def refractive_index_broadband_vapour(sigma):
    """

    Parameters
    ----------
    sigma : {float, numpy.array}, unit=µm-1
        vacuum wavenumber, that is the reciprocal of the vacuum wavelength

    Returns
    -------
    n_ws : {float, numpy.array}
        refractive index for standard moist air at sea level

    References
    ----------
    .. [1] Owens, "Optical refractive index of air: Dependence on pressure,
       temperature and composition", Applied optics, vol.6(1) pp.51-59, 1967.
    """
    cf = 1.022
    w_0, w_1, w_2, w_3 = 295.235, 2.6422, -0.032380, 0.004028
                                                            # eq. 13 in [1]
    n_ws = cf*(w_0 + w_2*(sigma**2)+ w_2*(sigma**4)+ w_3*(sigma**6))
    n_ws /= 1E8
    n_ws += 1
    return n_ws