Since things heat up when they move faster (i.e. particles and the definition of temperature), why does air feel nice and cool in a breeze, or from a fan? Why isn’t the air warmer than normal when it’s moving faster?

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Since things heat up when they move faster (i.e. particles and the definition of temperature), why does air feel nice and cool in a breeze, or from a fan? Why isn’t the air warmer than normal when it’s moving faster?

In: Physics

The air is usually colder than your skin.
A breeze can transport more heat away.

plus, if you have any kind of sweat on your skin, a breeze will make it evaporate faster (which takes additional heat away).

They are not moving faster, their chaotic movements just get a little less chaotic and start to go somewhere.

Imagine you have a bunch of kids in your living room, all the doors are closed and they are running like little demons around. Once you open a door, to the kitchen, for example, their speeds will stay the same but now you will see a flux of those little bastards into the kitchen and this is the only way we can feel the breeze.

When air gets hotter, it rises. The cold air nearby will fill the vacuum created and that’s the flux we feel. The speed of the molecules is not being raised, but now they have a direction to go.

The speed of wind is very small compared to the speed of individual air particles. The average speed of particles in the air is about 300-400 metres per second. Compare that to a normal wind speed of less than 3 metres per second. Adding 3 m/s from wind to 300 m/s barely makes a difference.

Wind feels cold because when you stand still, your body warms the air around it. The warmer air around your body prevents you from losing heat as quickly.

Wind replaces the warm air with cold air again, letting you lose heat more quickly and feel cold.

**Edit:** A fun consequence of how wind cools you down is that, assuming the air is saturated with water, or you’re not sweating (somehow), if the air is warmer than 37 C (body temperature), wind will actually warm you up, since it’s pushing hotter air in to replace the air that already lost some of its heat into your body.

Thermal motion is random. All particles move very fast (100s of m/s at room temperature), but they bump into each other and go in a random direction, so on average, they don’t move. Wind is the opposite, it’s quite slow but all particles move in the same direction.

For the temperature, the low velocity of the wind doesn’t account for much. But, wind constantly disturbs the hot air layer around your body and replaces it with fresh cold air, therefore it feels cool.

There are two things at work here. First, the breeze cools you because it is evaporating the water off your skin which is an endothermic reaction. It is why we sweat to cool off.

Second, there is a huge difference in the velocities of the molecules in the air and the wind itself. In an average substance at room temperature, the atoms and/or molecules are vibrating around the speed of sound. Now, that’s a very simplistic statement as it depends on the material, the size of the molecules, they types of atoms, it’s temperature, etc. But in any case it’s much, much faster than the velocity the breeze gives the molecules.

This one bugged me for years when I was younger, so it’s fun to see it here!

Very simply, your body heats up the air close to your skin. When you aren’t moving, that air doesn’t move, so you effectively have a thin cloak of warm, humid air surrounding you.

The breeze from a fan pushes this warm air away and replaces it with cooler, drier air.

* What you sense as “temperature” is actually heat flowing into or out of your body.
* Heat always flow from high concentration to low concentration.
* If there is no air flowing over your body, your internal heat will slowly make its way out of your skin into the air around it, heating that air up.
* Air can only hold so much heat so eventually that flow of heat will slow down.
* But if you move that heated air away and replace it with less heated air, the heat flow will pick up again.
* So a fan works by constantly replacing the slightly heated air around your body with air that isn’t as hot.
* It feels cooler than it is because your body is moving its heat away faster.