If you blow air against your hand with your mouth open, you feel warm breath. If you do with with your lips closed except for a small opening, you feel cold breath.
One explanation from here says "With your mouth open most of the air which reaches your hand has come out of your mouth. With pursed lips lots of cold air is entrained so the air reaching your hand is cold and moving quickly."
Another explanation from Paul Hewitt video,(go to min 24:00), is that air comes out cold it because it expands coming out of small opening.
So which explanation is right?
Best Answer
I think the expansion explains only part of it -- a small part I'd say.
I think the more likely explanation is that blowing with your mouth open results in an airflow that moves much slower than an airflow blown through a small opening in the lips. The faster moving air will cause a quicker rate of evaporation on your hand, which makes it feel colder.
But proper measurements are always more convincing than theories :)