Rit (Vísindafélag Íslendinga) - 01.06.1976, Blaðsíða 40

history of the air masses transporting the moisture, the fraction
already condensed from the clouds, the condensation temperature,
isotopic exchange between vapour and falling droplets, and re-
evaporation of falling droplets. It is clear, therefore, that the isotope
ratio of precipitation must vary from place to place as well as
with time.
In this work we are mostly interested in the stable isotope ratio
of the precipitation from the moment it has deposited as rain or
snow. The fractionation processes in the atmosphere will therefore
not be discussed in any detail, but for more detailed information
the reader is referred to the extensive work of Dansgaard (1953,
1954, 1961 and 1964), Friedman (1953) and Fridman et al. (1964).
Generally, the isotopic variations of the precipication can be sum-
marised as follows:
A. Geographical variations.
1. Latitude effect: Due to preferential removal of the heavier
isotopes from precipitating clouds moving towards higher lati-
tudes, the 8-values decrease (become more negative) with in-
creasing latitude. Dansgaard (1961) and Dansgaard et al.
(1973) give a latitude effect for 8018 on the west coast of
Greenland of the order of — 1%0 per degree of latitude. This
would give a latitude effect for 8D of — 8%0 per degree of lati-
tude. This is in agreement with what is generally to be expected
(Friedman 1964).
2. Altitude and inland effect: For the same reason as in A. 1, one
would expect the 8-values to decrease towards higher altitudes
and with distance from the coast. Data from Greenland show
an altitude effect of — 0.62%o per 100 m for 8018 (Epstein et al.
1959, Dansgaard et al. 1973). The corresponding change for 8D
should be — 4.96%0 per 100 m. Dansgaard (1961) also reported
much lower altitude effect in Norway, or only — 2%0 per 100 m
for O18, corresponding to — 1.6%0 per 100 m for deuterium. In
central Europe and North America an altitude effect of 8D
amounting to — 4%0 per 100 m has been reported, or similar to
that in Greenland (Friedman et al. 1970, Moser et al 1970 and
1974).