This two images shows the same aerosole ring leaving the Bocca
Nuova in eastern direction. This ring kept 15 minutes his exactly form, reached
maybe the height of 4000 - 5000 m, and was carried by the strong wind in east-direction to the
sea. In the days of 2-6 april when good weather- conditions has
been a rarity, the vent of the Bocca Nuova "produced" every 10 minutes
well-formed rings in the size of 20m up to 50m. This rings can get a
diameter up to 200m after a drifting time of may 5 - 15 minutes, before they
break up and disappear.

When you get the chance to have a closer view to this rings
you can see more details, and for me I had one big question. What mechanism of currency let this rings appears like home or pub made bocca
smoke- rings?

The beginning supposition is: a homogeneous atmosphere currency and the hot gases must leave the vent very
symmetric and fast. The involved condensing water caused by hot/cold air meeting makes this
'aerosole ring' visible. Then the hot driving up steam is pushing through the rough- ring, and the
colder air
is slipping at the outside down. The forming process starts while the whole thing begins
to rotate....at the end very fast. The cross-section of the rotation - form you maybe can
compare with a little inside closed tornado* because the whole thing looks like an inner tube...

May a tornado can only build up his inner low pressure because he has a solid resistance by the ground and can exist for longer times (the other end is
'open'). if you would join the ground part of the tornado with his upper (open) part, he will suck itself without resistance. In case of the ground-free rings of Etna, the inside lower- pressure will develop a screwing horizontal currency like a carousel. The gravity of horizontal rotation and the
itself contraction of the lower-pressure (inside the tube) makes the ring stabile for a longer time by supporting the 'vertical' rotation. May you can take into consideration the inside volume (diameter of the tube), which is steady decreasing in the cold environment, and the 'wall' parts are getting also faster coming nearer to the tube
axis.