An Isotopic Study on Meteoric Precipitation

Abstract

18O content in cloud droplets is higher in a closed system than in an open system. It is also higher under a non-equilibrium condition than under an equilibrium condition. In case of the frontal type of precipitation, the observed variation with time in18O content agrees fairly well with the calculation using an open model. The change in18O content in convective shower which gives snow pellets can be explained by an open model. The isotopic exchange between falling rain drops and vapor is more effective in enriching heavier isotopes than evaporation. The calculated rate constant of isotopic exchange reaction between falling rain drops and vapor is about half of that given by FRIEDMAN et al. (1962). A smaller amount of isotopic exchange than calculation was observed in rain water. The relationship between D and18O contents in precipitation under an equilibrium condition can be expressed as an approximate straight line starting from the original point with a slope less than (α1D—1)/(α18O-1) in the region of slight isotopic fractionation. The deviations from the straight line are explained by the effect of kinetic processes. The downward shift below the line is due to kinetic evaporation, while the upward shift is due to either the supply of source vapor under a kinetic condition or isotopic exchange between rain drops and vapor which has evaporated under a kinetic condition. A constant term in Craig's equation for the relation between D and18O contents is derived by the fact that18O content in source vapor is lower by 1.2% on an average as compared with D content. © 1968, Japan Meteorological Agency. All rights reserved.