Circular phenomena

Another curious coincidence: in two days I hear of two different odd natural phenomena, and both of them circular in nature.

One is Snow Doughnuts. It's not really explained how the holes appear.

The other is dolphins and bubble-rings. What the heck are those smoke-ring-like bubble phenomena? How do they appear, how do they stick around, and how to they move through the water like that? It seems the dolphins blow them somehow, and it's clear that they are made from air and nothing else from the way they break up, but beyond that it's puzzling to me.

Well, Wiki has an entry on it. Still too me, it's one of those things that are too weird, like ball lightning.

Bert writes:
Have you seen the movie Pushing Tin? If not, you could spend a good moment watching it. But if you did, you will probably remember the scene where Billy Bob Thornton is swept in the air by the wingtip vortex of a landing jumbo jet. Here is another (very dramatic) illustration of the phenomenon.
It may appear mysterious, but the explanation is really quite simple.

Simply put, in order for air to push upwards (e.g. generate lift) on an aircraft wing, there must exist a very significant difference between the pressure below the wing and the pressure above the wing, the former being of course greater than the latter. How this pressure differential is formed is quite frankly a very complex topic, but thankfully is not necessary for us to understand how this happens.

If you simply picture the pressure differential between the two sides of the wing, you will agree that it is only normal that the high-pressure air from under the wing will tend to travel towards the low-pressure area on top of the wing, by any path available. It does just that around the tip of the wing, and violently so I might add. This is what causes the vortex on the photo above, and nothing else. Simple enough, no? (if not, take the time to picture it in your mind, it really is quite simple)

[open parenthesis]
Wingtip vortices tend to move away from the aircraft at a constant yet surprisingly small speed (a few kph). An opposing wind of equal magnitude can therefore nullify such lateral motion and make a vortex remain stationary (i.e. over the runway) for several minutes. Since these phenomena are usually invisible in clear air, they can pose a serious threat to approaching aircraft. In case you ever wondered, this is why air traffic regulations strictly enforce a minimum delay between two successive landings or takeoffs (the smaller the approaching aircraft, the longer the delay). This of course applies mainly to the vortices created by large aircraft.

It is also interesting to note that, for jumbo jets flying at ~35,000 ft above sea level, the outside ambient temperature is less than -50C. However, the temperature measured at the tip of the wing is near +50C (if memory serves well); that's how violent wing tip turbulence is. This temperature variation is also part of what causes the white condensation trails left by airplanes.
[close parenthesis]

The same type of vortices are also formed in water around moving bodies. They can be observed, for example, around the edges of a paddle. They are also created by dolphins fins, but are usually invisible when completely submerged. It is these vortices that "trap" the ring-shaped air bubbles and makes them appear stationary.

This may seem incredibly complex, but keep in mind that the dolphins, like all underwater dwellers, spend their entire lives surrounded by such phenomena. It is therefore only normal that they should perceive vortices quite precisely. It is however not so obvious (downright staggering, if you ask me) that they are able to develop a sufficient understanding, albeit intuitive, of those vortices to use those in their playful endeavors... wow!