This photograph was taken by my dad, Frank, back in the late-1960s, and it shows nothing stranger than a smoke-ring at an air-show in England.
On three occasions in the last decade or so, however, people have sent me near-identical images to this one, claiming they have something to do with UFOs. They don't! At all!
You can see such rings all the time at air-shows - as evidence, click on this link.
And if anyone tells you the rings are linked with UFOs, tell them, no they are not. Then tell them to go away.