pvlib.clearsky.bird(zenith, airmass_relative, aod380, aod500, precipitable_water, ozone=0.3, pressure=101325.0, dni_extra=1364.0, asymmetry=0.85, albedo=0.2)[source]#

Bird Simple Clear Sky Broadband Solar Radiation Model

Based on NREL Excel implementation by Daryl R. Myers [1, 2].

Bird and Hulstrom define the zenith as the “angle between a line to the sun and the local zenith”. There is no distinction in the paper between solar zenith and apparent (or refracted) zenith, but the relative airmass is defined using the Kasten 1966 expression, which requires apparent zenith. Although the formulation for calculated zenith is never explicitly defined in the report, since the purpose was to compare existing clear sky models with “rigorous radiative transfer models” (RTM) it is possible that apparent zenith was obtained as output from the RTM. However, the implentation presented in PVLIB is tested against the NREL Excel implementation by Daryl Myers which uses an analytical expression for solar zenith instead of apparent zenith.

  • zenith (numeric) – Solar or apparent zenith angle in degrees - see note above

  • airmass_relative (numeric) – Relative airmass

  • aod380 (numeric) – Aerosol optical depth [cm] measured at 380[nm]

  • aod500 (numeric) – Aerosol optical depth [cm] measured at 500[nm]

  • precipitable_water (numeric) – Precipitable water [cm]

  • ozone (numeric) – Atmospheric ozone [cm], defaults to 0.3[cm]

  • pressure (numeric) – Ambient pressure [Pa], defaults to 101325[Pa]

  • dni_extra (numeric) – Extraterrestrial radiation [W/m^2], defaults to 1364[W/m^2]

  • asymmetry (numeric) – Asymmetry factor, defaults to 0.85

  • albedo (numeric, default 0.2) – Ground surface albedo. [unitless]


clearsky (DataFrame (if Series input) or OrderedDict of arrays) – DataFrame/OrderedDict contains the columns/keys 'dhi', 'dni', 'ghi', 'direct_horizontal' in [W/m^2].



R. E. Bird and R. L Hulstrom, “A Simplified Clear Sky model for Direct and Diffuse Insolation on Horizontal Surfaces” SERI Technical Report SERI/TR-642-761, Feb 1981. Solar Energy Research Institute, Golden, CO.


Daryl R. Myers, “Solar Radiation: Practical Modeling for Renewable Energy Applications”, pp. 46-51 CRC Press (2013)


NREL Bird Clear Sky Model




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