code: def angstroem(ssd, day_length, pot_rad_daily, a, b):
      
      if isinstance(a, pd.Series):
          months = ssd.index.month
          a = a.loc[months].values
          b = b.loc[months].values
  
      glob_day = (a + b * ssd / day_length) * pot_rad_daily
  
      return glob_day
docs: Calculate mean daily radiation from observed sunshine duration according to Angstroem (1924).
  
      Parameters
      ----------
      ssd : Series
          Observed daily sunshine duration.
  
      day_length : Series
          Day lengths as calculated by ``calc_sun_times``.
  
      pot_rad_daily : Series
          Mean potential daily solar radiation.
  
      a : float
          First parameter for the Angstroem model (originally 0.25).
  
      b : float
          Second parameter for the Angstroem model (originally 0.75).