A moist air parcel forms over the ocean and travels inland, losing water to precipitation as it goes. Each condensation event preferentially removes the heavier isotope (H₂18O) from the vapor, so the remaining vapor — and every raindrop after it — becomes progressively more depleted. This is Rayleigh distillation:
R_v / R_v0 = f^(α−1)
δ_v(‰) = [(1+δ_v0/1000)·f^(α−1) − 1]·1000
δ_p(‰) = [(1+δ_v/1000)·α − 1]·1000 (precip in equilibrium with local vapor)
where f is the fraction of the original vapor still airborne and α is the liquid–vapor equilibrium fractionation factor, which itself depends on local temperature (Majoube, 1971):
1000·ln(α) = 1.137×10⁶/T² − 0.4156×10³/T − 2.0667 (T in kelvin)
Because α varies with T along the path, the model integrates the distillation step by step rather than using a single closed-form α — exactly how real isotope-hydrology "rainout" models work.
- Sea-surface temperature — sets the starting vapor isotope ratio and how strongly α starts out.
- Rainout rate — how fast the air mass loses moisture per kilometre inland (the "continental effect": rain gets isotopically lighter the farther inland it falls).
- Mountain height — orographic lift over the range steepens cooling and rainout locally (the "altitude effect").
- Cross-section zoom — how many kilometres of the ocean-to-mountain profile are visible at once; drag the terrain view to pan manually.
- The strip chart below the terrain plots δ¹⁸O of vapor and of falling precipitation against distance travelled, so the continental effect (the downward trend) is visible directly as a graph, not just as a colour shift.
Real-world relevance: this is the physical basis for dating groundwater, reconstructing paleoclimate from ice cores, and tracing where a river's water actually recharged.