Smoke ring over Seattle

As I was returning home this evening, I got off the bus and saw that everyone was gazing up into the sky. This is what they were looking at. (Click to see it in the Gallery.)
Smoke ring over Union Station, Seattle
I don’t know how it got there, nor exactly how big it was… it looks to be a couple of seven or eight hundred feet up. It dissipated over a period of about a minute from when I took this.
Odd, innit?
UPDATE: I guess it was due to fireworks. (But who lets off fireworks in daylight hours on a cloudy day?) Check the comments for a link to video evidence.