The amount effect is a term applied to the relationship between isotopic composition of precipitation and monthly rainfall where months with heavy rainfall show different isotopic concentrations than do months with low rainfall.
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In high rainfall months, rain frequency is higher which entails a higher relative humidity in sub-cloud air, hence less evaporation from raindrops. Since the rate of evaporation determines the isotopic concentrations (the greater the rate the higher the heavy stable isotope composition), low rainfall months should show a higher heavy stable isotopic composition than high rainfall months.
- Cameron Wake and Michele Stievenard. THE AMOUNT EFFECT AND OXYGEN ISOTOPE RATIOS RECORDED IN HIMALAYAN SNOW. Glacier Research Group, Institute for the Study of Earth, Oceans and Space University of New Hampshire, Durham, NH 03824 USA
- Physical Oceanography Index