Answer to Question #4832 in Molecular Physics | Thermodynamics for Roshan Shaw
When we blow air from a small mouth opening we feel cold air while when we blow the air from a big mouth opening we fell hot air.
We know that heat moving is performed from hot bodies to cold ones. Hand and air are involved in heat transfer in our case. Heat transfer is going faster while the air flow increases. The expired air is warmed by our lungs. The distance between mouth and hand (if we blow on it) is sufficient paramenter: if the distance is small even the with small mouse opening we'll fell warm. When blowing from the removal the expired air mixes with environmental air and loses it's heat as a result. If we blow air from a small mouth opening the speed of an air flow is high and it interact rapidly with the skin surface. So, blowing from the distance from a small mouth opening cause fast cooling of the hand. When we blow the air from a big mouth opening the speed of an air flow is low and it give its heat to the hand cause it don't mix with environmental air and keeps warming. But if the distance is rather large we'll fell cool too, even if the mouth is opened from ear to ear. The are some more important phenomenon that take part in this experiment. The fact is that the skin always has some wet on its surface. Another fact is that the evaporation takes some heat while processing. Blowing on the skin cause fast evaporation of water from the skin surface and it's cooling down as a result. You can verify this by blowing on soaked hand.