Do smells travel farther in colder or hotter temperatures?

[pines-demon]

I have a not-so-well based intuition about this one, I think that in general odors travel farther in winter than in summer. I can smell the close-by bakery more intensively when is cold than when is hot. This might be a variety of reasons (like there being more smells to sense in summer) but I was thinking that molecules travelled father without colliding.

I have two ways to think about this that give me the opposite of my original intuition.

1) Brownian motion: per the usual formula the distance travelled of a particle in random field is $\langle x^2\rangle=2Dt$ where $D$ is the diffusion constant (proportional to temperature $T$). So this results says that in hotter environment a particle travels farther than in cold environment in a time $t$. However this is not the same that I wanted to calculate as it consider a single particle and it implies that there are many collisions moving the particle, I wanted to know how much a packet of particles can travel before collisions so:

2) Mean free path: the mean free path of a gas is proportional to temperature and inversely proportional to pressure. Meaning again that the hotter the environment, the father the particle travels. However I do not know how to account for pressure here as pressure lowers in colder environments.

So which is it? Do smells travel faster in cold environments or is it the opposite?

Note: I am not talking about volatility (how easily something emits some molecule), this clearly increases with temperature as things get rotten or evaporate more easily. I am talking of travel distance as a noticeable chemical signature before diffusing into the background.

 

[DaveC426913]

Presumably, the cold air will cool the otherwise warm exhaust from a building, causing it to propagate closer to the ground rather than rising into the sky and dissipating.

Not the best example, but it makes the point:

 

[berkeman]

Do smells travel faster in cold environments or is it the opposite?

Based on my hunting experience of a number of years, drafts make the biggest difference in scent transport. So lacking any obvious wind, you will get drafts of air that depend on the air temperature and the terrain. In the mornings, colder air tends to travel slowly downhill, and in the warmer afternoons, drafts tend to rise uphill.

 

[jedishrfu]

You could investigate odor dispersion:

https://www.biocycle.net/odor-dispersion-fundamentals/?utm_source=chatgpt.com

 

[DaveE]

In practice, odor transport is dominated by wind, not diffusion. Diffusion is very effective for short distances, but not effective at long distances. This is why you need so many capillaries in your tissues to deliver oxygen and why people are so sensitive to pulmonary edema with just a little swelling at alveoli.

Wind is complicated and virtually ever present. When we think there isn't much wind, it is often present and generated by temperature differences in the environment, or HVAC and such. Like the sun heating the ground, etc. Nearly every indoor space, where you think there isn't wind, has air movement that is much more important than diffusion. Air movement is almost always complex. You can see it with smoke generators, much more than you can feel it. There are always eddies and such except in the most pristine situations.

Weather in general creates other significant confounding features that can be significant, things like humidity or the temperature of the air at your scent receptors (except dogs, which have cool physiology to deal with that). Many claim that rapid barometric pressure changes are important, which I personally doubt. This creates an analytical problem IRL that often defies simple theories or confident conclusions.

Your question is complex in part because it's a complex system that ranges from odor source characteristics through olfactory processing in your brain. For example, search dogs that practice mostly in cold humid weather don't do well in hot dry weather, and vice-versa. Because they learn to work with the environment they are exposed to.

Detection, search, and tracking dog trainers really care a lot about this, but honestly, the practical results of science are often too difficult to be useful. People have written books about this, but they tend to be observational in nature, not too theoretical. Detection dog handlers care a lot about wind, but that doesn't mean they understand it in a particular scenario.

For me, as a sport detection dog instructor, trainer, and handler, the scariest weather is cold with truly, absolutely, no wind. In these cases the odor just doesn't move much away from the source and and you have to get very close for the dog to smell it. This is diffusion only, which just doesn't work well. It's rare, I've experienced that only 2 or 3 times in 15 years of working dogs frequently.

PS: Diffusion is important at most odor sources. It is what makes the odor move into the nearby airstream. In practice, this has more to do with how much odor is emitted than where it goes.

 

[A.T.]

Presumably, the cold air will cool the otherwise warm exhaust from a building, causing it to propagate closer to the ground rather than rising into the sky and dissipating.

Buoyancy depends on the density and thus temperature difference. The same warm exhaust will raise faster in colder surrounding still air.

 

[DaveC426913]

Buoyancy depends on the density and thus temperature difference. The same warm exhaust will raise faster in colder surrounding still air.

What I'm saying is that the emitted air rapidly cools to ambient temperature (it's a relatively small mass with little heat capacity ). The mass ceases to rise and the particulates hang low on the ground.

 

[A.T.]

What I'm saying is that the emitted air rapidly cools to ambient temperature ...

Just because the initial rate of cooling is greater in colder atmosphere, doesn't mean the exhaust reaches the ambient temperature in shorter time than in warmer atmosphere. The temperature change needed to reach the ambient temperature is different too. Also, the initially much greater temperature difference in winter also means, that it initially raises much faster.

Not the best example, but it makes the point:

View attachment 354503

It has nothing to do with your point. There is obviously strong wind right to left and turbulence caused by the house.

 

[pines-demon]

If odor transport depends mostly on wind and on the gas ability to rise or not, then is it possible that odors diffuse less due to temperature and thus a large concentration can travel more distance with the wind than when the air is hotter and is be less odor concentration due to diffusion?