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Any substance existing in the gaseous state at a temperature lower than that of its critical point; that is, a gas cool enough to be liquefied if sufficient pressure were applied to it.

If any vapor is cooled sufficiently, say at constant pressure, it ultimately reaches a state of saturation such that further removal of heat is accompanied by condensation to the liquid phase. Except for states quite close to that of saturation, vapors exhibit the general properties of all gases. Quantitatively, however, vapors exhibit measurable departures from perfect-gas laws even in states well removed from that of saturation. Since the critical temperature for water (374°C) is far above any atmospheric temperatures (except for the extreme upper air), all water substance found in the atmosphere in the gaseous state is appropriately called water vapor.

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