Rit (Vísindafélag Íslendinga) - 01.06.1976, Síða 39

On the other hand, condensation processes in the atmosphere
have not been found to be affected appreciably by kinetic effects.
Condensation is the most important process governing the isotope
ratio of the precipitation. When air saturated with water vapour
is cooled either by radiation or adiabatically hy expansion due to
rising, some vapour is condensed.
Assuming that the condensation proceeds under Rayleigh condi-
tions, i.e. that the condensation is very slow and the condensate is
immediately removed from the vapour after condensation, then
according to Dansgaard (1964), the resulting 8-values of the vapour
phase, 8V, and of the condensate, 80, will be as follows:
8V = — • Fv (“m ~ 1 ^ - 1
«0
8C = W • Ft (<Xm ~ 1 ' - 1
n„
The fractionation constants ao, a and am refer to the initial tem-
perature, t0, to the momentary temperature, t, and to tm = (t+t0) /2
respectively. Fv is the remaining fraction of the vapour phase.
On the other hand, if we assume that the condensation is a closed
two-phase system, i.e. the two phases are allowed to equilibrate by
exchange, the 8-values for the total liquid and vapour phase 8C* and
8V* will, according to Dansgaard (1964), change as follows:
The real situation for the condensation processes in nature will
lie somewhere between the two above extremes, i.e. the Rayleigh
conditions are more or less smoothed out by exchange.
Obviously the isotope ratio of the precipitation depends on vari-
ous factors, such as the origin of the water vapour, the paths and
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