
Standard Temperature and Pressure (STP) for a gas is defined by IUPAC as
0° C (273.15 K), 100 kPa. This differs from an earlier definition that used
1 atm (101.325 kPa) as the pressure reference. It also is different in
Imperial and USA systems of units.

Normal Temperature and Pressure (NTP) for a gas is defined
as 20 ° C, 1 atm. This standard is often more
convenient, e.g. for testing fans.

Other standards abound, and the definitions are not necessarily consistent.
See
www.engineeringtoolbox.com/stpstandardntpnormalaird_772.html
en.wikipedia.org/wiki/Normal_temperature_and_pressure
In these pages, we follow the early densityeffect papers of Sternheimer,
Barger, and Seltzer, who tabulate the gas density at NTP, (20°, 1 atm).
Note that the Handbook of Chemistry and Physics reference
conditions are 25°, 1 atm.
Indices of refraction are taken from
www.kayelaby.npl.co.uk/general_physics/2_5/2_5_7.html
where they are given for dry gases at 0° and 1 atm, evaluated at the
mean of the Na D doublet, _{} 589.3 nm.
A calculator for the index of air can be found at
emtoolbox.nist.gov/Wavelength/Documentation.asp,
where the wavelength, temperature, pressure, humidity, and
CO_{2} content can
also be varied.
Index and dispersion for some gases can be found at refractiveindex.info,
The reference pressure is usually not given
(assumed to be 1 atm?). T is usually (but not
always) 0° C. In some case, e.g. Kr, only UV data are given.
Written 2014 April 1
web pages as of Apr 2014