# Smoke Rising in Still Air: Physics Explanation

• Chris R
In summary, the conversation discusses the behavior of smoke rising from burning incense in still air. It is noted that the smoke, being a fluid, exhibits laminar flow at first before transitioning to turbulent flow. This transition is predictable and depends on various factors. The conversation also mentions the relevance of this phenomenon in understanding the behavior of air over a wing. Further information and equations are provided for reference.
Chris R
Hi all,
I have what I believe is a simple physics question. I was burning a sprig of incense in my apartment, and noticed that the smoke was rising from the burning section perfectly straight up into the air for a good meter, and then started to dissipate into the air. I know that a gas will disperse into the air, but I was slightly puzzled because the smoke is a solid. My question is if there was a room with perfectly still air, would the smoke rise straight up all the way to the ceiling, or would it behave more like a gas and dissipate into the air regardless? Also, what properties would explain this?
Thanks!
CR

Welcome to PF!

This is actually an interesting demonstration of laminar vs turbulent flow. Most fluid flow streams (the smoke is a fluid -- it's air and exhaust gases that you can see because of suspended ash particles in it) start of laminar (linear/coherent) and after a while transition to turbulent. The distance until transition is actually highly predictable and depends on velocity, size/length of the flow stream, and the properties of the fluid. It is also an important part of our understanding of how air behaves as it flows over a wing.

Here's more on the phenomena:
http://profs.sci.univr.it/~zuccher/research/blstability/

Here's the number/equation that characterizes the behavior of a flow stream:
http://en.wikipedia.org/wiki/Reynolds_number

Very interesting, thank you very much!

## 1. What causes smoke to rise in still air?

Smoke rises in still air due to a process called convection. As smoke is produced from a fire, it is warmer and less dense than the surrounding air. This causes it to rise and create a current of air known as a convection current.

## 2. Why does smoke rise in a straight column?

The smoke rises in a straight column due to the lack of wind or air currents to disturb its path. In still air, the convection current created by the rising smoke remains relatively undisturbed, allowing it to rise in a straight line.

## 3. Can smoke rise in any direction in still air?

No, smoke will typically rise in a vertical direction in still air due to the convection current created by the heat from the fire. However, in certain conditions such as high temperatures or strong winds, the smoke may be pushed and dispersed in other directions.

## 4. Why does the smoke stop rising at a certain height?

Smoke stops rising at a certain height due to the change in temperature and density of the surrounding air. As the smoke rises, it cools down and becomes closer in temperature and density to the surrounding air, causing the convection current to weaken and eventually stop.

## 5. Can the physics of smoke rising in still air be applied to other substances?

Yes, the physics behind smoke rising in still air can be applied to other substances such as hot air balloons. In this case, the hot air from the balloon is less dense than the surrounding air, causing it to rise and carry the balloon upwards.

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