A smoke ring is a visible vortex ring

Vortex ring

A vortex ring, also called a toroidal vortex, is a region of rotating fluid moving through the same or different fluid where the flow pattern takes on a toroidal shape. The movement of the fluid is about the poloidal or circular axis of the doughnut, in a twisting vortex motion...

 formed by sudden release of smoke

Smoke

Smoke is a collection of airborne solid and liquid particulates and gases emitted when a material undergoes combustion or pyrolysis, together with the quantity of air that is entrained or otherwise mixed into the mass. It is commonly an unwanted by-product of fires , but may also be used for pest...

. 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

Bubble ring

A bubble ring, or ring bubble, is an underwater ring vortex where an air bubble occupies the core of the vortex, forming a ring shape. The ring of air as well as the nearby water spins poloidally as it travels through the water, much like a flexible bracelet might spin when it is rolled on to a...

. Scuba divers often do this, and dolphins have also been observed performing this trick.

A vortex ring

Vortex ring

A vortex ring, also called a toroidal vortex, is a region of rotating fluid moving through the same or different fluid where the flow pattern takes on a toroidal shape. The movement of the fluid is about the poloidal or circular axis of the doughnut, in a twisting vortex motion...

 or smoke ring can be formed in the atmosphere by a rising (falling) mass of warm (cold) air, which is also called a thermal

Thermal

A thermal column is a column of rising air in the lower altitudes of the Earth's atmosphere. Thermals are created by the uneven heating of the Earth's surface from solar radiation, and are an example of convection. The sun warms the ground, which in turn warms the air directly above it...

 (microburst

Microburst

A microburst is a very localized column of sinking air, producing damaging divergent and straight-line winds at the surface that are similar to, but distinguishable from, tornadoes, which generally have convergent damage. There are two types of microbursts: wet microbursts and dry microbursts...

). One is occasionally at the core of a mushroom cloud

Mushroom cloud

A mushroom cloud is a distinctive pyrocumulus mushroom-shaped cloud of condensed water vapor or debris resulting from a very large explosion. They are most commonly associated with nuclear explosions, but any sufficiently large blast will produce the same sort of effect. They can be caused by...

, and they can be seen at fire eater

Fire eater

A fire eater is an entertainer, often a street artist or part of a sideshow. The performer places flaming objects into their mouth and extinguishes them by cutting off oxygen. They also practice controlling and transferring the flame....

 presentations.

The Exploratorium

Exploratorium

The Exploratorium is a museum in San Francisco with over 475 participatory exhibits, all of them made onsite, that mix science and art. It also aims to promote museums as informal education centers....

 has an interactive exhibit that blows large smoke rings.

Cigarettes & Cigars

There are five main ways of making a smoke ring with the mouth. Although all require the formation of an 'O' with the mouth, they differ in the way the smoke is physically expelled. The first technique is to let smoke into the mouth, then gently flick the tongue to expel a short burst of smoke. The second is to let smoke into the mouth, then while pushing the jaw out, slowly open and close the jaw to push smoke out. The third is to form the 'O' with the lips, then gently say just the very beginning of 'Oh'. The fourth technique is to let smoke into the mouth, and then tap your cheek. The fifth technique differs from the previous three in that one does not need to fill up the mouth with smoke beforehand because the smoke is actually being expelled from the lungs, not the mouth. Most smokers blow rings using some combination of these five techniques. A trick often performed in conjunction with mouth-blown smoke rings is the French inhale

French inhale

The French inhale is a trick one can perform while smoking. One takes a deep drag, then while the mouth is open and the jaw is slowly closed to force the smoke out, one inhales through the nose, thus inhaling the smoke drifting from the mouth....

.
A lit cigarette or stick of incense can also be used to create a smoke ring. To do this, hold it vertically and quickly raise it about 4 inches, then gently pull it back down. This creates the vortex needed to create a smoke ring.

Vapor ring

It is also possible to create a vapor ring by using the same techniques on a cold day with only one's breath.

Cooking

A smoke ring is also the name for the pink ring that forms around the edges of meat prepared by smoking, caused by myoglobin

Myoglobin

Myoglobin is an iron- and oxygen-binding protein found in the muscle tissue of vertebrates in general and in almost all mammals. It is related to hemoglobin, which is the iron- and oxygen-binding protein in blood, specifically in the red blood cells. The only time myoglobin is found in the...

 in the meat reacting with nitric oxide

Nitric oxide

Nitric oxide, also known as nitrogen monoxide, is a diatomic molecule with chemical formula NO. It is a free radical and is an important intermediate in the chemical industry...

 to form the heat stable pigment nitrosohemochrome, as nitrite salts do in cured meats.

Volcanoes

Smoke-ring eruptions from Mount Etna

Mount Etna

Mount Etna is an active stratovolcano on the east coast of Sicily, close to Messina and Catania. It is the tallest active volcano in Europe, currently standing high, though this varies with summit eruptions; the mountain is 21 m higher than it was in 1981.. It is the highest mountain in...

 have been documented in 1970 and 2000. http://gizmodo.com/5830880/extremely-rare-volcano-smoke-rings-only-documented-thrice

Eyjafjallajokull

Eyjafjallajökull

Eyjafjallajökull is one of the smaller ice caps of Iceland, situated to the north of Skógar and to the west of Mýrdalsjökull. The ice cap covers the caldera of a volcano with a summit elevation of . The volcano has erupted relatively frequently since the last glacial period, most recently in...

 in Iceland

Iceland

Iceland , described as the Republic of Iceland, is a Nordic and European island country in the North Atlantic Ocean, on the Mid-Atlantic Ridge. Iceland also refers to the main island of the country, which contains almost all the population and almost all the land area. The country has a population...

 sent up a smoke ring in May 2010.

External links

• How to puff a smoke ring?
• Blowing a smoke ring (wikiHow
  WikiHow
  wikiHow is a web-based and wiki-based community, consisting of an extensive database of how-to guides. wikiHow's mission is to build the world's largest and highest quality how-to manual. The site started as an extension of the already existing eHow website, and has evolved to host over 127,000...
  
  )
• Smoke rings generated by eruptions of Etna volcano
• Smoke rings from Stromboli volcano
• Official Smokerings website
• Instructions on building a smoke ring cannon
• Smoke rings at US Army disposals in Iraq (see images on bottom)
• Thomson, Sir William (Lord Kelvin), On Vortex Atoms, Proceedings of the Royal Society of Edinburgh, Vol. VI, 1867, pp. 94–105.
• Silliman, Robert H., William Thomson: Smoke Rings and Nineteenth-Century Atomism, Isis, Vol. 54, No. 4. (Dec., 1963), pp. 461–474. JSTOR link
• Movie of a modern recreation of Tait's smoke ring experiment