Why does air blown by a fan feel colder than the other air in the room?
In: Physics
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.
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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.
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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.
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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”.
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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.
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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.
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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.
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