Science Questions

How can you blow hot and cold?

Sun, 26th Feb 2012

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Shelley Woolf asked:

Hi Chris


My ten year old son Joshua Woolf asked me why the air that you breathe out when you blow out candles is cold as opposed to the arm air that you breathe out when you want to mist a window. I explained that the warm air comes from inside your body and the colder air is air that is being used from in front of your mouth but I'm not sure that this is correct. If I breathe in through my nose and hold it and then breathe out my mouth that air is cold or hot depending on how I breathe it out. Please clarify for us.



Shelley Woolf


Dave -   This is a really interesting one.  We did this as a Kitchen Science a couple of years ago.  If you blow with very pursed lips very quickly, it feels cold and then if you blow with an open mouth slowly on your hand, it feels warm. 

This is all to do with what's happening with the the air.  If you're blowing through very narrow pursed lips, the air is going very quickly and it tends to mix in very strongly with the air around it.  So, what's hitting your hand is mostly air from the room, itís moving quite quickly and quick moving air tends to feel cold because it moves heat away from your hand quicker.  So mostly, what's hitting your hand is air from the room. Whereas if you breathe slowly with your mouth open, that's a much smoother jet of air, so itís a much wider jet of air, it doesnít mix in nearly as quickly.  So what hits your hand is mostly air from your lungs which is warm.

You can actually get the warm effect from the fast moving air by putting your finger right up close to your mouth when you blow, it still feels quite warm and it also gets quite damp. That's because it doesnít have time to mix in with the air around it, so itís still warm and you can feel it.


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I think there are two effects.

The "jet" may not be mixing as much, as also picking up surrounding air and pulling it with it.  Would the venturi effect play a role?
The slow breath essentially displaces ambient air with the air from your breath, which presumably is warmer.

The second effect would be the wind-chill effect.  Assuming the air hitting your hand is cooler than the hand, then it the "jet" method will quickly wick away the heat like one would get with high wind speed and wind-chill.  The slow, method, on the other hand, surrounds your hand with air in a warmer than ambient temperature, and thus slowing down the transfer of heat to the ambient air. CliffordK, Thu, 1st Mar 2012

I also asked this a while ago now: Chemistry4me, Thu, 1st Mar 2012

Great question :). Most of the air previously in the vicinity of where you are about to blow gets moved. One does however have to perform an experiment in which you blow air at the same velocity with a bigger stream circumference. You can roughly gauge the velocity of the air by measuring the time it takes to hit your hand. What you will probably find is that the air is of different temperature in the two experiments. My explanation for this is that the new air from your lungs is filtered through your mouth at a high speed with a thin point of exit but ends up diverging quickly in the more closed mouth experiment. This quick divergence is probably the reason for the cooling as the further apart the atoms, the cooler the respective gas. This is also why when we put our hand close to the blowing close lips the air is warmer as it hasn't had a great chance to diverge and the air atoms are still more packed together. Most of the air in the vicinity will probably be your air at first but will then be replaced as it quickly diverges and spreads. The replacing air will probably be slightly warm due to the atoms crashing in from all directions.

The example in which we do same velocity air but open mouthed blowing sees some divergence of air but nowhere as much as in the close lip example. This explains the lesser temperature change. This is because the distance we hold our hand divided by the size of the originating air stream has a significant impact on how warm the air that hits our hand is going to be. Another element is that a small thin stream is more aerodynamic than a big one and can reach your hand more easily so there will be more actual air from your lungs in the close lip example and more relative time for the stream to diverge.

There are more elements to this but I won't regard them now as they are probably not that necessary to the understanding of this system.

This close together lip example, to my understanding, is a similar effect to an atomiser on a spray bottle. This is also why when you put hot water in a spray bottle it comes out cooler, or how your deodorant body spray always comes out cool. Same effect but in that case with liquid. We can observe where close lip air goes from the mouth in a smoker blowing the air out with pursed lips. The air is still the main constituent and we can observe the interaction of the smoky air with the surrounding air. This is all just a theory and I am no expert so please feel free to refute anything. I do however feel that this hypothesis is strongly backed up and probably true.

Once again great question and have a good day 4 years ago :). Sorry for the late reply. Only found this post right now. It will probably take you another 4 years to read this as I have written so much.

Good day :D! Tristan Viranyi, Wed, 2nd Mar 2016

Thank you Shelley Woolf for asking  Joshua's great question. I am not a scientist or really qualified to give you an answer, the truth is I do not know the answer so anything I suggest would be a pure guess and hopefully I will learn the answer from this thread also.  The only difference in the situation I can personally think of is the speed of the carbon dioxide/air mix , the difference being blowing or exhaling, having something to do with the  cause. 

Thank you for the question, I have no idea and am ''stumped'' for an answer.  Fascinating. Thebox, Thu, 3rd Mar 2016

Hiya there TheBox. I hope you are doing well!

I can recommend my post as an explanation to this problem :). I didn't find anything useful as an explanation in my quick scouring of the internet before I did that post so maybe give mine a read and tell me what you think :).

I can think of many cases that support my hypothesis. I am 99% sure it is a sound one.

In short, the divergence of atoms by natural release of high pressured gas out of a small opening, causing a lower average temperature of gas as a result of a lesser number of average particle collisions, is the key concept here to my understanding :).

Hope this helps :). Tristan Viranyi, Mon, 11th Apr 2016

does this have anything to do with pressure and temperature? because when the opening area reduces, velocity increases and hence pressure and temperature reduces and hence feels cooler. shaheen mongam, Tue, 14th Jun 2016

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