Precipitation contains fewer heavy isotopes than oceans and lakes, but this usually doesn't make a difference because the water cycles back to major bodies of water by running off into streams and rivers. When glaciers expand from additional rain and snow, however, the precipitation can remain locked in place for centuries, causing glaciers to act as reservoirs of isotopically light water. How different a glacier's isotopic ratios are from the ocean depends greatly on temperature; colder temperatures mean that fewer heavy isotopes are able to evaporate and the differences in isotopic abundances become more pronounced. As the temperature gets colder, therefore, the 18O/16O and D/H ratios of the ice decrease.
![Precipitation and condensation of water](images/glacier.png)
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