Bird and Hulstrom's Solar Irradiance Model Richard E. Bird and Roland L. Hulstrom A Simplified Clear Sky Model for Direct and Diffuse Insolation on Horizontal Surfaces. SERI/TR-642-761, Solar Energy Research Institute, Golden, Colorado, USA, February 1981. This model is widely used for estimating broadband clear sky irradiance at Earth's surface. It is based on empirical representations of radiative transfer equations and attempts to take into account the cumulative effects of aerosols, water vapor, ozone and other gases, and Rayleigh (molecular) scattering upon sunlight reaching Earth's surface. I have substituted my own calculations for solar zenith angle and Earth-sun distance for the calculations included in published versions of code for the Bird model -- not because they are necessarily better, but because I had already written the code. However, I have retained the original calculation for relative air mass, which will be significantly different from my own calculations only at solar zenith angles very close to 90°.

Inputs: Quantity Units Value Mean (1 AU) solar constant W/m2 Longitude degrees (West is "-") Latitude degrees (North is "+") Elevation meters Month 1-12 Day 1-31 Year 20xx Hour 1-24 UT Minute 1-59 Second 1-59 Sealevel ("weather report") pressure millibars Surface albedo (dimensionless) Total column ozone atm-cm Total precipitable water vapor cm H2O Aerosol optical depth @ 500 nm (dimensionless) Aerosol optical depth @ 380 nm (dimensionless)

Outputs: Quantity Units Value Julian Date Days Station pressure mbar Earth/sun distance AU Solar zenith angle degrees Relative air mass (dimensionless) Distance-corrected solar constant W/m2 Direct irradiance on horizontal surface W/m2 Diffuse irradiance W/m2 Total irradiance on horizontal surface W/m2