def _calc_num_density_conversion():
    """Calculate conversion factor from PWV * cross section to optical depth

    Returns:
        The conversion factor in units of 1 / (mm * cm^2)
    """
    n_a = 6.02214129e+23
    h2o_molar_mass = 18.0152
    h2o_density = 0.99997
    one_mm_in_cm = 10
    mm_to_num_dens = n_a * h2o_density / (h2o_molar_mass * one_mm_in_cm)
    return mm_to_num_dens
