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Why does the air we blow/exhale out from our mouths change from hot to cold depending on the size of the opening we make with our mouth?

It's not just a subtle difference, but significant in my opinion. I'm inclined to discredit the notion that it's just a matter of speed because I can blow fast with an open mouth and still, it's hot; and blow slow with an almost closed (tighter) mouth and again, it's cold.

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It is mainly due to air entrainment.

If you blow through a tight mouth, there is smaller volume of air but a higher velocity. This pulls in and mixes with a lot of ambient air (Venturi or Bernoulli effect) - in fact typically the air stream is only 40% body warmth and 60% ambient so it will be markedly colder. As an experiment you pucker and blow through a tube held to your mouth, this excludes the ambient air and you will get reduced airflow but at the higher temperature again.

With a wide mouth there is hardly any air entrainment. Its temperature will be almost same as its temperature in your lungs which is higher than the ambient temperature.

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  • $\begingroup$ How much pressure do you think you have in your lungs? $\endgroup$
    – Mark Eichenlaub
    Commented Apr 1, 2011 at 2:49
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    $\begingroup$ Pardon my ignorance, but I'm not following. I need a layman translation :) And you said "temperature decreases during the exhale with a big opening of mouth". In fact, the opposite is happening, no? The air is hot/warm when you exhale with an wide open mouth. $\endgroup$
    – John
    Commented Apr 1, 2011 at 2:52
  • $\begingroup$ +1 if you can put in some realistic numbers and predict a measurable effect :-) $\endgroup$
    – Sklivvz
    Commented Apr 3, 2011 at 15:57
  • $\begingroup$ @Georg: Yes, after some real thinking I came to the same conclusion as yours. I eliminated other "reasons". $\endgroup$
    – user1355
    Commented Apr 3, 2011 at 16:36
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    $\begingroup$ @Sklivvz I tried to predict a measurable effect on your request. You can try it as well :) I eliminated all other possibilities after thinking carefully. $\endgroup$
    – user1355
    Commented Apr 3, 2011 at 16:38
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I don't think it's quite that physical. The pressures involved aren't that high

When you breathe slowly on your hand the air is war, moist and 37C so feels warm compared to the surroundings, if you blow through a small opening the flow of air increases the cooling and evaporation from your skin.

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  • $\begingroup$ That's what I thought, but if you look at physics.stackexchange.com/q/2111 you see that lungs can create pressure a significant fraction of one atmosphere. $\endgroup$
    – Mark Eichenlaub
    Commented Apr 1, 2011 at 3:27
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    $\begingroup$ Easy to test, blow on a thermometer and see if it cools below ambient $\endgroup$
    – Martin Beckett
    Commented Apr 1, 2011 at 4:40
  • $\begingroup$ Cool experiment for kids, wet thermometers, dry thermometers, blowing different ways, etc. $\endgroup$
    – sigoldberg1
    Commented Apr 4, 2011 at 3:01
  • $\begingroup$ if the ambient temperature is below body temperature, then breath can't cool a dry bulb below ambient temperature. Skin can be cooled below body temperature (and thus feel cool) by moving ambient air. $\endgroup$
    – kevin cline
    Commented Apr 5, 2011 at 20:30
  • $\begingroup$ Yes - I was countering the argument that your breathe is significantly cooled by the adiabatic expansion as it leaves your mouth - compared to the cooling effect of an airflow $\endgroup$
    – Martin Beckett
    Commented Apr 5, 2011 at 20:56
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I think most reasons given above are false. It is our common experience that cold air comes when we exhale through a narrow opening of mouth. (Experience it yourself). It is due to adiabatic expansion of air. When a gas is allowed to expand suddenly, it does so by absorbing heat energy. When air is suddenly exhaled out into a larger volume through the narrow opening, air undergoes adiabatic expansion. When we place our hand near the out flowing air heat energy is being absorbed from our hand. Hence we feel cold. Opposite is in the case when a gas is compressed. Heat energy is liberated. (try this by tightly closing your mouth with your hand and exhaling air out into a small volume compressing it. You feel that the air is hot). So the exact reason is the adiabatic expansion or compression according to the size of the mouth.I wonder if the case is different in any other cold countries. In India it is as explained above.

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The total sense of hot or cold is set by the integration of the response of all the sensors in the skin, +- evenly distributed, involved in the experiment.

The sense of hot is proportional to the area and the sense of cold is proportional to the perimeter*$\delta r$ , for the same pressure (see the answer of user1355).
The outcome of the experiment can be described by the ratio hot/cold or _(area)/(perimeter*$\delta r$)_ $=\pi R^2/2 \delta r \pi R=R/2\delta r$, i.e proportional to $R$ .
Let Experiment open mouth have radius $10 R$ and Experiment almost closed mouth have radius $R$.
The ratio of the outcomes of the two experiments (open/closed) is 10 .
If someone has doubts, and I do have, that the perimeter can contribute significantly to a colder sensation than the overall outcome becames more contrasted (ratio of areas, proportional to $R^2$) and the above example will give the value 100.
An array of thermometers can set the question, imo. One measure can be defined as the sum of all the temperatures.

The description in the question is correct, supported by experimentation (I did it ;), and have a physical and rational interpretation.

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  • $\begingroup$ The sense of hot is proportional to the area and the sense of cold is proportional to the perimeter*δr , for the same pressure (see the answer of user1355). please cite another source as this one is gone. $\endgroup$
    – Eph
    Commented Feb 3, 2015 at 19:32

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