Seeing your breath

[PhDwannabe]

OK, ignore the magical space fairies. Also, apologies: this may be posted in the wrong area, but I'll be damned if I can find the right subforum for it.

 

So, I was walking to a meeting early this morning and, while crossing a busy street to walk onto campus, I noticed something that I think I've seen before and forgotten about. On a cold and bright morning like this, my breath is visible. I have a reasonably informed understanding of why that is.

 

Here's the thing, though: when crossing the street, it is much more visible. Dramatically so. I recalled that I've seen this in parking lots, around major streets, and anywhere around running cars.

 

So, what is it? I imagine it has to be a function of car exhaust--is the humidity just very locally higher? Is it a matter of local density of particulate matter, which provides for greater opportunity for condensation in the air? What are your guys' thoughts?

[zapatos]

Possibly the dark background of asphalt streets and parking lots makes it more visible to you.

[Blahah]

Perhaps it's just that away from the buildings the air is colder. I'd be interested if there is a real effect of cars' emissions though.

[imatfaal]

I would go along with Zapatos - perhaps you are also getting mixed up with car exhaust once in a while, so you tend not to notice where your breath ends and exhaust starts.

 

Would need help here; but would the black of the asphalt absorb more heat than the more reflective lighter colour paving stones? This might leave the areas of air above roads as cooler and thus more likely to cause condensation (I think there is something wrong here)

 

 

[SMF]

Dark areas that absorb more light get warmer and thereby conduct more heat to the air, and radiate more IR which is partially transferred to the air via greenhouse gasses, so that air above should be warmer. Lighter areas reflect more light and absorb less, so the air above should be cooler. SM

[PhDwannabe]

I swear I've seen this effect even without a black background (which, as I'm reading, could provide two very distinct effects: heat differences, and visual contrast differences), and running cars is the important difference.

 

Isn't there some precedent for my best (uninformed) guess?:

Is it a matter of local density of particulate matter, which provides for greater opportunity for condensation in the air?

Don't clouds form more readily when there are a lot of nucleation sites available from the particulates in air pollution? Couldn't this be a very similar effect?

[Mr Skeptic]

The visible part of the breath is of course water droplets formed by condensation of hot humid air from your lungs in the cold air. As to what would make it more visible: a dark background and proper lighting. The dark background will make a sharper contrast when light reflects/refracts/disperses from the droplets. Try playing around with a flashlight to see what effects extra lighting would have.

[PhDwannabe]

The visible part of the breath is of course water droplets formed by condensation of hot humid air from your lungs in the cold air. As to what would make it more visible: a dark background and proper lighting. The dark background will make a sharper contrast when light reflects/refracts/disperses from the droplets. Try playing around with a flashlight to see what effects extra lighting would have.

 

Again, the quality and direction of the light is certainly very important, but I feel like I've observed the effect fairly readily in a lot of different lighting and background contrast conditions, and there still seems to be a main effect difference driven by running cars.

 

The street I was talking about crossing, for instance, is the same color as the side street I walk down to get to it (very gray, not blacktopped), so the surface remains constant--the difference is traffic.

[imatfaal]

With respect PHDwannabe; it's time to strut your empirical stuff, and make some observations or even better get others to do it. I look forward to the publication of the results

[Greippi]

The street I was talking about crossing, for instance, is the same color as the side street I walk down to get to it (very gray, not blacktopped), so the surface remains constant--the difference is traffic.

 

Are they also the same width/direction? I really do think it's to do with lighting - larger space = more light.

[PhDwannabe]

I look forward to the publication of the results

 

Heh. May be a little out of my area, man. Psych is a ways away from... what the hell is this, anyway? Fluid mechanics?

 

Are they also the same width/direction? I really do think it's to do with lighting - larger space = more light.

The lighting is similar; I was paying attention to that today. Again, I strongly feel that I've observed this effect in a variety of different lighting and background conditions. (As somebody in psych, believe me, I'm well-aware of confirmation biases and other cognitive errors that interfere horribly with the process of drawing conclusions from these sorts of uncontrolled observations. So, while I strongly suspect something related to car exhaust is going on, I'm by no means screaming about how I know what's what.)

 

Again: particulate matter. Nucleation sites. Nobody's gone there yet. Isn't that plausible?