Blowing hot air

[Diagoras]

How come I can blow either warm or cool air? It feels like I'm moving something in my throat when I change from one to the other, but what the heck am I doing and how is that affecting the temperature of the air coming out of my lips? Does anybody here know?

[Madalch]

Have you actually checked the temperature of the air that you're blowing?

[Professor Yaffle]

Are you blowing with pursed lips to get cool breath, and breathing with a more open mouth to get warm breath?

If so, see here for some possible answers:

http://www.telegraph.co.uk/earth/main.jhtml?xml=/earth/exclusions/sciencequestions/scihar.xml

[Loss Leader]

How come I can blow either warm or cool air? It feels like I'm moving something in my throat when I change from one to the other, but what the heck am I doing and how is that affecting the temperature of the air coming out of my lips? Does anybody here know?


You're Superman.

[Ivor the Engineer]

How many people are now hurring and blowing at their hand to see if the OP is correct? (Just me, ok:o)

My theory is it's to do with the the speed and water content of your breath.

[Doctor Evil]

Here is my, possibly over simplified, explanation:

I would guess it is related to pressure differences. The air in your mouth is naturally hotter then the air in the room. If you blow slowly you will easily notice that.

However, if you build up pressure by narrowing your lips, the air you will blow will have a large change in pressure. Its volume will expand and it will get cooler. One needs to do an estimation to see if this effect is strong enough to explain your observation, but I would guess that it is.

[Dilb]

Here is my, possibly over simplified, explanation:

I would guess it is related to pressure differences. The air in your mouth is naturally hotter then the air in the room. If you blow slowly you will easily notice that.

However, if you build up pressure by narrowing your lips, the air you will blow will have a large change in pressure. Its volume will expand and it will get cooler. One needs to do an estimation to see if this effect is strong enough to explain your observation, but I would guess that it is.

Nope. The greatest possible temperature drop would happen if you have an isentropic expansion of the air in your lungs. For the ~5% difference in pressure that human lungs can create, this would cause a temperature drop of about 5C, so the air would be warmer than normal except on rather hot days (31C or greater).

[Doctor Evil]

Nope. The greatest possible temperature drop would happen if you have an isentropic expansion of the air in your lungs. For the ~5% difference in pressure that human lungs can create, this would cause a temperature drop of about 5C, so the air would be warmer than normal except on rather hot days (31C or greater).


Interesting. I would have guessed that I could create a much larger pressure difference than that. I would also guess that the expansion is best approximated as an adiabatic one.

[blutoski]

It's not expansion - it's rate of airflow over your skin. It's the skin that has different temperatures - not the air that's exhaled.

Our skin is moist, and evaporation draws heat from the skin. Faster airflow means faster evaporation, which means cooler surface.

As an experiment, try the same trick in a steam sauna - you can actually burn somebody with rapidly exhaled breath, because the air is fully saturated and evaporation cannot take place. The friction of moving air heats up the target skin instead of cooling it down.

[Diagoras]

It's not expansion - it's rate of airflow over your skin. It's the skin that has different temperatures - not the air that's exhaled.

Our skin is moist, and evaporation draws heat from the skin. Faster airflow means faster evaporation, which means cooler surface.

As an experiment, try the same trick in a steam sauna - you can actually burn somebody with rapidly exhaled breath, because the air is fully saturated and evaporation cannot take place. The friction of moving air heats up the target skin instead of cooling it down.

That makes a lot of sense. Thanks blutoski!

[XBoxWarrior]

As an experiment, try the same trick in a steam sauna - you can actually burn somebody with rapidly exhaled breath, because the air is fully saturated and evaporation cannot take place. The friction of moving air heats up the target skin instead of cooling it down.

Have you actually tested this?

Was it a coed sauna?

Did the other dude kick your #$* outta said sauna?

;)

p.s. On second thought, this could be a great topic for Playboy Forum...the benefits for the art of fellatio are numerous......

[Complexity]

Well, you see, you've got a colony of Maxwell's Daemons in your throat...

[Loss Leader]

I notice a distinct difference in my diaphragm position between the hot/cold air blowing. This lends evidence that pressure differences play a part.

[Dilb]

Interesting. I would have guessed that I could create a much larger pressure difference than that. I would also guess that the expansion is best approximated as an adiabatic one.

The isentropic expansion I'm giving the value for would be adiabatic, but you can also have an irreversible adiabatic free expansion, which probably would be a better model. But then you only get heating due to the Joule-Thompson effect, which is really small. As a first approximation, for example an ideal gas, it would be exactly zero.

In an isentropic expansion, the air does work against something, which means it loses thermal energy and drops in temperature. In a free expansion, it doesn't do any work, so the gas can only lose thermal energy if it can make some internal change.