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

def angstrom_alpha(aod1, lambda1, aod2, lambda2):
    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
    """
    return - np.log(aod1 / aod2) / np.log(lambda1 / lambda2)