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.
Heavier isotopes favor the less energetic liquid phase of water during evaporation and condensation. Water vapor is enriched with light isotopes relative to sea water. Clouds are depleted of light isotopes relative to water vapor. This results in higher latitude waters being isotopically "light". As water vapor is driven poleward by Earth's energy budget and rotating Hadley cells, the heavy isotopes are left behind as clouds and rain.
A generalized treatment of kinetic isotopic effects is via the GEBIK and GEBIF equations describing transient kinetic isotope effects.
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