I would have thought it was something to do with increasing the rate that the air is pushed through your lips? When you breath out gently with your mouth wide-open, you are doing so from further down in your gut too, so maybe that has a higher temperature, being closer to the core.

When you put your lips tightly together it is pushed out faster, and maybe has a cooling effect.

If you just "haaah" out some air at low velocity, almost 100% of it comes from inside you at 37 deg centigrade.

If you pucker and blow, there is a smaller volume but a higher velocity. This pulls in and mixes with a lot of ambient air (Venturi or Bernoulli effect) - you might find the airstream is only 40% body warmth and 60% ambient so it will be markedly colder.

If you pucker and blow through a tube held to your mouth, this excludes the ambient air and you then get reduced airflow but at the higher temperature again.

If you let the air out gently, it comes out at about atmospheric pressure, and at the same temperature as your lungs, which in most times and most places is warmer than ambient. What is more, it is saturated, or supersaturated, with moisture, which gives up warmth on condensing.

However, if you blow hard through pursed lips, the air emerges at a pressure significantly higher than atmospheric. It expands abruptly as soon as it leaves your lips. This expansion amounts to doing work, the energy for which comes largely from the heat in the compressed air, and causes adiabatic cooling. It is in fact a simple example of part of the cooling mechanism used by most commercial refrigerators.

Incidentally, this is the basis of the expression "blow hot, blow cold", which as far as I know, originated with Aesop's fable of the man who blew hot and blew cold, thereby alarming his naive fellow-traveller.

Go well,

Jon

I aksed this of my friend who is a cardiologist and here is the answer:

topa aa physics che,tane khabar na hoy,

compressed air is cool, the one which comes out with open mouth is CO2 and therefore hot.

atul

The first part, lined through, is telling me off for presuming knowing a physics question when I am not even qualified!!

I think there are two more things to be said in addition. Firstly, the air coming out at high speed forms eddies and mixes with the surrounding air, so what you feel at some distances is hardly that same air, though you feel the ENERGY you put into blowing.

Secondly, air moving fast has a cooling effect on the skin even if it is slightly warmer than the skin itself. I myself experienced that a breeze still cools when the air is 38 degrees hot. A friend witnessed that a fan helps even when it's 53 degrees, which I find rather hard to believe, though. Anyway, I suppose this is because moving air helps to evaporate more water.