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In the past I thought a breeze of wind was "made" from a large amount of air molecules going all the same direction in the same speed. But it's not like that, actually this voted answer, this site, this other site say actually air that seems still (no wind), the particles are moving at 500 m/s average. This dwarfs completely the biggest winds on earth as said by the voted answer linked before.

I'm sure wind speed and direction (velocity, in one concept) is made out of somehow "averaging" velocity of air molecules. I'm not sure how exactly.

Let's suppose at a certain location I measure 20 m/s speed of wind (say pointing to north, doesn't matter). Would it be correct to say that if you take the linear momentum of all particles in the air at that location (adding the momentum of every particle) and then divide by the total mass (to get velocity units) that would be approximately what one would call "wind velocity".

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Yes, you take the net momentum of all the molecules that net momentum will be (sum of masses of the molecules) * (wind velocity). If there is no wind, then net momentum in any direction should be zero for a given volume.

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  • $\begingroup$ Can you back up this answer with a source? It is written in some book or website? $\endgroup$ – Santropedro Feb 23 '16 at 13:42
  • $\begingroup$ It aint that simple : pep.uni-bremen.de/downloads/bremenlecturesabstract.pdf $\endgroup$ – anna v Feb 23 '16 at 13:56
  • $\begingroup$ @annav thanks, I will pospone reading this after my final, wich is in 15 days. The lectures materials seem to be this, I wonder if you can read and understand them without going to class: pep.uni-bremen.de/service/lecturematerials/index.html. $\endgroup$ – Santropedro Feb 23 '16 at 16:22
  • $\begingroup$ @Santropedro too many lectures. Which is the one relevant to your question? $\endgroup$ – anna v Feb 23 '16 at 17:02
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The 500 m/s is probably the square root of the mean square (rms) velocity and so it is a mean of a quantity with direction not being counted as squaring a vector produces a scalar. So if you have still air the average velocity of its molecules is zero, that is the centre of mass of the air is stationary. When a wind is blowing the velocity of the centre is mass of the air is the wind velocity so the momentum of the air is mass of the air times the velocity of the centre of mass of the air (wind velocity).

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  • $\begingroup$ Can you back up this answer with a source? It is written in some book or website? $\endgroup$ – Santropedro Feb 23 '16 at 13:41
  • $\begingroup$ @Santropedro look at en.wikipedia.org/wiki/Kinetic_theory_of_gases . The temperature of the gas is proportional to the rms kinetic energy of a molecule , v^2 $\endgroup$ – anna v Feb 23 '16 at 17:00

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