The liquefaction behaviour of the temporary gases like ${\mathrm{CO}}_2$ approaches that of permanent gases like ${\mathrm{N}}_2,{\mathrm{O}}_2$ etc., as we go

Critical temperature of a gas is highest temperature at which liquefaction of gas first occur. Liquefaction of so called permanent gases (i.e. gases which show continuous positive deviation in Z value) requires cooling as well as considerable compression.
But temporary gases like ${\mathrm{CO}}_2\text{ and }{\mathrm{CH}}_4$ approach that of permanent gases as we go above the critical temperature.