By narrowing the opening of the lips, the air forms a faster jet. This jet pulls in room air and mixes with it. Thus, what reaches your hand isn’t just the air you’ve just exhaled: the surrounding air also participates, which can be cooler. With the mouth open, by contrast, the flow is wider and slower, it mixes less, and the warm air reaches the skin in a different way.
There is a second effect. A fast jet disrupts the thin layer of air close to the skin and promotes heat transfer by convection. It can also speed up the evaporation of moisture from the hand, a process that consumes heat. Therefore, the thermal sensation depends on several factors: the speed of the puff, how much ambient air it entrains, and the room temperature. A commonplace gesture thus becomes a small experiment in fluid physics and thermodynamics that you can test without instruments.
Santiago Campillo (@Scruzcampillo) is a biologist and science communicator specializing in health, nutrition and nature.
