 The second way in which oxygen isotopes are fractionated is via kinetic processes, for example, evaporation. Evaporation acts on oxygen isotopes because the light isotope of oxygen, oxygen 16, is more readily evaporated from water than oxygen 18. Thus clouds contain more oxygen 16 than oxygen 18, and the rainfall that comes from these clouds also contains more oxygen 16 than oxygen 18, as does the snow that forms from the rain. When you have a warm climate without snow, most of this oxygen 16 that is included in these clouds evaporates, precipitates, and is returned to the oceans. When you have a cold climate, most of the oxygen 16 becomes locked up in ice, and therefore the water, the seawater from which this evaporation ultimately derives, becomes more enriched in oxygen 18. Thus when we have calcite, foramniferal calcite, that forms from seawater, in a cold climate, the foraminifera will tend to be enriched in oxygen 18, whereas in a warm climate, the foraminifera will tend to be enriched in oxygen 16. Likewise, when we have snow and ice forming in a cold climate, it will tend to be enriched in oxygen 16, whereas when we have a little bit of snow or ice forming in a warm climate, it will tend to be more enriched in oxygen 18.