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.
