def calc_rso(ra, elevation):
    """Clear-sky solar radiation [MJ m-2 day-1].

    Parameters
    ----------
    ra: pandas.Series, optional
        Extraterrestrial daily radiation [MJ m-2 d-1]
    elevation: float, optional
        the site elevation [m]

    Returns
    -------
    pandas.Series containing the calculated Clear-sky solar radiation

    Notes
    -----
    Based on equation 37 in [allen_1998]_.

    """
    return (0.75 + (2 * 10 ** -5) * elevation) * ra