Why is it when you breathe out with your lips together its cold? And with your mouth open its warm???

Question

Why is it when you breathe out with your lips together its cold? And with your mouth open its warm???

Answer 1

It's a question of the speed of the air-stream. A tight-lipped breath is a faster current. Open your mouth wider and the same volume of air will leave, but at a lower velocity.

Now, faster winds have a cooling effect on skin (wind chill), caused mainly by faster evaporation of moisture from the skin, which draws heat from the body. On the other hand a slow breath has much less of this effect, and so allows you to feel the body temperature of the breath, which is slightly higher than that of the skin.

Hence the difference in sensation.