def relative_shortwave_radiation(rs, rso):
    """
    Calculate the ratio of the solar radiation to the clear-sky solar
    radiation (Allen et al. 1998).

    Parameters
    ----------
    rs : float
        Solar radiation (MJ m-2 day-1).
    rso : float
        Clear-sky solar radiation (MJ m-2 day-1).

    Returns
    -------
    float
        Relative shortwave radiation (-).

    """
    return rs / rso