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I learned that our atmosphere has ~78% Nitrogen and ~21% Oxygen. If I fill a glass with 21% water and 78% Canola Oil, the more dense (and thus heavier) water will take a full bottom later and Canola Oil will sit on top and take the rest of the space.

Why don't we see the same in our atmosphere? Troposphere is the bottom layer, Oxygen is Heavier than Nitrogen; but why is the composition of the bottom layer of our atmosphere still same as the overall composition? What obvious clue am I missing?

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    $\begingroup$ The spoon that keeps stirring? $\endgroup$
    – CuriousOne
    May 6, 2016 at 19:13
  • 1
    $\begingroup$ Does the ethanol float to the top of your beer? $\endgroup$
    – M. Enns
    May 6, 2016 at 19:15
  • $\begingroup$ By way of comparison: Chlorofluorocarbons rising $\endgroup$
    – user59991
    May 6, 2016 at 19:52

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In simple terms, the glass is being constantly shaken by weather.

There isn't very much difference in mass between Oxygen and Nitrogen and the air at low altitudes is dense enough that molecules collide after a very short distance and so the atmosphere is very well mixed.

At very high altitudes where the density is very low there is no weather, and little mixing and so molecules do stratify out by mass.

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