[Un article de The Conversation écrit par Coralie Thieulin – Enseignant chercheur en physique à l”ECE, docteure en biophysique, ECE Paris]
Contrary to what one might think, a fan does not cool the air: it is content to set it in motion. Moreover, an electric fan emits a little heat due to its engine which converts electrical energy into heat.
In an isolated room of 1 cubic meter held at -30 ° C, a fan can raise the temperature of 1 ° C: it is clear that the effect is minimal and without significant impact in most situations.
What the fan changes is our perception. It creates a feeling of freshness, without really lowering the temperature. This impression comes from our own body, which reacts to air flows by activating its natural thermal regulation mechanisms.
To understand this feeling of freshness, we must therefore be interested in the way in which our organism manages its internal temperature. Because it is there, in the constant exchanges between our skin, the air and our nervous system, that the real refreshing mechanism is played out.
Indeed, the human body works a bit like a thermal machine: it produces heat permanently (when you move, digests …).
The role of perspiration to keep our internal temperature at 37 ° C
To avoid internal overheating, the body activates a very effective cooling system: perspiration.
When you are hot, your skin releases sweat. By evaporating, sweat consumes energy (called the “latent heat of vaporization”): it absorbs the heat of your body. Sweat somehow steals calories to him, which cools him.
But this mechanism depends a lot on external conditions. If the ambient air is warm and humid, the evaporation of sweat becomes less effective, because the air is already almost saturated in humidity and is less likely to absorb that of your sweat. Result: you continue to sweat, but without effective evaporation, sweat stagnates on the skin and no longer extracts heat. In other words, she does not stolen The more calories to your skin that would allow your body to cool.
This is where the fan comes in! By stirring the saturated air around the skin, the fan replaces it with drier air, which promotes evaporation and helps your body to cool.
In addition, the use of a fogger provides additional cooling by projecting thin droplets of water on the skin.
By adding droplets, the amount of water is available for evaporation, which makes it possible to extract more heat from the skin and intensify the cooling.
The combination of the fogger and the fan optimizes thermal comfort in hot climates by maximizing evaporation. On the other hand, when the air is very humid and saturated with water vapor, this mechanism is ineffective, because the evaporation is limited.
Thus, the fogger is particularly efficient in dry environments, where air can easily absorb humidity, while the fan promotes the renewal of wet air around the skin, thus avoiding local saturation and now a gradient favorable to evaporation.
Stir up
But that's not all. Even without sweat, your body transfers heat to the ambient air: it is convection.
This means that the air in contact with your skin warms slightly. When the air is motionless, this layer of hot air remains stuck to the skin like a thin blanket.
By setting the air in motion, the fan dissipates the thin layer of hot air that surrounds your skin. This allows the heat to be evacuated faster, which causes an almost immediate feeling of freshness.
More specifically, researchers have shown that when air speed increases, evaporation of sweat can grow by almost 30 % for an air speed of 2 meters per second (which is equivalent to about 7 kilometers per hour). At the same time, the heat loss by convection also intensifies thanks to the constant renewal of hot air close to the skin.
This phenomenon is called the “wind cooling effect”, or Wind Chill. He explains why, in winter, a strong wind can make you feel a cold much more intense than the real temperature: for example, a 0 ° C accompanied by wind can be perceived as -10 ° C, because your body loses its heat faster.
Actual temperature, temperature felt
In summer, it is the same principle: the breath of the fan does not lower the temperature of the room, but it promotes the loss of body heat, giving the illusion that the ambient air is cooler. It is a lower temperature, not a real temperature.
A fan is therefore an optimal ally. It does not drop the air temperature, but accelerates the heat loss of your body. It thus facilitates your natural cooling mechanisms such as the evaporation of sweat, convection of heat, sensory perception of air in motion.
In reality, the air remains at the same temperature: it is you who cool faster … and your brain translates this loss of heat with a pleasant feeling of freshness!
This process is not trivial. It is based on a complex interaction between sensory receptors located in the skin and specific regions of the brain, in particular the posterior island cortex. These receptors detect body temperature variations and transmit this information to the brain, which integrates it to generate a sensation of freshness.
Thus, what you feel as a pleasant freshness is actually a fine and sophisticated brain perception of the real decline in the temperature of your body.
With an unwavering passion for local news, Christopher leads our editorial team with integrity and dedication. With over 20 years’ experience, he is the backbone of Wouldsayso, ensuring that we stay true to our mission to inform.
