Your question is a very tough one, and we can't tell you the exact answer because we just don't know. We have talked a bit with a local expert in non-linear dynamics, and here is what he had to say: "In convection patterns the flow lines near the center of the upward draft are often chaotic even if the flow is laminar. I could imagine that this might be quite typical for cigar smoke but I have never studied cigar smoke."
We will try to find a better answer to this excellent question, so please be patient.
-Mike [see answer to follow-up question below]
answer to this question is given below. But like the expert above said, this may not be right...
The way that gases like smoke move is part of a science known as 'fluid dynamics'. (Smoke isn't really a gas in the strict definition of the word, but in this case, you can safely think of it that way.) Fluid dynamics is a very complicated thing to study because there are so many different things that can influence how something like smoke will move (which is why this answer may not be correct). Another part of fluid dynamics is studying how liquids like water move, and they often work the same way as gases. So before I explain the answer to your question, let me explain something a little simpler. Try going to your kitchen sink and turning on the water. As the water falls, gravity makes it speed up. The water that is close to the faucet flows very smoothly. This smooth flow is called 'laminar flow' and requires that the different parts of the fluid be moving slowly with respect to each other, as they do near to the faucet. But farther down, the flow of the water becomes very rough and convoluted. This is called 'turbulent flow'. Turbulent flow occurs when parts of the fluid move quickly with respect to each other. Now on to the cigar smoke... The smoke that comes out of a cigar is extremely warm, and it is being continually heated by the cigar itself. I expect that you have heard of the phrase 'warm air rises'. As the smoke rises higher and higher, it speeds up. Close to the cigar, when it first comes out, the smoke's flow is very smooth, or laminar. But since it's speeding up as it rises, it will eventually reach a speed at which its flow becomes turbulent. [The speed up should vary a lot from the middle of the plume out to the sluggish edges.] This is when you see the smoke spreading out and curling around. To go into more detail than this about the exact nature of the turbulent flow would require a pretty solid understanding of fluid dynamics. And in some cases, it can actually be impossible to predict everything that happens. But I think that this should be enough for what you're looking for.
-Tamara [some mods by mw]
(republished on 07/31/06)