Simple Model of the Atmospheric Radiative Transfer of Sunshine

The Simple Model of the Atmospheric Radiative Transfer of Sunshine (SMARTS) is a computer program designed to evaluate the surface solar irradiance components in the shortwave spectrum (spectral range 280 to 4000 nm) under cloudless conditions. The program, written in FORTRAN, relies on simplifications of the equation of radiative transfer to allow extremely fast calculations of the surface irradiance.

Source: Wikipedia — Simple Model of the Atmospheric Radiative Transfer of Sunshine (CC BY-SA 4.0)