code: def angstrom_alpha(aod1, lambda1, aod2, lambda2):
      r
      return - np.log(aod1 / aod2) / np.log(lambda1 / lambda2)
docs: r"""
      Calculate Angstrom alpha exponent.
  
      Parameters
      ----------
      aod1 : numeric
          first aerosol optical depth
      lambda1 : numeric
          wavelength in nanometers corresponding to ``aod1``
      aod2 : numeric
          second aerosol optical depth
      lambda2 : numeric
          wavelength in nanometers corresponding to ``aod2``
  
      Returns
      -------
      alpha : numeric
          Angstrom :math:`\alpha` exponent for AOD in ``(lambda1, lambda2)``
  
      See also
      --------
      angstrom_aod_at_lambda