|MadSci Network: Earth Sciences|
Howard, this question has a very simple answer if I take it literally. The PERCENTAGE of oxygen in the atmosphere remains exactly the same at all heights up to about 85 km! It is just over 21% by volume (just under 23% by mass) whether we measure it at sea level, or at the top of Mt Everest! But that does not mean that there is just as much oxygen available to breathe at the top of Mt Everest as there is at sea level! The air as a whole gets thinner as we go higher -- there is less nitrogen and argon as well as less oxygen. As a rough approximation, the AMOUNT of oxygen present goes down by a factor of 10 for each 17 km of altitude increase, a factor of 2 for each 5000 m, or 10% for each 650 to 700 metres. Tables of values for the "Standard Atmosphere" are available in the CRC Handbook of Physics and Chemistry, around about page F200. The quantity that will exactly parallel the amount of oxygen present is the "density". "Pressure" is also a reasonable guide, but not exact because, as well as the amount of gas present, changing temperature also affects the pressure .
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