It's not as slow mo as you might think; closer to real time actually. The smoke ring is only about 50 meters wide, and was caused by heavy artillery firing through a fog bank. The first part of the shot is edited out.

...but I see no way of calculating "The smoke ring is only about 50 meters wide..."

It's not a matter of "calculating" it, but of remembering by an old vet whose ears still ring after many years, and of recognizing the smoke ring from heavy artillery. In fact, 50 meters is a bit too large. Make that more like 20, or 30 meters.

Since we see the ring come up through a low fog bank, I'm guessing the video was taken from one of the artillery ranges on the west coast, in southern Ca.; Most likely, it was US Marines practice firing from the beaches near San Diego.

If the first couple of seconds hadn't been edited out the video you'd have seen the muzzle flash, and perhaps even the streak of the round itself across 1 or 2 frames.

Toroidal vortice from mt. Vesuvius, footage recycled. About half way through video after the dolphins and whales.
[Link]

Yes, hobbyborse is right, except its footage of mt. etna, youtube search toroidal votex - etna. same video.

My mistake, mt Etna. I have had a few today.