Kimberley Robinson was in the car with her roofer boyfriend Danny Cooper, 26, when she looked up and saw a mysterious black ring.
The 22-year-old photographed the strange black mark near Birkenshaw on the M62 on her Samsung phone.
The circle resembles a giant smoke ring but West Yorkshire Fire Service say there had been no reports of fire at the time of the sighting on Sunday just after 1.30pm.
The Met Office said that the ring did not appear to be weather-related.
But Nick Pope, who used to investigate UFOs and other mysteries for the Ministry of Defence, thinks he may have debunked the phenomenon.
He said: “Despite exotic theories about UFOs, aliens, or portals to other dimensions, I believe there’s a decidedly down-to-earth explanation here.
“A so-called smoke vortex can arise when you have a blast through a circular structure like a smokestack.
“So these weird effects can be caused by accident - or indeed by design.
“On this latter point, they can be created as a sort of pyrotechnic effect, and it would be interesting to know if there were any outdoor festivals being held in the area concerned.
“I realise that by debunking this I’ll probably be accused of working for the Illuminati or something, but I genuinely think we can close the case file on this one.”
Full-time mum Miss Robinson described the circle as “the weirdest thing I have ever seen”.
She said: “Me and my boyfriend were driving today and noticed a black flying ring in the sky.
“It looked a bit smoky but the shape was a solid ring.
“We hadn’t ever seen anything like this before do we have looked into it and no one knows what it is.”
The pair were travelling back home to Bury from Norfolk after spending the night at an RAF base to see Mr Cooper’s godson.
A spokesperson for the Met Office said on Monday: “We have had a look at it and cannot think of anything metrological that would have caused such a phenomenon.
“It looks reasonably clear, with fair-weather cloud that looks like it has bubbled up. There is a little bit of rising air in the atmosphere.
“It does not appear to be anything at ground level that is causing the smoke to rise.
“It looks as if it’s a small feature rather than something several miles across.”