>> Atmospheric pressure and, consequently, Oxygen partial pressures,
>> halve every 14,000 feet or so.

> So does that mean that the air is ~85% Nitrogen at 6000 feet? I
> thought that the percentages would stay pretty much the same, just 
> the
> overall pressure dropped with altitude.

No.
Yes indeed.

I'm assuming, quite possibly incorrectly, that it's the absolute 
oxygen levels available (eg O2 partial pressure) that's important. It 
could be that the relative percentage is in fact important as well as 
the absolute PP. I'm basing my assumptions on things I've read on the 
effect of increased O2 levels on burning, and I may have it wrong.

Burning a candle or gas flame or wood or paper in a sealed container 
may provide some guide to this, although increased CO2 percentage is 
going to confuse things. A burning object on a floating platform in a 
'bell jar' inverted over water would give a rough indication of how 
long combustion would continue.


        Russell


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