So I had a fan on the other day and got to wondering, why does a fan cool me down? The air being blown around is the same warm air I’m sitting in without the fan on, so why does it feel cooler when it moves around? Why doesn’t the friction of the air moving past me have the opposite effect and warm me up? After all isn’t it friction that causes meteors to burn up in the atmosphere?
I’m probably gonna smack my forehead in disbelief at the simplicity of the answer, but for now I’m stumped.
It’s a combination of evaporation (of sweat) and increased convective cooling (just like your PC CPU fan). Moving air carries away heat more effectively than still air can conduct it away.
When you sweat, the mooisture of your perspiration evaporates into the air. It takes energy to evaporate water, and when you sweat the water steals this energy from the heat in its environment. You, being in its environment, lose heat to this process.
But is you sit still and sweat for a while, the air very close to you becomes rather full of water (we say it becomes more ‘saturated’) and so the rate at which evaporation can occur is reduced. Less or no evaporation means less or no heat loss to power the evaporation.
The less heat you lose to the environment, the hotter you are.
Now, if we blow away that wet air next to you, and replace it with drier air from a part of the room where you weren’t sweating, then the evaporation can once again occur at a faster rate. A fan keeps a consstant upply of fresher air.
That process is essentially ‘Adiabatic Saturation’
Then there’s Newton’s Law of Cooling. (Not one of the famous Newton’s Three Laws). The cooling law says that the rate at which a hot body (37-degree you) cools down when it’s next to a cold body (20-ish-degree air) is related to the difference in temperature between them. (in fact, the the square of the temperature. Same thing, really, only the effect is amplified.)
If you and the air are only one dergee apart, you will cool very slowly, and the air will warm very slowly, to an intermediate temperature.
But if you and the air are at very different temperatures, then you cool faster, and the air warms up faster.
Now, if you sit still in a room, not only do you moisten the air right next to your skin, but you heat it up. When it’s closer to your temperature, you lose heat to it more slowly. Blow that air away, and put relatively cool air in its place, and the rate of heat loss from your body increases.
If, of ocurse, the air is hotter than you, then you can only cool off by sweating. And if the air is also too humid (for the experts, if the Wet-Bulb temperature exceeds your body temperature) then you can’t lose heat by sweating. So you cook.
Ok, so maybe I won’t be slapping my forehead after that description. Thanks for the answers.