def density_relative(ro_st, ro_air_st):
    """
    Natural gas density relative to standard air density
    :param ro_st: (float) Density of the natural gas at standard parameters, kr/m3
    :param ro_air_st: (float) Density of the air at standard parameters, kg/m3
    :return: (float) Relative density of the natural gas, dimensionless
    """
    return ro_st / ro_air_st