Why is there cold wind?

  • #1

Main Question or Discussion Point

So awhile ago I was thinking about wind.

Sometimes wind is hot but when I think of wind I usually think of cold wind.
Wind seems to be fast moving particles and I know based on the kinetics theory of gases

[tex] KE = \frac{1}{2}mv^2 = \frac{3}{2}kT [/tex]

Maybe, I'm making a wrong assumption but to me this seems that if a particle of gas has a high KE and high v that it will have a high T

So why would there be cold wind. Shouldn't all high velocity wind be hot?

Through my investigation of this I have gotten some info on how wind is created.

From what I can gather

The sun heats the earth unevenly
The atmosphere of the earth that is getting heated
and the hot air rises because it becomes less dense

(here is where I get a little confused and maybe making wrong assumptions)

this causes less pressure in that area of the atmosphere
causing nearby particles of air to fill the uneven pressure (partial vacuum the rising hot air created?)

and so you have wind!

To me it seems if the gases filling the uneven pressure have a low T that it would seem plausible that we would have cold wind BUT wouldn't that air over time heat up because of it's increase in KE?

I know this looks like a wall of text now but this is my thought process
 

Answers and Replies

  • #2
russ_watters
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Welcome to PF!

1. Bulk motion is not included in the temperature.
2. Can you calculate at what speed, the KE would make a noticeable difference if converted to heat?
 
  • #3
SteamKing
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Also, the pressure of the atmosphere is not evenly distributed at any one time. If a region of low pressure develops, a portion of the atmosphere from a region of higher relative pressure will move in. This movement will also create wind.
 
  • #4
CWatters
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Sometimes wind is hot but when I think of wind I usually think of cold wind.
Clothes and hair trap a layer of warm air next to your skin. Wind can remove this layer of warm air making it feel colder than it would otherwise.

More..

http://en.wikipedia.org/wiki/Wind_chill
 
  • #5
D H
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Wind seems to be fast moving particles and I know based on the kinetics theory of gases

[tex] KE = \frac{1}{2}mv^2 = \frac{3}{2}kT [/tex]

Maybe, I'm making a wrong assumption but to me this seems that if a particle of gas has a high KE and high v that it will have a high T
You're right. You are making a wrong assumption. Velocity and energy are frame-dependent quantities -- change the frame of reference and you get different velocities, different kinetic energy. So in which frame are those velocities expressed? The answer is that it is a rest frame of the gas, a frame in which the total momentum is zero.

In other words, wind velocity doesn't come into play in determining temperature in the kinetic theory of gases.
 
  • #6
rcgldr
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Clothes and hair trap a layer of warm air next to your skin. Wind can remove this layer of warm air making it feel colder than it would otherwise.
There's also the effect of evaporation if a person is sweating, and the effect of relative humidity.

If the air temperature is around 100°F ~= 38°C or higher, the wind will make it fill hotter still.

http://www.zunis.org/at_least_theres_a_breeze.htm
 

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