MATHEMATICAL MODELING OF VARIATIONS ON STABLE ISOTOPIC RATIOS IN FALLING RAINDROPS^*

The variations of stable isotopic contents in falling raindrops are not only influenced by the humidity conditions, but also by the stable isotopic contents in atmospheric vapor to a certain extent. If there is a difference between the isotopic contents in the vapor of the surrounding air and at the surface of the raindrops, the move of the isotopic contents from high to low values will be produced. Usually. influenced by the evaporation process, the stable isotopic ratios in raindrops are constantly increased in the unsaturated atmosphere. The less the atmospheric humidity, the more obvious the increased range. As the enrichment rate of stable isotopes in raindrops is equal to the outward isotopic move rate. the "pseudo-equilibrium state" appears. The influence of evaporation on stable isotopic contents disappears in the saturated atmosphere, so that the magnitude of isotopic ratio in raindrops is dependent on the isotopic exchange between the raindrops and the surrounding atmosphere.