This is a common phenomenon in life, but when you think about it seriously, you will find it interesting. Why on earth is this? Let me give you an answer!
1. Why is the breath coming out hot when breathing? According to the description of Naville-Stokes equation, when the total amount of gas is constant, the gas velocity will change due to the difference of gas outflow aperture and blowing distance.
When people breathe, if the mouth is close to the skin and the caliber is large, at such a short distance, the friction process between the exhaled gas and the surrounding air is short and there is no heat loss. Moreover, in the process of gas transmission, there is not much cold air brought in, and its heat cannot be neutralized, resulting in the exhaled gas being hot.
Second, why does it blow out air when it is cold? Different from Harbin gas, when blowing, the diameter of gas outflow is small and the lead of gas column is long, which leads to the fast gas flow rate. According to the principle of pressure, the faster the gas flow rate, the smaller the air pressure. At this time, other cold air will flow from the place with high air pressure to the place with low air pressure, resulting in more cold air to neutralize the hot air blowing. This gas will get cold when it reaches the skin.
Third, the principle of water evaporation will also lead to this phenomenon. As mentioned earlier, when you blow, the air column will be brought into a lot of cold air. When these cold air comes into contact with moisture on the skin surface, it will accelerate the evaporation of moisture, and evaporation is an endothermic process, which will lead to the cooling of the skin surface, thus bringing coolness.
On the contrary, the hot air you breathe does not bring a lot of cold air. Although there is evaporation, the effect is not obvious.
This is a small phenomenon in life, but it contains knowledge of fluid mechanics. It seems that physics is everywhere in life!