def eta_ratio(M2, M4, M6):
    """Compute the ratio eta using the method of moments

    Ulbrich and Atlas (JAM 1998), Eqn 8

    Parameters
    ----------
    M2: float
        Second moment of the DSD [m^-1]
    M4: float
        Fourth moment of the DSD [m]
    M6: float
        Sixth moment of the DSD [m^3]

    Returns
    -------
    Eta: float
        Ratio of M4^2/(M2*M6) [unitless]

    Notes
    -----
    This particular methodology uses the 2nd, 4th, and 6th moments.
    """
    Eta = M4 ** 2 / (M2 * M6)
    return Eta
