Kinetic fractionation
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Kinetic fractionation is a process that separates stable isotopes from each other by their mass during unidirectional processes.
One naturally occurring example of kinetic fractionation is the evaporation of seawater to form clouds. In this instance, isotopically lighter water molecules (i.e., those with 16O) will evaporate slightly more easily than will the isotopically heavier water molecules with 18O.
During the course of this process the oxygen isotopes are fractionated: the clouds become enriched with 16O, the seawater becomes enriched in 18O. Thus, rainwater is observed to be isotopically lighter than seawater.
