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cooling of matter to temperatures below those of equilibrium phase transition that brings the matter to a new state of aggregation, or another crystalline modification that does not result in a phase transition. A supercooled material is in a state of metastable equilibrium.
The phase transitions that are associated with loss of heat— condensation, crystallization, and polymorphic transformations —usually involve supercooling in their initial stages; the supercooling helps generate nuclei of the new phase in the form of tiny droplets or crystallites. It is difficult to form nuclei at the phase-transition temperature because the nuclei feature higher pressure or solubility and thus cannot be in equilibrium with the initial phase at this temperature. Under conditions where the generation and growth of nuclei of a new phase are inhibited, for example, during recrystallization of a solid and during crystallization of a highly viscous liquid, deep supercooling can be used to obtain a practically stable phase whose internal structure resembles that of phases at higher temperatures. This procedure is the basis for steel-quenching processes and for the production of glass. The degree to which water vapor is supercooled in the atmosphere determines whether precipitation will occur in the form of rain, snow, or hail.