![Precipitation and condensation of water](images/waterCycle.png)
The majority of water (H2O) molecules are composed of only 16O and 1H isotopes, but a small percentage contain the heavier stable isotopes 18O and D (2H, or deuterium). When water in oceans and lakes evaporates into the atmosphere, water molecules composed of lighter isotopes evaporate more readily than those with heavier isotopes. As a result, water vapor in the atmosphere is isotopically lighter than ocean water.
When water vapor then condenses to form precipitation, heavier isotopes condense more readily than lighter ones, causing the 18O/16O and D/H ratios of precipitation to be less than that of ocean water, but greater than that of the water vapor. This process is often referred to as isotopic fractionation.
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