Shelley Woolf asked:
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.
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.
I think there are two effects.
I also asked this a while ago now: http://www.thenakedscientists.com/forum/index.php?topic=20394.0 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.
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.
Hiya there TheBox. I hope you are doing well!
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