Blowing air out your mouth question.

Why is it that when you breath out, with your mouth wide open, the air is hot. But when you blow out with your lips making a whistle shape, the air is cooler?

I try to control my breathing so that I am making the same exact breath for each way. Same depth and same intensity of exhale. It always turns out as different temps.

I have some theories:

The wide-open-mouth breath has moisture coming straight out while the semi-closed-mouth causes the mouth lining to soak up moisture (and the heat in the moisture) or,

the wide-open-mouth breath does not linger in the mouth long enough to cool down.

These are some cute little theories, but I would love to know the actual technical reason!


This question was asked exactly one month ago, see the details here. Billy’s second post hit the nail on the head, whilst most of the other posts were arguing about factors that had very little to do with the question.

If you don’t want to wade through all this, here’s the excerpt that answers your question:

"*Here is what is happening:
Air coming from mouth is same temperature (within the accuracy of my test setup ~.1 degreeC) no matter what you are doing with your lips or how hard you blow.

The air hitting your hand is drastically different temperature depending on what you do with your lips.

When blowing hard through a small hole in your lips, the high speed air creates lots of trubulence, mixes with room air, and the mixure hits your hand about 1 degree C higher than room temp.

When blowing with your mounth wide open, there is little mixing going on so the warm air makes it to your hand relatively pure.

It’s a dimension of opening vs distance to detector thing*".

That’s answers’ good enough for me. Thanks mike.

I’m still not convinced that that’s the right answer, but I don’t have anything to say new since that thread.

I’m convinced. Try this admittedly unscientific experiment:

Purse your lips tightly and blow hard on the back of your hand from about two inches away. It feels cool, right? While blowing, move your hand towards your mouth until it eventually touches your lips. As you get closer, the breath feels warmer since room air has had increasingly less time to mix with your breath. If there were any significant cooling due to expansion of compressed air, you would expect it to be cooler the closer you got, but that’s not the case.

Yeah… I guess so… does that explain why air from a fan feels cold, though? It’s already room temperature, so the mixing with air should have no effect, right?

There is some evaporative cooling as well, which is what the fan is doing. The air from a fan blowing on you feels cooler because it causes sweat to evaporate faster, carrying heat away with it.

Another thought, which may have been covered in the previous thread.

When you are breathing out slowly, the breath is more likely to retain more of the heat it picked up from within the persons body.

I meant to add:

From doing some ‘tests’ myself (ie blowing on my hand in various ways, as we have all been doing no doubt) I find that the the temperature of the outgoing breath is similarly warm wether blown out slow or fast.

More likely, people doing this test will naturally hold their hands closer to their mouths when checking the temp of the light breath (to measure it before it disperses), and further away when blowing out hard.

At which point I refer to my previous post.

Added to this, the evaporation.

The faster the air hits your hand, the more effective the evaporation - and your hand doesnt need to be sweaty or covered in water, it just makes the effect stronger.

And why is that?

Kindly disregard my last post. Thank you.

Which bit are you referring to ?

I was referring to your first post, but your second post explained it, and it showed up after I picked Submit. Sorry for the confusion.