They undoubtedly rank amongst the most eccentric sky phenomena on the planet: the metres-wide rings of vapour or smoke that can be seen floating through the air on occasion.

The physics behind these "vortex rings" is very complex. Moreover, vortex rings do not always form in the same way. In most instances, the rings are formed by air mixed with smoke or steam that is forced out of a (relatively) small circular or cylindrical opening (this can be a chimney, the barrel of a canon, or a vent in a volcano crater). Because of the drag of the surface of the opening, the air expelled from the centre will move faster than the air exiting the opening near the edge. The air from the sides is sucked in, and a circular motion is created. In this way, a doughnut-shaped vortex is formed, just like a smoke ring that is blown from a smoker's lips. The ring-shape is maintained due to the rotational motion of the air flowing in the vortex ring.

Another mechanism by which vortex rings form, is when a spherical mass of fast moving air is pushed into a mass of stationary air. Upon impact, the outer layers of the ball-shaped air mass will peel off and curve round back into the main mass. This inward curving flow then splits the already flattened air mass into a doughnut shape.

The number one cause of the latter type of vortices is explosions. These can be either accidental (for instance by a short circuit or a lightning impact) or induced (for instance during bombings, mining activities and the clearance of mines). A fine example of a deliberately induced explosion that sometimes produces dark-coloured smoke rings, is the miniature atomic bomb simulation, conducted from time to time at military bases to amuse or impress visitors. During such bomb demonstrations large drums are filled with - for example - a mixture of gasoline, diesel fuel and explosives. The drums are then arranged in a circle and detonated. The result is a mushroom-shaped cloud that, upon impact with the ambient air masses, can create a stable black smoke ring in the way described above.

Various types of aerial vortex rings can be distinguished. The most common type is the horizontally oriented, dark-coloured smoke ring already mentioned. But there are several other types of ring- or doughnut-shaped clouds that can easily be mistaken for an unworldly flying object.

At air shows, large white rings have been observed floating through the air in a vertical position. Such rings form in the wake of airplanes and are not to be confounded with the black smoke rings rising up from controlled ground explosions that are often set off during such shows. The mechanism that produces the white rings is not fully understood. Possibly there is a link with the condensation rings that sometimes form around aircraft flying at low altitudes under humid conditions and at speeds just below or above the speed of sound (an effect known as the Prandtl-Glauert effect).

A particularly well-documented type of rings are the steam rings ejected from circular vents on the sides of volcanoes. Dependent on the orientation of the vents, these rings can take a horizontal or a vertical position. Volcano rings can be very large (photos exist of specimens that measure up to 200 m or 650 feet across). They can last up to 15 minutes, drifting upward to heights of more than 1 km (3,048 ft) above the volcano.

Normally, smoke and vapour rings travel at a constant speed, maintaining the same horizontal or vertical position until they break up. However, some of the reports we collected describe rings in slanted positions, diving towards the ground or climbing into the air. The explanation for these sometimes unexpected manoeuvres is that vortex rings are part of the air and therefore follow the air currents.

A typical property of all vortex rings is that the smoke or vapour inside the ring curls around the edge, sometimes forming new, smaller rings around the initial ring. From a certain angle, this can create the impression of rods pointing toward the centre of the ring (some observers compared smoke rings to "twisted bicycle wheels"). In other cases, smoke or vapour assembles in places where the ring is buckled, creating node-like areas.

One of the strangest by-products of smoke rings, is the formation of small cumulus-type clouds in the circular space inside the ring. These ball-shaped clouds are often of a white or light grey colour and give the phenomenon the appearance of a Saturn-shaped object, a type of UFO that is often described in the literature. CAELESTIA is currently studying the possible relationship between Saturn-shaped UFOs and smoke rings.