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So I asked my science teacher how wind could feel colder than normal air when the particles would be moving faster than normal air particles, meaning that they would have more kinetic energy and a higher temperature. My teacher told me that the faster rate of particles hitting your body would mean that the heat would leave faster than in normal air, but I was confused as to why they would take energy.

Eventually, I figured out that the air particles have kinetic energy separate from their velocity and my teacher confirmed this. He also confirmed that the particles would vibrate in place while they are moving.

How is this possible, and how does the vibration dictate the heat transferred, and not the velocity?

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  • $\begingroup$ What does "kinetic energy separate from their velocity" mean? $\endgroup$ Jan 22 '20 at 2:16
  • $\begingroup$ I thought they were separate but I guess my science teacher told me wrong $\endgroup$
    – Novarender
    Jan 22 '20 at 2:26
  • $\begingroup$ Related video $\endgroup$ Jan 22 '20 at 2:42
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    $\begingroup$ Does this answer your question? Why does a breeze of wind make us feel cooler? $\endgroup$ Jan 22 '20 at 6:05
  • $\begingroup$ I did upvoted the answer by Aaron Steven. Just I want to say that the separation is a tentatively way to say something more than correct. You can't take the average kinetic energy of molecules in a volume as an index of the kinetic energy of that volume. A more concrete example: you can trow a stone at different speed. This has no relation to the stone T. The teacher or you, that is not clear, was right in this point. The speed of molecules is not the speed of the body they compose, of course. $\endgroup$
    – Alchimista
    Jan 22 '20 at 9:44
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Your science teacher was wrong. Wind feels colder to your skin on a hot day because it evaporates your sweat faster, carrying away more heat from your skin than can conduction to still air at the same temperature.

Kinetic energy is equal to 1/2 * (mass) * (velocity^2), so the assertion that kinetic energy is not connected to velocity is also false.

Furthermore, the molecules in air are vibrating around and bouncing off one another far faster (~750 miles an hour) than the movement of the wind. So the additional kinetic energy they have because the wind carrying them is going 20 miles an hour isn't important in this context.

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  • $\begingroup$ Ah, really? I didn't know that. $\endgroup$
    – Novarender
    Jan 22 '20 at 2:29
  • $\begingroup$ I was thinking about that while reading the first answer! $\endgroup$ Jan 22 '20 at 2:49
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While I am not sure of what "kinetic energy separate from velocity" means$^*$ (I will wait for clarification), there seems to be some confusion as to why wind makes one feel colder, even though the air is moving faster relative to your skin than still air would be.

First, if we are stationary in a location with no wind and air that is at a temperature that is lower than our body temperature, then we essentially will reach a steady-state situation where the air right next to our body is at the same temperature as our body, and the air will become colder as we look farther from our body. Thus, we will essentially develop a "protective layer" of air that insulates us from the colder air around us.$^{**}$

However, if we start moving, or if the wind starts blowing, then this warm protective layer of air will become thinner/non-existent, and the air closer to your body becomes colder, hence you feel colder as well.

It sounds like your science teacher just doesn't know what is happening in this situation, and so they are just saying things that might sound correct until you appear to be satisfied with the explanation so they don't have to be on the spot anymore.

Aside: Something to keep in mind is that humans are horrible thermometers. The classic example showing this is how conductors like metals feel in comparison to insulators like fabric. If we have a conductor and an insulator both at the same temperature, say lower than body temperature, we will feel the conductor to be colder than the fabric. This is not because they are at different temperatures, but it is because the conductor absorbs energy at a faster rate, and so we feel it as "colder".


$^*$ Kinetic energy is defined as $\frac12mv^2$, so I cannot see how kinetic energy can be separate from velocity.

$^{**}$ It is not a perfect insulation, of course, but it is better than nothing.

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  • $\begingroup$ Thanks. Also, wouldn't that protective layer of air slowly dissipate the heat anyways? Probably slower than if you were moving though. $\endgroup$
    – Novarender
    Jan 22 '20 at 2:26
  • $\begingroup$ @Novarender Yes, that is correct. $\endgroup$ Jan 22 '20 at 2:28
  • $\begingroup$ Not that I want to confuse things... but the temperature does change for things other than humans when the fluid is moving. For a stationary thermometer in a stationary flow, the temperature is the stagnation temperature. But if you were to move the fluid and the thermometer moves with it, you'll get the static temperature. The static temperature plus the kinetic energy of the motion gives you the stagnation temperature. So moving things are colder, it's not just a human perception thing. $\endgroup$
    – tpg2114
    Jan 22 '20 at 2:56
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    $\begingroup$ As for kinetic energy separate from velocity... when I first read that, I thought of statistical mechanics where temperature is the average translational kinetic energy of the molecules, which is different from kinetic energy of coherent motion. I don't know if that's what is being referred to here, but it could be a confusion between the thermal motion/kinetic energy and the kinetic energy of bulk motion. $\endgroup$
    – tpg2114
    Jan 22 '20 at 3:01
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When there isn't wind blowing, all the air molecules are moving in random directions, but with no average motion. For example, the $x$-components of velocity might be mostly within $-100$ to $100$ m/s. When air molecules of this kind hit you, they pick up energy, but they only slowly carry it away because each individual molecule doesn't go anywhere on average.

When there is wind blowing, there is a nonzero average. For example, the $x$-components of velocity might be mostly within $-99$ to $101$ m/s. This carries heat away faster. These particles do have very slightly more energy than in the no-wind case, but that doesn't really matter.

So your instinct and your teacher are correct, in this case "average wind speed" and "temperature" are basically independent variables, and the former has a big impact on how cold the air feels.

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These other answers are correct, but there is another way to think of it I want to share.

Blowing wind causes forced convection. What this means is that the energy is carried away by the wind. The faster the wind blows, the faster it is able to carry that energy away from you.

Since the heat loss depends on the temperature difference, the faster the wind is able to move that heat away, the cooler the air around you, and thus the higher the temperature difference between you and the air around you; therefore the rate of heat loss is greater and you feel colder.

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