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So I was thinking... If heat I feel is just lots of particles going wild and transferring their energy to other bodies, why am I not burned by the wind?

When I thought about it more I figured out that wind usually carries some humidity, and since particles of liquid are moving same speed as the wind, they are basically static relative to each other, so no energy is transferred between them (wind and water particles). And if that water sticks to my skin and wind blows, it'll evaporate thus taking energy from my skin and make me feel cold.

Thing is, I don't think that's really the case but even if it is, if I somehow dry out the wind, will it burn me if it's strong enough? And winds can reach some pretty high velocities (though I must admit I'm not sure if they are comparable to movement of atoms in warm bodies etc...).

So. Bottom line. Can I be burned by wind in some perfect scenario?

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Nice title and framing of the question. –  dmckee Feb 1 at 20:37
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So I initially closed this as a duplicate of "Why isn't temperature frame dependent?", but now I'm not so sure (though I still think it is relevant). Over the the crowd for group consideration. –  dmckee Feb 1 at 20:38
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I was in the process of writing an answer when this question was closed. I would like the question re-opened because I find it extremely unlikely that the OP will understand the linked question to be equivalent, and also because my answer takes a considerably different tack than it would when answering the linked question. –  Mark Eichenlaub Feb 1 at 20:39
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An interesting fact is that wind over 40 mph actually has a negative wind chill. –  hosch250 Feb 1 at 23:36
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Well, I learned something today: windburn is really just sunburn. Very strange, as my only experience with windburn felt like sunburn, but healed up with no blistering or peeling and in a very short amount of time. –  Patrick M Feb 2 at 9:22

5 Answers 5

up vote 85 down vote accepted

Air molecules $(\require{mhchem}\ce{N2_}$ and $\ce{O_2})$ have an average velocity of around $500\text{ m/s}$, varying some depending on the temperature. This means that a nice $5\text{ m/s}$ wind is a hundred times slower, and the energy represented by wind is 10,000 times smaller than the thermal energy. Therefore, wind does not have considerably more energy than calm air and will not burn you.

Very high-speed winds, such as those in tornadoes, hurricanes, or the wind you would experience while sky-diving, are still only around $50\text{ m/s}$, so the energy density in the wind is still just 1% of the thermal energy density. Likewise, the ram pressure the air exerts on you would be small compared to the homogenous atmospheric pressure, so no large effects should be observed. Thus, one would not expect even high winds to burn you.

The transfer of heat between you and the air is fairly complicated, and does not depend solely on the energy density of the air. Wind usually makes you feel colder, in fact. Heat travels across gradients of temperature. The air right next to your skin will be at the same temperature as your skin, but the air a small distance away will be at the ambient temperature. This creates a gradient of temperature, and heat travels across the gradient. When there is wind, the difference in temperature between your skin and the ambient air is the same, but the temperature falls down to the ambient temperature a shorter distance from your skin. This increases the temperature gradient, so that you cool down faster with a wind.

Humidity also plays a role; heat transfer is not very simple. However, I think this suffices to explain why we should not expect wind to burn you. You will burn up if you travel through the air at extremely-high velocity. This happens to meteors and other astronomical objects moving at orbital velocities ($\sim10^4\text{ m/s}$) when they enter Earth's atmosphere. It is also relevant for fast-moving aircraft, which do experience winds as fast as the thermal velocities of the molecules in the air. I've heard it said that the SR-71 Blackbird, the fastest airplane ever built, heated up so much due to aerodynamic heating that it had to be built to be loose at low speed so that the parts would fit together at top speed. See "Aerodynamic heating" for more.

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I think the SR-71 actually leaked fuel on the runway, and circulated fuel beneath the skin to cool itself. –  lionelbrits Feb 1 at 20:59
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I think this is a great answer, but I'm questioning the part about fast-moving objects like the SR-71. Is the heating effect really due to high velocity relative to the motion of air molecules? Or is it due to the extreme compression of air in front of the object (ideal gas law comes to mind). Or are these kind of saying the same thing? –  andy holaday Feb 1 at 23:42
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@lionelbrits It leaked so much fuel that it would take off, fly supersonic to heat up the skin so the leaks closed, and then had to be refueled because it used up it's entire fuel load just doing that. –  tpg2114 Feb 1 at 23:55
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@andyholaday It's effectively the same thing -- the kinetic energy of the molecules from the air must go somewhere when the air is brought to rest against the surface of the body. That somewhere is into thermal energy (and possibly into chemical or electronic modes). So the heating of air from compression is due to the conversion of kinetic energy of the molecules being converted to thermal energy. –  tpg2114 Feb 1 at 23:57
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@Anixx As the answer states, I am referring to energy density, not velocity. –  Mark Eichenlaub Feb 2 at 2:25

The other answers address your question quite well. Just as a reminder of the ability to be burned by a strong enough wind, the image below shows the Chelyabinsk meteor during entry into Earth's atmosphere last year over Russia. :)

Chelyabinsk meteor

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That would be more of a friction thing, no? Since air condenses and forms some sort of a wall. –  Jinx Feb 2 at 12:29
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@Jinx Not really. Aerodynamic heating is usually (though dependent on conditions) due to the adiabatic heating of the air in front of the projectile as it is compressed in the flow. To achieve the sort of heating on that meteor, though, you need supersonic 'wind', which (although not impossible) is not really observed on Earth. –  Emilio Pisanty Feb 2 at 20:13

If heat I feel is just lots of particles going wild and transferring their energy to other bodies, why am I not burned by the wind?

I think the most direct answer to your question is that heat is the random movement of molecules, with speeds on the order of $v_{rms} = \sqrt{\frac{3RT}{m}}$ which is in the hundreds of meters/second range, whereas the layer of air surrounding you moves in a not-quite random way (there is a boundary layer, for example), and not usually as fast.

If we think of friction, on the other hand, I think the problem is that while friction /drag will heat you up and the air surrounding you, the stream of air is also very efficient at removing heat from you, so depending on the temperature of the wind, the two effects will partially cancel each other out.

Now, when you start getting to transonic speeds, the (adiabatic?) compression of the air in front of you will heat it up significantly (I believe this is why the SR-71 was made out of titanium). At hypersonic speeds, this can give you a nasty case of plasmification.

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"Can I be burned by wind in some perfect scenario?"

Drag Effects

Drag Effects are the main source of the effect you are after in your primary question.

There is always an effect of heat from the wind drag, but in most circumstances the heat transfer is a net loss to your skin due to moisture evaporation and the air temperature being below your body temperature.

See also:

http://en.wikipedia.org/wiki/Drag_%28physics%29

http://en.wikipedia.org/wiki/Parasitic_drag#Skin_friction

Moisture Effects

You mention the effect of moisture in the air, keep in mind that it condensing on you would heat you as it loses energy converting from gas to liquid state, and this would rarely happen, unless your skin is very dry and the air very moist (hot and humid days are no fun, are they?), or unless you're exposed to steam (hence the more terrible effects of steam burns versus boiling water burns.) It's better to consider your wind where the air-moisture to skin-moisture balance is such that no energy is gained or lost. Since stronger wind would increase the evaporation rate, that balance would vary based on the wind speed, so consider that at higher speeds your skin would need to be very dry or the air to be very moist to maintain that balance.

See also:

http://en.wikipedia.org/wiki/Enthalpy_of_vaporization

The Bounds of Reason

Since terminal velocity is 120 mph, or about 54 m/s, it is unlikely you would ever be exposed to relative wind very much faster than that, since it would pick you up and carry you along with it. Also, your body needs moisture for you to live, so for it to become very dry, you would have to be dead. Altogether, I would say for you to ever feel a net positive heating from wind would be very rare.

However, in conclusion, yes, you can be burned by the wind.

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minor nitpick - 120mph winds are unlikely to pick up an adult. They are hard to walk in, but a constant 120mph wind is okay. Gusts, tornadoes etc are a whole different kettle of fish :-) –  Rory Alsop Feb 2 at 22:41
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"Very much faster" was the qualifier. –  Aaron Hall Feb 2 at 22:47

it is possible to be burnt by a combination of very warm air (~ 46°c or maybe lower ) and a moderate fan. People in ill health, drunk or unconcious are at most risk. Eg. http://www.sciencedirect.com/science/article/pii/S0003497500013229

Edit: more thoughts. I use these "forced air warming" devices every day at work. The other situation that is assocated with burns is if the hosing delivering the warm air is in contact with an unconscious patient this may lead to a burn - so this is a conduction burn, not a convection burn - but the heat is delivered via conduction. To tie this back to the OP - if a hot air wind is heating a highly conductive surface then contact with this surface may lead to a burn more readily - effectively this conductive surface concentrates the heat. Also tissue that is compressed is more more vulnerable to burns - so if the hot surface is pressed against the skin this speeds up the burning because the body part is unable to carry the heat away with increased blood flow.

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That seems bogus to me. The Bair Hugger that paper quotes looks like it warms through conduction. –  Emilio Pisanty Feb 2 at 20:16
    
it's a warm air fan - with a tissue paper blanket to dissipate the heat - burns have been caused when people have just stuck the warm air hose under the drapes - in this case you can see the burn marks because the blanket had holes letting out little jets of warm air –  ErichBSchulz Feb 2 at 23:30

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