When blowing, the opening area of the mouth is small, so the flow rate of the blown gas is faster. Due to the existence of friction, the blown gas moves with the nearby static cold air, and the proportion of cold air blown on the hands is large, which will cause the evaporation of water or sweat on the hands to accelerate. Evaporation is an endothermic process, so it will feel cold when blowing.
When breathing with the mouth open, the mouth area is larger and the exhalation speed is relatively slow. When the exhaled gas touches the skin and returns, it basically does not accelerate evaporation. People feel more hot air, so they feel that the exhaled gas is hot.
In addition, if you put your hand close to your mouth when blowing, you will basically feel hot. This is because it is too close, and the air blown out of the mouth has no time to entrain the cold air nearby.
Extended data:
Blowing balloons can exercise vital capacity. Balloon blowing is a kind of deep breathing exercise, which can increase people's vital capacity, promote metabolism, consume energy and fat, and play a role in slimming.
Generally speaking, try to inhale and exhale to form vital capacity, which is related to the height, weight, chest circumference and physique of the human body. Breathing is a chest-expanding exercise. A series of breathing exercises the muscles attached to the chest, which not only increases the vital capacity, but also makes the chest muscles plump and more attractive.