there is a high frequence of flyovers by various sized aircraft), this contrail was shaped like "smoke rings" from a giant cigar. I whipped out the camera and took a few shots. A close-up of the phenomenon is seen here
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Some searching online tells us that this is caused by the so-called Crow Instability in the vortexes caused by aircraft; whilrling the otherwise straight contrails into spirals then donut-shaped rings.