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Published: 2006-07-10 12:54:00
Updated: 2006-07-10 12:54:00
Posted July 10, 2006
Cindy, The phenomenon involved is an example of a "ring vortex," the most familiar of which to most people is the "smoke ring" that some smokers like to create. The ring vortex occurs when a parcel of air is accelerated rapidly through a surrounding airmass with similar properties, in such a way that the air at the interface of the two airmasses creates a shear zone in which a circular, or toroidal, vortex develops. This vortex may retain its structure for some time, and involves a region of somewhat lower pressure along the ring. This reduces the interchange of air in the spinning ring with the air around it, so if there is some visible material like smoke that becomes involved with the vortex, that which happens to be in the ring as it develops will tend to remain concentrated for some time while smoke outside the ring more rapidly diffuses and becomes less visible, making the ring itself visible against its surroundings. In a very still environment the ring can maintain itself for quite a while. Stronger surrounding winds and greater ambient turbulence can distort or disrupt the ring more quickly, or in some cases prevent an organized ring from developing in the first place.
In the case of the ring you saw, it would appear that the lighting strike hit some object that produced a sizable puff of smoke, perhaps an exploding electrical transformer. In addition to producing smoke, the lightning strike and resulting explosion would have produced a concentrated parcel of greatly heated air. Being so much hotter than surrounding air, it would become highly buoyant and rise rapidly (the "rapid acceleration" mentioned above) and in this case happened to be the correct size and shape to interact with the smoke released to create a nice airborne ring.
I hope that's a reasonable explanation - here are some links that may help visualize what's happening a little more, and also show that this wasn't an entirely unique case!
These next two links are to photos from the "burning man" festival in Nevada. Someone there has built a machine that more or less replicates the process I described above in order to create giant smoke rings that look quite similar to what you observed. In one of these cases, winds are moderately strong and in the other, very light, allowing for an especially well-formed and stable ring…
Enjoy, and congratulations on witnessing such a cool phenomenon! Here's a cropped-in view of the ring in Cary...