The Joule-Thomson effect is a physical process in which the temperature of a gas is decreased by letting the gas expand adiabatically. It finds application in the Linde technique as a standard process in the petrochemical industry for example, where the cooling effect is used to liquefy gases.
In general, when a gas expands adiabatically, the temperature may either decrease or increase, depending on the initial temperature and pressure. For a fixed pressure, a gas has a Joule-Thomson inversion temperature, above which expansion causes the temperature to rise, and below which expansion causes cooling. For most gases, at atmospheric pressure this temperature is fairly high (above room temperature), and so gases can be cooled by expansion. One notable exception is helium, whose Joule-Thomson inversion temperature at one atmosphere is about 40 K (−233 °C). The only other gas which warms upon expansion at standard conditions is hydrogen.
The change of temperature (ΔT) with respect to change of pressure (Δp) in a Joule-Thomson process is the Joule-Thomson coefficient 
; the Joule-Thomson inversion temperature is the temperature where it changes sign.
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