smoke ring is a visible vortex ring formed by sudden release of smoke. It can be created by blowing smoke from the mouth, quickly lighting a cigarette lighter and putting it out or holding a burning incense stick or a cigarette vertically, pushing it with the burning side up and suddenly pulling it back (or just stopping it).
In general, a smoke ring occurs when a mass of fluid from the mouth
is impulsively pushed through a roughly circular opening, forming a
vortex ring just outside of the opening. The smoke ring travels roughly
straight from the opening and maintains its shape until dispersed by
turbulence or other interference. The smoke serves to make the flow
pattern of the air visible, and does not significantly affect the fluid
behavior. Another method of creating a smoke ring involves releasing a
mass of fluid with a different density than the surrounding fluid. This
can be done in several ways:
Releasing air underwater forms rings of bubbles. Scuba divers often do this, and dolphins have also been observed performing this trick.
A vortex ring or smoke ring can be formed in the atmosphere by a rising (falling) mass of warm (cold) air, which is also called a thermal (microburst). One is occasionally at the core of a mushroom cloud, and they can be seen at fire eater presentations.