[pyrrole]

Why is this?

[Dan]

The air you blow is the same temperature, the reason it feels colder is wind chill.

[SirRoderick]

Meaning it moves more quickly, which affects the way heat is transferred.

[nj_bartel]

The air you blow is the same temperature, the reason it feels colder is wind chill.

Are you sure?  I've always attributed the majority of the effect to the depressurization of the air upon leaving your mouth (i.e. interpreting it through work done on/by the gas).  If you breathe hard against your hand, it is still warm, but a soft blow is cold.

[SirRoderick]

The air you blow is the same temperature, the reason it feels colder is wind chill.

Are you sure?  I've always attributed the majority of the effect to the depressurization of the air upon leaving your mouth (i.e. interpreting it through work done on/by the gas).  If you breathe hard against your hand, it is still warm, but a soft blow is cold.

Depressurization?
The air in your body will be at around 1 atmosphere, just like the air outside.

[nj_bartel]

If that's so, then why is it moving?  Your lungs apply a pressure to exhale the air, and when you constrict your lips, pressure can build significantly between your cheeks.  How else would you be able to fill a balloon?

[enahs]

It is hotter. Blow slowly on your finger, still cold.

When you breath out, the air is coming directly from your lungs which are at a nice 37 C. The inside of your mouth cavity is well below that . When you blow, you are talking the cooled down air form your mouth. That is why you can not just stick thermometer in your mouth, but it has to go under the tongue.

[SirRoderick]

If that's so, then why is it moving?  Your lungs apply a pressure to exhale the air, and when you constrict your lips, pressure can build significantly between your cheeks.  How else would you be able to fill a balloon?

That is.....just such an example of misfiring logic.
Because you can create pressure in a closed container like a balloon does not mean the air you exhale is at higher pressure. Seriously, you're pushing the air out one side, the other one is wide open. That's the way breathing works.

[nj_bartel]

Ok.. example with no closed containers.  If you pull a basketball pump plunger to the top, then push it all the way to the bottom very quickly, does the air pressurize before coming out the hole then depressurize upon leaving the hole?  Same concept

[John009]

When we pucker up our lips it creates less area for the air to move through and causes greater pressure and faster speed. Similar to when you close off part of a garden hose nozzle with your finger and the pressure increases making the water come out faster and harder.

[ATMyller]

When you blow hard the air stream from you mouth gets mixed with surrounding air which is cooler, causing the chilling effect similar to a regular fan. Try the same in room with temperature above you body temperature (e.g. sauna) and you'll get burning sensation instead of cooling.