def _fcd_daily(rs, rso):
    """Daytime cloudiness fraction (Eq. 18)

    Parameters
    ----------
    rs : ee.Image or ee.Number
        Measured solar radiation [MJ m-2 d-1].
    rso : ee.Image or ee.Number
        Clear sky solar radiation [MJ m-2 d-1].

    Returns
    -------
    fcd : ee.Image or ee.Number
        Output data type will match "rs" data type.

    Notes
    -----
    fcd = 1.35 * min(max(rs / rso, 0.3), 1.0) - 0.35

    """
    return rs.divide(rso).max(0.3).min(1.0).multiply(1.35).subtract(0.35)
