Experiments were conducted on low-density supersonic axisymmetric freejets of dry air flowing past cylindrical models of flat and hemispherical heads to relate the shock formation, Knudsen number and the Bird's freezing parameter. Glow discharge images showed that for different Knudsen numbers, the shock formation was not considerably different at constant operating pressure. Results revealed that Bird's freezing parameter can be used along with the Knudsen number for the determination of shock formation in low-density flows.