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.

Researchers have created equations that quantify the relationship between isotopic ratios in glaciers and the temperature at the time that layer of the glacier formed. These equations are very complicated, but have proven to be fairly accurate when compared to actual temperature records. Different equations are required for each region of the world due to climatic variations.