I was working out this morning. There were a few times where I stopped to catch my breath, and as I bent over and breathed, I could feel my breath on my skin and it felt cool on my skin. I tried Googling it, but I’m still not sure exactly why this is. I read that our breath is slightly cooler than our body temperature, but not by much – only by about three degrees. Exercising can raise your body temperature, but I would think it would raise the temperature of your breath.
Also, if your breath is 94 degrees or so, and you’re working out in a 70 degree room, wouldn’t your breath feel warmer than the air in the room? Is it just because I was breathing hard, which made my breath feel more like a breeze than just static air?
It feels cool because the breath makes the air move faster which whisks heat away from the body faster than still air can. Plus not all the air that is moving is from your breath, it pushes a lot of other air along with it, I wager that after a few inches the wind is not much warmer than the surrounding air. When it’s up close it is probably closer to body temperature but the wind is a lot faster.
The air you inhale is almost always at something less than 100% relative humidity, but the air you exhale is always at something very close to 100% relative humidity. There’s a lot of evaporation taking place in your respiratory tract, which results in a lot of evaporative cooling. So even though your body is heating that air by direct convection as the air flows across warm respiratory tissues, evaporative cooling is having an opposing effect. If you are breathing at a rapid rate, you don’t give much time for convective warming of the air, so the air you exhale won’t be quite up to body temperature. If you take a breath and hold it for 30 seconds, you will saturate that air with moisture, ending the evaporative cooling effect and giving time for convective heating to have its way with things. If you measure the temperature of that exhaled air, you’ll find it’s at or close to your body temp.
Air movement is part of it. If you’re in a room full of stagnant 70-degree air, and you’re smoldering hot because you’ve been working out, a 94-degree breeze might cool you better than 70-degree stillness.
The other issue is entrainment of ambient cool air by the jet of warm air coming from your mouth. If you want to remove entrainment from the situation, position your mouth very close to your skin; the jet of air hitting your skin won’t feel nearly as cool, and may even feel warm. This is perhaps most noticeable on a cold winter day: if you want to warm your hands/fingers, you don’t blow on them from 12 inches away, you put them right in front of your mouth.
OK, so outside your mouth, you’ve got 14.7 psi. Inside your mouth, you’ve got 16.1 psi. If you blow as hard as you absolutely can, adiabatic cooling could could produce a temperature drop of about 14.3 degrees. Of course, the problem is that when you initially pressurize your respiratory tract (RT) to commence blowing air at high speed, adiabatic heating will cause the temperature to rise by the same amount. So the sequence goes like this:
-inhale lungful of air
-body warms air to X degrees F
-purse your lips and blow HARD, raising RT pressure to 16.1 psi
-adiabatic heating causes air in RT to increase in temp to (X+14.3) degrees F
-air passes across pursed lips, pressure decreases to 14.7 psi
-adiabatic cooling causes just-exhaled air to decrease in temp to X degrees F
If the air in your RT was initially at 94F (per the OP), then it’s conceivable that the initial pressure increase could adiabatically heat it to above body temp, at which point it would begin to cool by convection across RT tissues. Then you could end up exhaling air that is a bit cooler than you would at lower blowing speed. However, this would require a large RT pressure to generate the large adiabatic temperature increase that would be needed in order to facilitate meaningful convective cooling of the air in your RT. It would also require adequate residence time in your RT for that convective cooling to take place. In other words, you’d need to be blowing like a world-class trumpet player, and you’d need to keep it up for a bit. The perceived cooling effects being discussed in this thread are occurring at much lower pressures and can be felt immediately after blowing commences.
Under the circumstances I described - that is, uncomfortably high respiratory tract pressures sustained for a significant length of time - yes, you might get a slightly reduced exhaled-air temperature due to adiabatic cooling, and it might even be measurable if you can somehow eliminate entrainment of ambient air.
Under the circumstances posited by the OP - very modest respiratory tract pressures sustained for brief intervals to blow a jet of exhaled air through cooler ambient air against warm, possibly sweaty skin - adiabatic cooling is an insignificant contributor to the perceived cooling effect, as scr4 stated.
If you were sweating, even just a little (Sometimes it’s not always aparent that you are sweating. Especially on you torso), that would make a considerable difference.
Lick the back of your hand and blow on it, and see if that’s the kind of “coolness” you felt.
Thanks for the explanations! I’ve been setting here at my desk trying to blow on my arms very quietly and trying not to get caught so I won’t have to explain what I’m doing. I tested out what Machine Elf said about your air getting cooler as it’s further from your body, but I just trusted you about the holding your breath because I didn’t feel like doing it myself! What’s also interesting to think about is that the only time I could ever even feel my breath that far away from my mouth is when I’m breathing particularly hard, so my breath probably normally couldn’t feel that cool! Another thing I wonder about is what you’re talking about with the humidity. I know that air is humidified when you breathe it in through your nose, but when you’re breathing through your mouth you don’t get that effect. When you’re breathing hard, you’re using both your nose and your mouth to draw in and expel air, so I wonder if that is also enhancing the evaporative cooling effect.
Terminus Est, you’re right as well. I was pursing my lips when I was breathing hard earlier, though I had to Google why pursing your lips makes the air cooler! Though I do still wonder why I was instinctively pursing my lips. A quick Google search says that it’s a way to control shortness of breath, but I can feel myself going down this rabbit hole. Does anyone know why pursing your lips helps control shortness of breath? Machine Elf, you were good with the last question, maybe you can answer this one, too?
**Grrr! **I never would have thought of that, but you’re absolutely right! It was extremely humid this morning (even though it was only 6 AM!), and I was drenched in sweat. I licked my hand to test your theory, and you’re right, that makes a big difference.
Agreed, I’m referencing static pressure. Maintaining such a high pressure while blowing requires pursing your lips tightly to severely limit the flow rate. A small but non-zero flow rate should still allow a person to produce something very close to this static pressure.
However…
This is what I’ve been saying.
If you’ll purchase an anemometer and ship it to The wind of my soul, she can report back with airspeed measurements under the circumstances about which she was inquiring in her OP. From that, we’ll be able to back-calculate respiratory tract pressure, and estimate an absolute upper bound for adiabatic temperature drop.
Absent any real data obtained thusly, I’ll claim that we’re probably talking about a pressure of just a few inches of water, resulting in a potential temperature change of perhaps 1-2 degrees F. And I will stand by my claim that this temperature drop is largely canceled out by the earlier temperature increase that was incurred when pressurizing the respiratory tract to begin blowing.
Do let me know if you think my claims are in error.
If you’re looking for a demonstration rather than a claim, try this:
Position the back of your hand six inches from your mouth, and blow. Assuming you’re in a room that’s about 70F, you will observe a cooling effect.
Now continue blowing at the same rate, and gradually move your hand closer to your mouth until until your lips are almost touching the back of your hand. Your hand should actually feel slightly warm doing this. With your lips this close to your hand, you’ve eliminated entrainment of cool, dry ambient air, and now all of the air impinging on that one spot is coming from your lips - nice, humid, warm air that transfers heat to the back of your hand and eliminates the evaporative cooling effect from any sweat that may be present. This demonstrates that entrainment of cool, dry ambient air is the dominant factor in the cooling effect observed under the typical circumstances in which one blows on one’s skin.
High enough to force the air from the inside of my mouth to the outside of my mouth. When the air is outside my mouth it expands and adiabatic cooling occurs.
I just tried this online calculator that calculates air flow through an orifice for a given pressure difference. When I plugged in 8mm orifice diameter and 1/1000 atmosphere for the pressure difference, I got about 0.46 liter / second. Which sounds about right for flow rate for blowing on a hot coffee. So that should result in no more than 0.3C temperature drop.
But that’s assuming that the air comes to thermal equilibrium while it is compressed. I’m not sure I buy that assumption. If you inhale, pause, then exhale, the air probably comes to thermal equilibrium with the lung at neutral pressure, then gets adiabatically compressed before the adiabatic expansion. So you’re just heating the air above the body temperature (by ~0.3 deg) and then cooling it back down again. As Machine Elf already said.