In article ,
wrote:

The relation between altitude and pressure is actually logrithmic, but
up to about 10,000 feet or so the rule of thumb is pretty close.

For a chart of altitude versus pressure up to 100,000 feet, see:

http://www.engineeringtoolbox.com/ai...ure-d_462.html

Everybodies altimeter is the same and the nonlinear nature of the
atmosphere is one of the reasons everyone sets the altimeter to
29.92 in the flight levels.

Now make it even more interesting and convert the two main component of
the air we breath to their partial pressures.