Isotopes differ from atoms in the number of neutrons contained in the atomic nucleus.
The concentration of these isotopes in the water molecules of polar snow and ice depends largely on the temperature of the snow when it fell. For example, the lower the temperature the less O 18 there is in the snow, and consequently there is relatively more O 16 . This leads to a seasonal variation in the isotopes, depending on when the snow accumulates at a site, which has distinct similarities to the area's temperature cycle. Identification of this seasonal variation is one of the simplest ways to date an ice core. The air temperature history for a polar site can also be interpreted from the isotope profile through a snow pit or ice core.