Why is it ...?

Question

when we breath out the air is warm and yet when we blow the air is cold (for example to blow on your food to cool it down)?

Answer 1

It has to do with heat transfer and the temperature of what you are blowing on.  There is little adiabatic cooling of your breath as you exhale, the pressure differential is only 1 mmHg, far too low for there to be any measurable effect on average.  

What does happen is if you blow on an object that is warmer than your breath, the motion of the air thins the thermal boundary layer around the object, increasing the rate of heat transfer from the object to the air.  Since the object is warmer, it cools down.  But it is the thinning of the thermal boundary layer than speeds the cooling.  

When you blow on something hot and moist, there is also a lot of cooling that occurs from increasing the transfer of latent heat (or water vapor).  Blowing on a cup of hot coffee cools the coffee mainly by this mechanism.  Latent heat transfer from a liquid increases linearly with the velocity of the air close to the liquid surface.  So blowing across a hot liquid cools it a lot.

Answer 2

If you breathe out, your breath is at body temperature which is warm.

But if you constrict your lips and puff, you are first of all compressing the air in your lungs, and then letting it expand against atmospheric pressure which means that it is doing work and since the energy to do its work of expansion has come from somewhere, it cools down.

If you have studied thermodynamics, think of this is an application of the First Law.

Answer 3

when we breath out air it is worm cos ur body has heated it up. When we blow we are using power behind it so as it travels faster and further the air cools it down.