The stream is smooth and shaped like a column because it is laminar, and all the smoke particles travel in parallel. The flow is laminar because the cigarette's burning area is small, and the energy output that is driving the stream upward is only about one watt. Larger sources of smoke such as smokestacks or bonfires clearly do not produce laminar streams. The scattering of the smoke marks the transition of the flow from laminar to turbulent, and is caused by the growth of small, invisible disturbances to the smoke stream that eventually create scattering.
Turbulent flow is characterised by random fluctuations of speed and direction.
Careful measurements using lasers and special imaging techniques have revealed that the smoke stream is only a small part of a much larger invisible plume that rises from the cigarette, with the hottest and fastest moving part of the plume actually a few millimetres in front of the smoke stream. As the plume rises, it takes the surrounding air with it and cools the air down which, surprisingly, makes it move faster. In very still air, I have observed stable plumes that are as long as 30 centimetres, which become wavy just prior to scattering. Philip Morris USA Richmond Virginia a. The effect is caused by a transition from a laminar to a turbulent air flow.