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Because the speed with which the air moves is different in the two situations. When you just open your mouth for the breath to come out, the air velocity is low. When you set the “beak” to blow, the air comes out much faster. The temperature difference we feel is just because of that. Both breath and breath air have the same temperature before exiting through the mouth.
opening the beak
With the mouth closed, air comes out faster and creates a low pressure area.
breath
1. When we blow on a body part, we displace small layers of air that are next to the skin. As the blowing speed is high, the pressure in the affected area decreases, since the higher the speed, the lower the pressure.
2. Other ambient air molecules then tend to occupy this low-pressure space. As these molecules are at a lower temperature than our body, there is a cooling sensation caused by the breath.
Breath
1. The air that comes out when we open our mouth to cause the breath has a low velocity. With this, a region of low pressure is not created in the area affected by it. Thus, air molecules from the environment are not attracted there.
2. The only thing that rolls is a mixture between the layer of air that was around the skin and the air molecules in the breath. Because they came out of our body, the molecules of the breath are at a slightly higher temperature, which generates the feeling of heat.
Important!
The breath is only cold if the ambient temperature is lower than that of our body. Inside a sauna, for example, the breath will be hot.