Why does air blown by a fan feel colder than the other air in the room?
In: Physics
Moving air makes moisture on your skin (like sweat) evaporate more quickly.
Water evaporating takes energy and therefore reduces the temperature in the surroundings (this is why humans sweat, to cool the body).
You can also observe the same cooling effect when putting a wet cloth in front of a fan and measuring the temperature with a thermometer.
Doing the same experiment with dry cloth won’t result in any measurable cooling.
You have hairs all over your body that traps air and makes it roughly body-temperature. So when you have a fan directly blowing at you, that air moves away from those hairs, making it seem cooler. The actual temperature of the room does not change…
Hover your hand a half centimeter above your arm right now in a windless room. Feel the warmth? There is a warm pocket of air just hovering in a thin layer just above the skin. The wind basically removes this layer and exposes your skin directly to the cooler air instead of the warm air layer. This happens in addition to evaporation as others pointed out.
You are not comparing the average air in the room with what is blown by the fan. You are comparing the air close to your skin with the air the fan blows.
If we assume the air temperature is below you body temperature then you heat up the air around you. The air in the rest of the room will be colder. The fan replaces warmer air around you with cooler air around you.
If it is warm and you sweat the air close to you will have a higher moisture content than the rest of the room, so the fan replaces it with air with lower moisture content. This means the sweat evaporates faster and that cools you down.
If you were in a room where the air was warmer than your body temperature then you cool down the air close to you, When the air heats you up it gets colder. So the fan will replace cooler air with warmer air.
If it feels warmer or not will depend on the moisture content, if it is dry air sweat on your skin can evaporate and that cools you down. If the air is saturated with water the sweat will not evaporate and you feel warmer. An example where you can have air warmer than you that is very moist is in a sauna, then a fan heats you up
There’s kinda 2 parts to this.
1. The human sense of hot vs cold.
2. The rate of transfer of heat determined by physics.
—
Firstly, the human body can’t actually directly sense temperatures. We can’t actually tell that something is a certain temperature. We can only feel heat loss or heat gain to our skin.
When something feels hot to us, the heat is transferred at a high rate from the thing into our skin.
When something feels warm, the heat is transferred at a slow rate from the thing into our skin.
When something feels cool, the heat from our skin is lost to the thing at a slow rate.
When something feels cold, the heat from our skin is lost to the thing at a high rate.
—
Secondly , in our current universe, the rate of “transfer of heat”(aka gain/loss) is determined by the difference in temperatures between A and B.
If the difference in temperature is small, the heat moves to the other object slowly. If the difference is large, the heat moves very quickly to “compensate” the large gap.
—
We combine the 2 concepts mentioned above.
When we feel boiling water is scalding, it’s because the difference between our skin e.g. 30 °C and boiling water 100°C is a **whopping 70°C difference**. When 30 touches 100, the heat from the 100 rushes at an extreme rate towards the 30.
Because the heat is *gained super quickly* by our skin **due to a large temp difference**, we sense this as “fucking hot”.
When you feel room temperature water is slightly cool, it’s because the difference between your skin 30°C and the water at e.g. 25°C is a **small -5°C difference**.
When 30 touches 25, the heat from 30 slowly moves to the 25.
Because the heat is *lost only gradually* by our skin **due to a small temp difference**, we sense this as “slightly cooling”.
—
Now, when you idle inside a room with no air movement, what is happening is actually you are constantly losing the heat produces by your body; heat from your skin is being transferred to the air directly beside your skin.
– Imagine your skin is 30°C and the room is 25°C.
This thin layer of air directly beside your skin is warmed up by your skin. If you don’t really move much too, your skin and the layer of air kinda becomes the same temperature.
– E.g. 30°C skin and 28°C air = difference of 2°C = small difference in temps = slooooooow heat transfer.
When the air gets disrupted or you move, that thin layer of skin-temperature air is moved as well and gets replaced with a new layer of air of 25°C. By itself, 25°C isn’t considered cold by any means, but because your skin is 30°C vs 25°C air, that greater “difference of 5°C” in temperature makes heat move from your skin towards the air at a higher rate(vs a difference of 2°C).
Heat lost from a 5°C diff in temp will always feel cooler than a 2°C diff in temp.
—
disclaimer: There’s a lot more very important variables to this “transfer of heat from skin-to-air”, like others have mentioned. But without diving in depth into relative humidity, latent heat, rate of evaporation, I think it would only serve to further confuse a layman.
—
TL;DR if you don’t want to learn physics concepts.
A fan works by constantly replacing the air directly beside your skin. It forces air to keep moving, and there isn’t a chance for any air to stay still beside your skin to get warmed up.
edit: lmao I just realised my whole essay kinda breaks down if you consider the room reaches wet bulb temp. welp heatstroke here I come.
My daughter, studying to become a civil engineer, had an exam where a question was (paraphrasing) “in a static room with a fan, will the temperature of the air increase or decrease?” They also gave some pertinent numbers to work with so they could do the relevant math work, but that was the gist of the general question. She said several wrote that the air would cool down and then they came up with some math to make it work in their heads, IDK. Of course we know that the temperature of that room will increase because of the heat the motor gives off, and the only cooling that is happening is by removing the boundary layer off the skin of a person.
First, understand that heat is constantly radiating out of your body. This is why wearing clothes helps you stay warm – they trap the heat coming out of your body, and keep it close to you.
The heat radiating out from you radiates into the air around you, heating it up. You can test this yourself – hold your bare hand still in the air in front of you. Now rapidly move it up or down.
See how your skin feels slightly cooler during the movement, from the “wind” of moving your hand?
A fan is essentially doing that to an extreme degree. The air from the fan blows away the little bubble of warm air that hangs around you, helping you cool down in the process.
Latest Answers