Determine absolute (pressure corrected) airmass from relative airmass and pressure
Gives the airmass for locations not at sea-level (i.e. not at standard pressure). The input argument “AMrelative” is the relative airmass. The input argument “pressure” is the pressure (in Pascals) at the location of interest and must be greater than 0. The calculation for absolute airmass is\[absolute airmass = (relative airmass)*pressure/101325\]
airmass_relative (numeric) – The airmass at sea-level.
pressure (numeric, default 101325) – The site pressure in Pascal.
airmass_absolute (numeric) – Absolute (pressure corrected) airmass
 C. Gueymard, “Critical analysis and performance assessment of clear sky solar irradiance models using theoretical and measured data,” Solar Energy, vol. 51, pp. 121-138, 1993.