Altitude Compensation

[Chuck Tomlinson]

Robert J. Harris wrote:
>
>Does anyone do any altitude compensation?  Because, as altitude
>increases, not only does the air mass density decrease, but the
>percentage of oxygen decreases (simple physics - check books
>before questioning or flaming) and the oxygen decrease is independent
>of the density decrease.  

OK, I did a little research on the Web; everything I saw that 
described atmospheric composition said that the composition of the 
troposphere is roughly constant (up to ~10 km) because of thorough 
convective mixing. 

This makes sense to me, and since the highest roads I know of are 
lower than 5 km (16,000') and subject to winds that travel from 
valleys to peaks, I'd expect the air composition to be essentially 
the same as sea level (but less dense, of course).

I'm not flaming you; I'm just curious about *why* the percentage 
of oxygen would change from sea level to 5 km, and if it does, 
by how much. 

Thanks.
--
Chuck Tomlinson.