As would a man in his right senses on a sweltering afternoon, I switched the fan to rotate faster. And as would a man in his right mind, I wondered why should the fan make one feel cooler.
Consider this. When the blades rotate faster, they smash the air molecules harder, which increases the speed and hence the energy of the molecules. Now, when these energetically jiggling molecules ricochet off your body, they slow down (a steel ball bouncing off the floor would be analogous). Since slowing down means losing energy, there is some energy given off which appears as heat energy*. That’s the catch! How in heaven’s name can a fan heat you up?
The implications I have so lucidly sequenced, though correct, are incomplete and slightly irrelevant. Now, as you may know, humans sweat. So, at any given time, there are water molecules glued to your skin. These are not exactly glued because molecules have a tendency to wiggle about their position continuously. This wiggling endows the molecules with inherent energy or heat. This extra heat makes you feel warm.
Every once in a while, an air molecule** whizzing past you with high-enough speed knocks off one such energetically wiggling molecule off your skin into your surroundings. With the molecule, you have lost the heat that it carried. Losing the heat makes you feel cooler.
Your fan accelerates this knocking-off process. Since there are a greater number of fast-moving air molecules capable of removing the water (and its heat) glued to your skin. This makes you chilly quicker.
This entire operation is analogous to blowing over your soup to cool it. Like any mechanism, this too has a name. The Wind-Chill factor. Something that makes stuff feel cooler when in fact the milieu is actually warmer.
Everything sounds too good to be true, give it some time to digest.
*Allow me to iron out a few details. Firstly, when these jiggling molecules bounce off you, the heat energy given off is itsy-bitsy. Why? Because collisions on such a tiny scale are almost perfectly elastic, and the kinetic energy lost is minuscule. However, energies from all the collisions combined would definitely cull up appreciably. Secondly, the heat energy we have been discussing all this while is essentially infrared radiation. How? That’s a topic for another time.
**An air molecule does not imply a molecule of air. It might be a molecule of Nitrogen, Oxygen, Carbon dioxide or anything which might have made its way through your window.