As ¹⁶O-enriched vapor moves from the tropics toward the poles, it cools and condenses. Each precipitation event preferentially removes H₂18O from the remaining vapor (because it condenses more readily). By the time air masses reach polar regions, the residual moisture is extremely depleted in 18O.
This progressive depletion follows Rayleigh distillation: with each condensation step, the vapor becomes lighter — more negative in δ18O. Adjust the sliders below to see how source temperature and number of rainout events determine how depleted polar precipitation becomes.
Rayleigh distillation law: As vapor condenses, the isotopic ratio of the residual vapor follows:
Rv = R0 · f (α−1), where f is the fraction of vapor remaining and α is the fractionation factor. Each condensation step further depletes the remaining vapor in ¹⁸O, so snow falling on polar ice caps can have δ18O values as low as −55‰ (Antarctica) or −35‰ (Greenland).
δ¹⁸Ovapor(step n) ≈ δ¹⁸Osource − n × Δ
where Δ ≈ 4–6‰ per major rainout event depending on temperature. Each step removes the heavy isotope preferentially.