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If you turn on a fan in a warm room, it feels as if cold air is being pushed from the fan out in the direction that it's facing, but what's actually happening on a molecular level?

When an object is heated, its molecules are excited and they can in turn excite other molecules around them thereby passing on heat through the air (or another medium). But the opposite of this - molecules vibrating less and passing on a lack of vibration - doesn't seem to make as much sense, intuitively at least.

So is the apparent effect of cold air being pushed out by a fan or air conditioner caused by molecules passing on their relative lack of movement to their neighbours or is there something else going on?

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I believe the air feels cold because when it comes into contact with your body, the moisture on your skin evaporates. Evaporation is endothermic (it absorbs heat), and this makes your skin feel cool. –  wgrenard May 7 at 15:38

3 Answers 3

up vote 9 down vote accepted

As mentioned in the comments, air from a fan feels colder because of two reasons. Firstly, the air is cooler than your body so as it passes, your body heat transfers in part to the air, which is then carried away. Second, as the air passes, it evaporates moisture on your skin, which takes absorbs heat in the process.

As for the propagation of cold temperatures in general. If you put an ice block out on the counter, the air around it will get colder and this coldness will seem to radiate outward. This is not because the air molecules are passing on a lack of vibration, it is because they are passing their heat to the colder molecules of the ice block. The excited, warm air around the ice block bumps into the molecules of the ice block and transfers energy to it. This causes that air to lose heat and be colder. Then the air immediately around that transfers heat to the first layer of air and loses heat itself. Repeat ad nauseam. This is all because objects tend to want to be in thermal equilibrium. To make everything the same temperature, heat tends to flow towards colder areas, which can seem like cold flowing outwards to warmer areas.

In short, what seems like cold flowing outward is actually heat flowing inward.

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So, if you are cooled down by evaporation, then continuing to stay under the fan should eventually do 1 of 2 things. The fan should either evaporate all water in your body, or lose the coolness that comes from evaporation, correct? –  David Starkey May 7 at 21:42
    
@DavidStarkey Eventually, the air will saturate with water and it won't cool you anymore. That or when your body feels cool enough, it will stop sweating, so there won't be any moisture on your skin –  Jim May 8 at 12:47

The temperature of the air didn't change. What you are experiencing is an increase in rate of evaporation from your skin - and this evaporation is what makes you feel cooler.

Why does the rate of evaporation increase? Well - evaporation is a balance between water molecules going from liquid to vapor, and from vapor to liquid. In a stationary air environment, water vapor concentration close to the skin will be greater - slowing down the rate of evaporation. When you blow "fresh" air (not so moist) against your skin, you can start cooling down more quickly.

There is also a very minor effect that the Bernoulli effect means that air pressure for moving air is slightly lower - I think that is secondary though.

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A fan turned on in a closed hot room will eventually increase the temperature of the room because the energy from the fan will turn into kinetic energy of the molecules plus direct heat from the inherent inefficiency of the motor. A 300 watt fan is like a 300 heater eventually, so the kinetic energy of the molecules will increase at the end of the story.

ceiling fan

The cooling felt is due to the air flow established by the impetus of the blades. The air flow passing over human skin increases heat transport from skin contact (if temperature less than 36C or so) and also evaporation of perspiration. Over 37C only extra evaporation provides cooling.

The molecules within the air flow have the collective motion of the mass in the flow, which does not change the microscopic kinetic energy which defines the temperature of the moving mass. As I said above eventually all the energy dissipated will turn to extra heat in a closed room.

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