<p>It's because gases also diffuse. If you separate two gases of different densities by a horizontal membrane, and then slowly remove the membrane, then the interface will diffuse. You can try this with bromine and air, for example&nbsp;&ndash; the bromine will stay at the bottom (easily visible because it's brown) and the air will stay at the top, but the interface will be diffuse. The bromine will largely stay at the bottom because it's <em>significantly</em> heavier than the air, but the diffusive mixing will be much stronger if the density difference is smaller&nbsp;&ndash; as is the case with nitrogen and oxygen, which are of course not very different in density.</p>

<p>While the effect above just considers mixing due to molecular diffusion, the atmosphere of course also vigorously convects due to the effects of thermal heating (and humidity differences). Both also turn over the atmosphere and mix it.</p>

<p>That said, the composition of the atmosphere does change with altitude. See, for example, <a href="http://wordpress.mrreid.org/2014/08/01/the-composition-of-earths-atmosphere-with-elevation/" rel="nofollow">here</a>. The thing is that the thickness of layer corresponding to the blurry interface between bromine and air is about as thick as the entire atmosphere, and therefore hard to distinguish from other effects (such as ionization) that change its composition with altitude.</p>

CC BY-SA 3.0
2016-02-01T05:08:00.857Z
Point taken that O2 and N2 have very similar mass. However, looking at the chart you linked to only makes me marvel all the more. How in the **world** does the composition mixture ratios stay so similar all the way out to 100 km and more? Violent winds weather only exist in the troposphere AFAIK. Do convection cells really extend up to 100+ km? Can diffusion and convection really mix it up that far out?
DrZ214
CC BY-SA 3.0
2016-02-01T10:47:07.083Z
@DrZ214 The stratosphere and mesosphere have plenty of time.
gerrit
CC BY-SA 3.0
2016-02-01T10:48:23.647Z
@gerrit What does that mean exactly? If it has plenty of time to mix, then it has plenty of time to stratify by gravity too.
DrZ214
CC BY-SA 3.0
2016-02-01T10:50:22.043Z
@DrZ214 True, but up to the [homopause](http://scienceworld.wolfram.com/astronomy/Homopause.html) (by definition), mixing wins.
gerrit
CC BY-SA 3.0
2016-02-01T16:33:31.517Z
@DrZ214 the atmosphere above the troposphere has plenty of strong wind. Speeds above 100 km/h are no rarity. The true reason for the demixing probably lies in the overall loss of interaction due to macroscopic free paths. Major vertical convection does indeed extend up to the Mesopause (~95 km).
Chieron
CC BY-SA 3.0
2016-02-01T16:37:06.160Z
BTW, that linked blogpost about the atmospheric composition should be taken with a grain of salt, casually stating that the Ozone layer lies around 100km altitude is truly an astounding error. (it's in the stratosphere around 25-35 km)
Chieron
CC BY-SA 3.0
2016-02-01T23:11:09.540Z
@Chieron I didn't see any O3 in the charts. I did see in the text it says stuff like an increase in ozone after 100 km, and a "domination" of ozone by 200 km. It's not talking about by mass. Everything it says is by **relative concentration.** So for all I know, above 200 km there is 99% ozone, just super super thin, but at 30 km, there is maybe 5% ozone but much much thicker. You would have to go by mass to see that the greatest number of ozone molecules indeed lay around 30 km.
DrZ214
CC BY-SA 3.0
2016-02-02T00:38:59.557Z
@Chieron: "Major vertical convection does indeed extend up to the Mesopause (~95 km)."  This can't be true for the stratosphere, as defined by its inverse temperature gradient. Convection only works for positive temperature gradients. Also strong winds are unstable which leads to turbulent mixing.
AtmosphericPrisonEscape
CC BY-SA 3.0
2016-02-02T08:01:10.253Z
@AtmosphericPrisonEscape the Brewer-Dobson circulation extends to the Mesopause (it's the reason, why it's so cold up there in the summer). Vertical winds are much slower than horizontal winds, though. Contrary to what one would expect, the stratosphere is actually quite turbulent.
Chieron
CC BY-SA 3.0
2016-02-02T08:05:08.477Z
@DrZ214 in great heights, there will be atomar oxygen, ozone would require collisions and those just aren't happening frequently enough. In any case, the so called ozone layer has nothing to do with the homosphere/heterosphere and mixing, as the linked site suggested (I'm speaking about the text, not the graph, which seems about correct).
Chieron
CC BY-SA 3.0
2016-02-02T15:26:44.860Z
@Chieron  thank you, didn't know about this BD circulation. It is not a convection-driven circulation however, unlike stated earlier.
AtmosphericPrisonEscape
CC BY-SA 3.0
2016-02-01T03:15:31.553Z
2016-02-03T20:29:57.190Z
2016-02-03T20:29:57.190Z
Wolfgang Bangerth
<p>Diffusion is important, but much more important is the turbulent stirring from all manner of processes such as convective clouds, hurricanes, Hadley cells, frontal systems, jet streams, etc. This is more than enough to keep the atmosphere's component gasses well mixed.</p>

CC BY-SA 3.0
2016-02-02T08:23:52.320Z
2016-02-02T08:23:52.320Z
Gordon Stanger
<p>Consider a gas centrifuge holding two different gases. After spinning for a while, the heavier gas will move to the outside, and the lighter gas will be on the inside. In other words, we have stratified the gases.</p>

<p>Why hasn't this happened in Earth's atmosphere for N$_2$ and O$_2$?</p>

<p>Note: I'm <strong>not</strong> considering Earth a centrifuge because it's rotating. I'm comparing it to a centrifuge because there's a weight (gravity) on the gases. In this case, the oxygen should settle to the bottom, and the nitrogen above it.</p>

<atmosphere><atmospheric-chemistry>
What keeps the different gases mixed in the atmosphere?
DrZ214
# What keeps the different gases mixed in the atmosphere?

(Asked by: username_0 on 2016-02-01)

Consider a gas centrifuge holding two different gases. After spinning for a while, the heavier gas will move to the outside, and the lighter gas will be on the inside. In other words, we have stratified the gases.

Why hasn't this happened in Earth's atmosphere for N$\_2$ and O$\_2$?

Note: I'm **not** considering Earth a centrifuge because it's rotating. I'm comparing it to a centrifuge because there's a weight (gravity) on the gases. In this case, the oxygen should settle to the bottom, and the nitrogen above it.

**Tags:** atmosphere, atmospheric-chemistry


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## 2 answers

### Answer by username_1 on 2016-02-01. Score: 15

It's because gases also diffuse. If you separate two gases of different densities by a horizontal membrane, and then slowly remove the membrane, then the interface will diffuse. You can try this with bromine and air, for example – the bromine will stay at the bottom (easily visible because it's brown) and the air will stay at the top, but the interface will be diffuse. The bromine will largely stay at the bottom because it's *significantly* heavier than the air, but the diffusive mixing will be much stronger if the density difference is smaller – as is the case with nitrogen and oxygen, which are of course not very different in density.

While the effect above just considers mixing due to molecular diffusion, the atmosphere of course also vigorously convects due to the effects of thermal heating (and humidity differences). Both also turn over the atmosphere and mix it.

That said, the composition of the atmosphere does change with altitude. See, for example, [here](http://wordpress.mrreid.org/2014/08/01/the-composition-of-earths-atmosphere-with-elevation/). The thing is that the thickness of layer corresponding to the blurry interface between bromine and air is about as thick as the entire atmosphere, and therefore hard to distinguish from other effects (such as ionization) that change its composition with altitude.

> **Comments:**
> **username_0**: Point taken that O2 and N2 have very similar mass. However, looking at the chart you linked to only makes me marvel all the more. How in the **world** does the composition mixture ratios stay so similar all the way out to 100 km and more? Violent winds weather only exist in the troposphere AFAIK. Do convection cells really extend up to 100+ km? Can diffusion and convection really mix it up that far out?  
> **username_2**: @username_0 The stratosphere and mesosphere have plenty of time.  
> **username_0**: @username_2 What does that mean exactly? If it has plenty of time to mix, then it has plenty of time to stratify by gravity too.  
> **username_2**: @username_0 True, but up to the [homopause](http://scienceworld.wolfram.com/astronomy/Homopause.html) (by definition), mixing wins.  
> **username_3**: @username_0 the atmosphere above the troposphere has plenty of strong wind. Speeds above 100 km/h are no rarity. The true reason for the demixing probably lies in the overall loss of interaction due to macroscopic free paths. Major vertical convection does indeed extend up to the Mesopause (~95 km).  
> **username_3**: BTW, that linked blogpost about the atmospheric composition should be taken with a grain of salt, casually stating that the Ozone layer lies around 100km altitude is truly an astounding error. (it's in the stratosphere around 25-35 km)  
> **username_0**: @username_3 I didn't see any O3 in the charts. I did see in the text it says stuff like an increase in ozone after 100 km, and a "domination" of ozone by 200 km. It's not talking about by mass. Everything it says is by **relative concentration.** So for all I know, above 200 km there is 99% ozone, just super super thin, but at 30 km, there is maybe 5% ozone but much much thicker. You would have to go by mass to see that the greatest number of ozone molecules indeed lay around 30 km.  
> **username_4**: @username_3: "Major vertical convection does indeed extend up to the Mesopause (~95 km)."  This can't be true for the stratosphere, as defined by its inverse temperature gradient. Convection only works for positive temperature gradients. Also strong winds are unstable which leads to turbulent mixing.  
> **username_3**: @username_4 the Brewer-Dobson circulation extends to the Mesopause (it's the reason, why it's so cold up there in the summer). Vertical winds are much slower than horizontal winds, though. Contrary to what one would expect, the stratosphere is actually quite turbulent.  
> **username_3**: @username_0 in great heights, there will be atomar oxygen, ozone would require collisions and those just aren't happening frequently enough. In any case, the so called ozone layer has nothing to do with the homosphere/heterosphere and mixing, as the linked site suggested (I'm speaking about the text, not the graph, which seems about correct).  
> **username_4**: @username_3  thank you, didn't know about this BD circulation. It is not a convection-driven circulation however, unlike stated earlier.  

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### Answer by username_5 on 2016-02-02. Score: 2

Diffusion is important, but much more important is the turbulent stirring from all manner of processes such as convective clouds, hurricanes, Hadley cells, frontal systems, jet streams, etc. This is more than enough to keep the atmosphere's component gasses well mixed.


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