Gives the relative (not pressure-corrected) airmass.
Gives the airmass at sea-level when given a sun zenith angle (in degrees). The
modelvariable allows selection of different airmass models (described below). If
modelis not included or is not valid, the default model is ‘kastenyoung1989’.
zenith (numeric) – Zenith angle of the sun in degrees. Note that some models use the apparent (refraction corrected) zenith angle, and some models use the true (not refraction-corrected) zenith angle. See model descriptions to determine which type of zenith angle is required. Apparent zenith angles must be calculated at sea level.
model (string, default 'kastenyoung1989') –
Available models include the following:
’simple’ - secant(apparent zenith angle) - Note that this gives -inf at zenith=90
’kasten1966’ - See reference  - requires apparent sun zenith
’youngirvine1967’ - See reference  - requires true sun zenith
’kastenyoung1989’ - See reference  - requires apparent sun zenith
’gueymard1993’ - See reference  - requires apparent sun zenith
’young1994’ - See reference  - requries true sun zenith
’pickering2002’ - See reference  - requires apparent sun zenith
airmass_relative (numeric) – Relative airmass at sea level. Will return NaN values for any zenith angle greater than 90 degrees.
 Fritz Kasten. “A New Table and Approximation Formula for the Relative Optical Air Mass”. Technical Report 136, Hanover, N.H.: U.S. Army Material Command, CRREL.
 A. T. Young and W. M. Irvine, “Multicolor Photoelectric Photometry of the Brighter Planets,” The Astronomical Journal, vol. 72, pp. 945-950, 1967.
 Fritz Kasten and Andrew Young. “Revised optical air mass tables and approximation formula”. Applied Optics 28:4735-4738
 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.
 A. T. Young, “AIR-MASS AND REFRACTION,” Applied Optics, vol. 33, pp. 1108-1110, Feb 1994.
 Keith A. Pickering. “The Ancient Star Catalog”. DIO 12:1, 20,
 Matthew J. Reno, Clifford W. Hansen and Joshua S. Stein, “Global Horizontal Irradiance Clear Sky Models: Implementation and Analysis” Sandia Report, (2012).