Unlike ideal gases, the temperature of a real gas will change during a Joule expansion. At temperatures below their inversion temperature gases will cool during Joule expansion, while at higher temperatures they will heat up. The inversion temperature of a gas is typically much higher than room temperature; exceptions are helium, with an inversion temperature of about 40 K, and hydrogen, with an inversion temperature of about 200 K. Since the internal energy of the gas during Joule expansion is constant, cooling must be due to the conversion of internal kinetic energy to internal potential energy, with the opposite being the case for warming.