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Cooling Fans

This experiment came from Diane in South-central Pennsylvania. She and her son were discussing ceiling fans, and how they make you feel cooler. They wanted to know if the fan actually cools the room, or does it just feel that way?

To find out, you will need:

  • a fan
  • a thermometer
  • water
  • cooking oil
  • rubbing alcohol

Start by turning the fan off. Also turn off any air conditioning or heating. Take a seat in the path of the fan's air and wait a few minutes. Notice the temperature and how your skin feels. Then turn the fan on and compare the sensations. You probably found that the breeze from the fan made you feel cooler. Why?

Lets start by looking at the fan. It is simply a motor attached to some rotating blades that are angled so they will push the air forwards. No refrigeration unit, no compressor coils, nothing that would cool the air. Then why does a fan make you feel cooler? The fan is moving air across your skin. To understand why that cools you, we have to think about heat.

Heat moves from warmer things to cooler things. If the air next to your skin is cooler, heat will move from your skin to the air, making the temperature of your skin lower. If that air is not moving, then you quickly reach a point where you are surrounded by a layer of air that is the same temperature as your skin. At that point, the air around you stops feeling cool. Your skin gets warmer, which makes the air around you warmer, which lets your skin get even warmer. Not a comfortable thing on a hot day.

By moving the air, the fan provides you with a constant supply of cooler air. The air next to your skin absorbs some of the heat, cooling your skin. Then it is blown away by the fan and replaced with more air that is cooler. This constant supply of cooler air keeps you comfortable.

But, what if it is a hot day, and the air is warmer than your skin? Why doesn't the fan make you warmer instead of cooler? In that case, the answer is evaporation. Put a drop of cooking oil on your skin. In another spot, put a drop of rubbing alcohol. Blow gently on both. Do you notice a difference? The alcohol should feel quite a bit cooler. As things evaporate, they absorb heat from their surroundings. If that heat is absorbed from your skin, it makes you cooler. The faster a liquid evaporates; the cooler it will feel.

So, how does the fan make liquids evaporate faster? Again, the answer is moving air. As you are sitting there, reading this, you are sweating. You may not have streams of perspiration running down your face, but moisture is moving from your skin into the surrounding air. If the air next to your skin is still, it is quickly saturated with water vapor. Once that happens, you no longer get the benefit of evaporative cooling.

Again, the fan comes to the rescue. By moving the saturated air away, and replacing it with drier air from the surrounding room, you continue to be cooled by the evaporation of moisture from your skin.

So does the fan actually change the temperature of your room? That depends on where you place your thermometer. As air gets warmer, it expands, which causes it to rise. Cooler air is denser, so it tends to sink. That means that in a room where the air is still, the air near the ceiling will be quite a bit warmer than the air near the floor. A fan can mix the air, giving you a room with a fairly even temperature. The upper part of the room will be cooler in a room with a fan, while the air near the floor will be warmer. The overall amount of heat is the same, but it is more evenly distributed.

Still, that difference is important. Since your brain needs a very large supply of blood, we tend to lose a lot of heat from our heads. Our heads tend to be in the upper part of the room, unless you are really into yoga and standing on your head. With your head in the warmer, upper air, there is less difference in temperature between the air and your skin. That means less heat transfer and less cooling. If you make the air in the upper part of the room cooler, you lose more heat from your head, and you feel cooler, even though your feet may be warmer.

So although a ceiling fan does not make the overall temperature of a room any cooler (in fact the motor changes some of the electrical energy into heat, so it is actually adding heat to the room), it can make the air feel quite a bit cooler by moving it around.

That brings up the question of fans and pets. Dogs and cats do not sweat as we do. Most of their skin lacks sweat glands, so they only sweat from their noses and the pads of their feet. That means that a fan will not do much to help them with evaporative cooling. The fur on their skin also traps air, so especially for long haired pets, a fan won't help much there either. Even worse, most cats and dogs spend much of their time near the floor, where the air in a still room would be cooler. If the fan evens out the temperature of the room, the air near the floor winds up warmer, not cooler. So your pet is probably not getting nearly the relief from your fan that you are getting. Of course, you could help evaporative cooling by spritzing their fur with a little water, but I find that most pets don't appreciate that at all. Better to give them some cool water to drink. In fact, eating something cold will help you cool down too. I wonder what we could find in the freezer that is cool, and pleasant to eat? Could it cream!