This will eventually be a proper equation of state, but for now
it's just returns something from wikipedia (i.e. not trustworthy), and
not using humidity.
References
National Oceanographic and Atmospheric Agency, 1976. U.S. Standard
Atmosphere, 1976. NOAA-S/T 76-1562.
(Available as of 2010-09-30 at http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19770009539_1977009539.pdf).