# What is cold wind?

As per my understanding, temperature is the movement of particles in an environment. A highly energetic environment where particles possess high energy has a high temperature, and low energy means low temperature.

Wind in general means speed and energy in my opinion, so what does cold wind mean? How can it exist? Is it a bunch of super stable particles moving together? Does it move slowly? Air conditioning transfers heat okay, but what happens at the particle level? Do particles suddenly stop moving/less movement when they hit the heat transferring element-how?

• Are you just asking the difference between macro-scale motion and the molecular motion associated with temperature? Nov 7 '21 at 2:08
• @DKNguyen, I believe so. I'm not very sure myself, if it is purely macro scale, what happens at heat exchangers? Nov 7 '21 at 2:13
• I don't know where to put this comment. But it integrates the answer. After reading them, you should have also concluded that a warm wind (unusual it could be, otherwise think of a hair dryer) isn't warm on your skin because blowing fast. Nov 7 '21 at 13:31
• Related (and possibly answers the question): Why am I not burned by a strong wind? Nov 7 '21 at 15:18
• Nov 9 '21 at 0:51

Technically, it all is the same motion. The difference is magnitude and direction and how you separate out the superposition of them.

Temperature is a result of the components of motion (vectors) of each individual air molecule with a net translational movement of zero over time. This motion is random and all over the place in many directions as they move around, collide and bounce off each other and objects. This speed is apparently around 500m/s, just to give you an idea: https://pages.mtu.edu/~suits/SpeedofSound.html

Wind is the collective motion of a mass of randomly moving air molecules: the average velocity of the actual motion of the individual air molecules. Averaging their velocities cancels out the vectors that point in "random" directions which lead to net zero motion, leaving only the collective translational motion remaining. Obviously, wind is usually not 500m/s.

So, typically, the random net-zero impact vectors between air molecules and objects are much higher speed than those due to the collective/average motion of the air molecules (wind). This means the net-zero impact vectors dominate over the translational impact vectors in kinetic energy transfer (heat) in either direction into or out of your body.

But if you're an SR-71 flying through the air at Mach 3, the relative wind speed is so high that the vectors of the translational motion of the air molecules is much larger than that of the net zero, random motion. The result is a net heating of the aircraft skin due to impacts from the wind even if the random velocities of the individual air molecules make for very cold air that would otherwise cool down the airplane's skin.

So a cold wind would be one where the kinetic vibratory motion (thermal motion) of the molecules in your body is higher than the random motion of the individual air molecules such that energy is transferred out of your body into the air from the collisions rather than into it, and impacts vectors for the average motion of the air (wind) is too slow to to transfer energy from the air into your body and only replaces the air molecules heated up by your body with fresh, cold air molecules.

• Fun fact: SR-71 pilots used to heat up their meals by pressing them against the windshield. Nov 7 '21 at 9:47
• @JörgWMittag Still seems safer than blowing on pitot tubes so the aircraft in the hangar thinks it's flying so you can activate the radar dish to heat your lunch. Nov 8 '21 at 2:46
• There is still some scale separation in between and that's wht the concept of continuum is so handy. Even in the tiniest vortices in a typical turbulent flow are much larger than the molecule mean free path (=> Knudsen number). You term air particles is extremely confusing as it often means the same as an air of fluid parcel, tracked in the Lagrangian decription of fluid flow, which indeed chaotically moves in turbulence, but is still well described as continuum with a well defined temperature. Nov 8 '21 at 8:34
• @VladimirF I meant to say molecule, not particle. Would that remove the confusion? Nov 8 '21 at 14:13
• @VladimirF: The above-answer's use of the term "particle" was correct. You're thinking of a fluid-parcel.
– Nat
Nov 8 '21 at 17:39

Temperature is related to motion, yes. And wind is motion. But numbers matter.

A particle with temperature $$T$$ (in absolute units, like Kelvin) will typically have a kinetic energy of approximately $$kT$$, where $$k$$ is Boltzmann’s constant. For nitrogen and oxygen molecules at room temperature, the typical speed associated with this typical kinetic energy is a few hundred meters per second. That’s an order of magnitude faster than the winds in Earth’s fastest storms. Thermal motion is closer to the speed of sound than the speed of wind. (Why is the speed of sound close to the mean thermal speed? That’s a fun question to try and figure out on your own.)

We don’t ordinarily care very much about the speeds of individual molecules in the air, because their mean free path is very short: less than 100 nanometers for air at room temperature. Even though those molecules are zooming around, they never go very far before they change direction.

If I light a match across the room from you, you’ll hear the noise within milliseconds, because the pressure wave to which your ear responds travels at approximately the mean thermal speed. But you might not smell the smoke from the match for many seconds, because the smelly molecules to which your nose responds have to make their way over to you by diffusion.

A “wind” is collective motion of all the molecules in the gas. Each molecule in the wind has a thermal motion whose direction is almost random, with only a slight preference for the downwind direction. Because the thermal motion is so much faster than the wind speed, there is plenty of room for the temperature of the collectively-moving air to be lower than the temperature of the collectively-stationary air which it is displacing.

• "Why is the speed of sound close to the mean thermal speed? That’s a fun question to try and figure out on your own." Is it because sound travels via the vibrations of air molecules, and so the vibrational speed(thermal speed) is almost equal to the speed of the sound wave? Nov 7 '21 at 13:27
• That’s enough of the gist to look for a derivation of sound speed and see how the temperature shows up.
– rob
Nov 7 '21 at 14:07
• My limited knowledge as a 10th grader just lets me get the "gist" of the question as of now :') Nov 7 '21 at 14:12
• Hmmm, yes, the derivation (e.g. from Feynman, or in a more Newtonian way from NASA, both summarized at this Physics.SE question) is not very 10th-grader-friendly. You can see $v_\text{sound} \propto \sqrt T \propto v_\text{thermal}$ at the end, but there is a fair amount of calculus-magic in the middle.
– rob
Nov 7 '21 at 21:26

There are already two amazing answers above, but I think explaining it more simply might be helpful:

• Molecules in wind have vibrational motion as well as translational motion.

• The vibrational speed is higher than the translational speed(in general situations).

• The more the vibrational speed, more is the kinetic energy in the molecules, hence more the heat content and temperature of the wind.

• So the temperature depends on the vibrational motion and not the translational motion, the latter which we feel as blowing wind, and we feel the vibrational motion as heat. The translational motion is independent of vibrational motion in the wind molecules.

• If the vibrational speed is more than the vibrational speed of molecules in our skin, we feel warmer as heat travels from wind to our skin, and the other way round for the opposite.

• I would have to check the heat capacities to be sure, but I believe that, at room temperature, nitrogen and oxygen molecules are all in their vibrational ground states. However, the rotational degrees of freedom are available.
– rob
Nov 7 '21 at 14:06
• On re-read, I think you’re using “vibrational speed” in a way that is appropriate for a solid, but not for a gas.
– rob
Nov 7 '21 at 14:10
• Oh, I just wanted to explain the gist in an extremely simplified way @rob Nov 7 '21 at 14:12
• there is also berownian motion, which is translational and is experienced as air pressure, but is not necessarily directional. Nov 8 '21 at 1:58
• @DKNguyen because temperature is the translational KE. Molecules can be complex systems that store energy in other modes, and energy is shared between modes. Water, in particular, can spin and can vibrate in several different ways. You can see that the spinning H of the asymmetric molecule can act like a baseball bat when it bumps into another molecule. It soaks up energy in the other modes, as collisions set it spinning and vibrating rather than moving through space. Nov 9 '21 at 16:09