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An atmosphere's cold trap is a layer that is cold enough to freeze a gas rising into it that would otherwise escape. The frozen gas descends again and remains. On Earth, a cold trap at about 20 km traps water vapor.
The term cold trap is also used for a region on the surface of a planet or moon that remain cold enough to keep volatiles frozen, particularly those caused by a permanent shadow.
The term cold trap is also used for technological devices that remove or isolate some gas species from a gas mixture by condensing or freezing it through cooling it and/or changing its pressure.