Deuterium and oxygen 18 in rain water

The D and O18 contents of rain water are known to vary considerably with time and location. Dansgaard found a good correlation of the mean O18 content of rain with mean air temperature, which causes a latitudinal as well as a seasonal variation of O18 and a similar one for D. Under dry climatic conditions, as in South Africa, the D content appears to be strongly influenced by re-evaporation of the falling raindrops. This has the effect of obscuring the normal seasonal variations. The evaporation obviously takes place under kinetic conditions, thus increasing the separation factors especially for the oxygen isotopes. A detailed study of a thunderstorm in Heidelberg shows that here too evaporation causes an enrichment in D.