Why Do Fans Make Us Cooler?

Good Monday Morning everyone. We’re going to kick off this week with SCIENCE! Specifically we’re going to look at fans and the reason they can cool us down (but not always).

Fan In a Box

Let’s start with simple science. Can a fan physically cool a room? We can’t rely on our senses for this, we need actual, concrete numbers. As we covered last week, we can trick the senses into feeling cooler with well designed environments. People make for very poor thermometers.

For our experiment, we’ll put a fan in a plastic box with a temperature sensor. We’ll also monitor the room for any changes in temperature. We’re going to discover something very important, very quickly: the temperature inside the fan box steadily increases when the fan is on, and the room temperature remains constant.

We can discern two important facts from this:

  1. Fans do not create cool air or change the temperature of the air on their own.
  2. Fans contribute heat to the environment.

The heat is easy enough to explain. The fan contains copper windings for a motor and there are spinning parts. Whenever something moves, it experiences friction. Whenever we pump electricity into something, there’s a certain amount of it lost to the environment. The fan creates heat as a matter of functioning.

Let’s try something else. We’ll take a computer fan, and put it through a little home-made duct to blow over the tip of our temperature probe. We could do this with a box fan or a window fan, but lead sales guy Scott is didn’t want us blasting his invoice papers around the office or sending any of them off to Dorothy on the Yellow Brick Road.

The temperature probe with a concentrated stream of air over it IS cooling. This now begs the question, what’s going on? We’re not creating any inherent ‘coolness’ and yet we are able to create a decrease in temperature.


Physics of Cooling

There are two factors to moving air cooling us down. The first involves the dissipation of energy, entropy, and the other involves water changing state. These are fundamental ideas in physics with big implications.

Energy wants to equalize wherever possible. All the atoms and molecules want to achieve an equal state. If you have two containers of equal pressure and attach them together, the pressure will attempt to equalize. If you mix hot water with cold water, the temperature will equalize somewhere in-between. Heat, thermal energy, spreads from areas of high concentration to low concentration.

So long as the air is even a little cooler than something, we can radiate heat into that air. Stationary air will quickly reach equilibrium and cease to take on new heat. Moving air however, provides a constant supply of new, cooler air to take away heat. Moving air provides instant cooling capability. With the human core body temperature sitting at 98 degrees, most air is capable of providing cooling, but that isn’t the only factor.

We naturally sweat to cool down. Sweating takes advantage of physics. The water we excrete conducts heat. When that water evaporates, it takes a great deal of energy with it. It turns out when state-changes occur, it’s not a linear relationship. Water doesn’t turn to 32 degrees and instantly turn to us, nor instantly vaporize at the boiling point. Although the temperature remains constant, energy continues to transfer. The change of state itself actually requires a significant amount of energy. This allows us to pack energy into the water, evaporate it, and experience significant cooling.

When air moves over a body of water, it speeds up the rate of evaporation. This has two major effects. On the one hand, the air takes heat from the water, on the other the increased rate of evaporation increases the amount of heat our bodies can lose to the environment.


When Physics Works Against Us

There are however, times when our cooling strategies don’t work. When the ambient air temperature is over our body temperature, such as 105 degree days, we are the cooler thing, so we end up absorbing energy from the air. We can have issues with sweating too. Water that evaporates is humidity. If the air is saturated with evaporated water, our sweat can’t go anywhere and we cannot be cooled as effectively.

Depending on your environment, these may be rare situations or a fact of daily life. If this is your day to day life, then an air conditioner is not a luxury, but a necessity to remain cool and healthy in the long-term. You’re in situations that your body struggles to cope with.

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