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. (A PDF of this document may
be available at
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19770009539_1977009539.pdf
or
http://www.dtic.mil/cgi-bin/GetTRDoc?Location=U2&doc=GetTRDoc.pdf&AD=ADA035728
although neither link has proven to be reliable.)