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?
Best Answer
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.