def stress_radiation(ra_24):
    """
    Computes the stress for plants when there is not sufficient radiation

    .. math ::
        S_{r}=\\frac{S^{\\downarrow}}{\\left(S^{\\downarrow}+60.\\right)}
        \\left(1+\\frac{60}{500}\\right)

    Parameters
    ----------
    ra_24 : float
        daily solar radiation
        :math:`S^{\\downarrow}`
        [Wm-2]

    Returns
    -------
    stress_rad : float
        stress factor for radiation
        :math:`S_{r}`
        [-]


    Examples
    --------
    >>> import ETLook.stress as stress
    >>> stress.stress_radiation()
    0.0
    >>> stress.stress_radiation(500)
    1.0
    >>> stress.stress_radiation(700)
    1.0
    >>> stress.stress_radiation(250)
    0.90322580645161288
    """
    stress = ra_24 / (ra_24 + 60.0) * (1 + 60.0 / 500.0)
    stress = stress.clip(0, 1)
    return stress
