Atmospheric Refraction Correction
This function computes the apparent shift in the solar elevation angle caused by atmospheric refraction. Note that the output is a positive number; i.e. near the horizon, the solar disk appears higher in the sky than its true position. The apparent elevation is the true (geometrical) elevation plus the correction factor.
Reda, I., Andreas, A. Solar Position Algorithm for the Solar Radiation Applications. NREL Technical Report NREL/TP-560-3402