KnowHOW team explains: Smoke rises from a cigarette because it is lighter than air. In the initial stage, the stream of smoke is smooth and shaped like a column ? all the particles travelling in parallel in an upward direction. The flow is laminar ? taking place along constant streamlines ? because the cigarette?s burning area is small, and the energy driving the smoke upwards is extremely small. This does not happen in larger sources of smoke ? such as smokestacks or bonfires ? which produce turbulent and scattered smoke from the very beginning.

Because the surrounding air is stationary in a cigarette, a boundary layer exists between the moving air and the still air surrounding it. Air on the inner wall of this boundary layer is moving upwards at the same speed as the column, whereas air on its outer wall is stationary. Between these two extremes, air rise at a range of intermediate speeds. At the bottom of the column, the boundary layer is thin and the flow within it is smooth.

Each molecule of air is confined to a thin sheet of lamina of air. The laminae slide past one another quite easily. Movement of air between the laminae is minimal. At a critical point, however, this situation cannot be maintained and the air scatters.

The question was sent by Subhadra Singh from Jamshedpur