# Why does the composition of the air does not change with altitude?

Air contains about 78% nitrogen and 21% oxygen independent of altitude (up to 100 km). Why is this? Shouldn't the concentration of nitrogen increase with higher altitudes since nitrogen has a lower density than oxygen?

• why do you think, that it doesn't? The picture is not sooo precise, is it? – Ilja Apr 5 '16 at 19:34
• Because there are large mixing terms from, e.g. updrafts, weather systems, etc. – Jon Custer Apr 5 '16 at 19:34
• Like Ilja, I doubt the accuracy of those plots; keep in mind that O$_2$ is only 10% heavier than N$_2$ (and this gradient is fighting against entropic forces) so you wouldn't expect a large difference in any case. – lemon Apr 5 '16 at 19:39
• the air molecules only widen there gap between each other as the altitude heightens. – user5434678 Apr 5 '16 at 19:48
• @SeanLake -- Apparently it's wordpress.mrreid.org/2014/08/01/… Note that the ordinate in this graph is percent composition and that the total falls below 100% at 100 km or so. That means our atmosphere is something else than molecular nitrogen, molecular oxygen, argon, and other trace species above 100 km. The author of that blog post used the NASA MSIS E-90 model (there are others) as the basis for his plot. – David Hammen Sep 20 '16 at 10:28