Sciencemadness Discussion Board

Smoke blowing in the wind

DDTea - 20-10-2009 at 10:54

Close your eyes and set your minds eye onto a fire, a distant fire, and the subtle, suggestive movements it makes. Notice the smoke rising up, expanding, playing in the wind. If your concentration wanes, imagine the sound of one hand clapping...

Although this is a peaceful thought, I don't mean to open a thread about pyromancing. I'd like to talk about the physical behavior of smoke. There are numerous descriptions of it depending on the angle you're approaching from: environmental chemists might be interested in the enrichment of metal particles carried in smoke, toxicologists might be concerned about the harmful effects of smoke, while a stoner might simply want to the immediate area of he and his friends with smoke.

One thing that I have always enjoyed doing when smoking (bad habit, trying to quit) is observing the behavior of the smoke coming off of the cigarette and trying to use that to gauge the weather (with limited success). In calm, windless weather though, my understanding is that smoke rises adiabatically from its origin/heat source. As it cools, it expands. The intensity of the original heat source determines how high the smoke will rise (which I'm sure can be described using the zeroth law or the second law of Thermodynamics).

I've noticed that some particles carry in the air for long distances, while others rise and fall somewhat, before "playing in the wind" a bit more. Is this simply due to particle size? What role does Brownian motion play? Is there a way to describe the smoke using the Boltzmann Equation? :P

I can't be the only smoking chemist who thinks this way... I'd like to have a conversation about smoke movements, and perhaps what they can tell us about local meteorological conditions.

psychokinetic - 20-10-2009 at 11:35

I find smoke amazing too, and I don't smoke!
I too find it amazing how far things travel, and I'm inclined to agree with you on the size of the particles, but maybe also the concentration has a part to play.

It's solidy and gassy, and considering it often has water in it - wet too :D

JohnWW - 20-10-2009 at 14:40

Being a finely-divided product (soot, composed of carbon, aromatic polyenes, and buckminsterfullerene) of incomplete combustion (or of complete combustion of an inflammable substance containing non-combustible ashy substances), smoke rises because it is entrained by hot CO2 and CO formed by combustion, and heated N2/Ar and any O2 left over from combustion, which have (because of their temperature) a lower density than the surrounding air.

DDTea - 20-10-2009 at 15:57

Quote: Originally posted by JohnWW  
...smoke rises because it is entrained by hot CO2 and CO formed by combustion, and heated N2/Ar and any O2 left over from combustion, which have (because of their temperature) a lower density than the surrounding air.

So the initial movement of the smoke has to do with adiabatic expansion of the gasses the particles are entrained in...that's a bit of important clarification.

I don't remember the derivation of adiabatic expansion off hand, but the gist is that as the gas expands, it cools down and in turn, its density increases relative to that of air, and the particles rise at an ever-decreasing rate. Now if I recall correctly, particles of about 10 microns stay in the air "indefinitely"--at least until some outside force brings them down (e.g. rain). Larger particles, then, should eventually fall out due to gravity. However, more important forces like the wind initially prevent them from falling straight down...

So then: using the zeroth law (specifically, the rate of heating is proportional to the difference in internal energy...kind of like Newton's Law of Cooling--which can, in fact, be derived from the zeroth law), the rate of cooling of the carrier gasses should be slower in warm weather and faster in cool weather. Smoke then, should rise to a higher altitude in warm weather than in cool weather because the rate of cooling (proportional to difference in temperature between the carrier gas and the atmosphere) is slower. I feel like I'm on to a new way to use a cigarette: as a thermometer!

hodges - 22-10-2009 at 15:28

Yes a bit off topic, but related to smoke. Here is an interesting story about how someone's smoking habit lead to a major discovery in physics.


psychokinetic - 22-10-2009 at 21:19

Chemists aren't meant to be healthy anyway :P

E-tech - 24-10-2009 at 21:09

Observing larger amounts of smoke can help you to read atmospheric conditions, such as inversion layers- which turned out to have a big influence in chemical weapons usage.

Aurus - 25-10-2009 at 11:06

Smoking and meteorology, interesting conjunction. I think the differences in height, although quantitative, must be too small to be noticeable, particularly as the smoke is not visible after some distance.