Warmer air holds more water vapor — a real physical relationship, the Clausius–Clapeyron equation, that governs how the saturation vapor pressure of water changes with temperature:
e_s(T) = e_s0 · exp[ (L/R_v)(1/T0 - 1/T) ]
L = 2.5×10⁶ J/kg (latent heat of vaporization)
R_v = 461 J/(kg·K), T0 = 273.15 K, e_s0 = 611 Pa
This 2D view draws that curve directly — drag anywhere on it to read off e_s(T) at any temperature. Since water vapor is itself a greenhouse gas, more of it traps more outgoing infrared radiation, warming the surface further, which holds still more vapor. That closes a positive feedback loop, drawn here as a ring of flowing particles between forcing, surface, vapor and space:
No-feedback warming: ΔT0 = ΔF / λ_planck (λ_planck ≈ 3.2 W/m²/K)
Feedback fraction: f = λ_wv / λ_planck
Equilibrium warming: ΔT_eq = ΔT0 / (1 - f) = ΔT0·Σ f^n
The bottom panel builds that sum term by term — T_n = ΔT0 + f·T_(n-1) — so you can watch the geometric series climb toward the dashed equilibrium line instead of just reading the closed-form answer.
- CO2 forcing ΔF — the radiative imbalance driving the initial, feedback-free warming ΔT0. 3.7 W/m² is the standard value for a doubling of atmospheric CO2.
- λwv — the strength of the water-vapor feedback itself, in W/m² of extra trapped radiation per K of warming. Observational and model estimates cluster around +1.6 to +2.0 W/m²/K, the single largest amplifying feedback in the climate system.
- Relative humidity — the fraction of saturation the air actually holds; the vapor increase shown scales with it, since climate models find RH stays roughly constant as the planet warms (Clausius-Clapeyron sets the ceiling, not the floor).
- Feedback ON/OFF — toggles whether the loop is allowed to close, so you can directly compare the raw forcing response to the amplified equilibrium one.
- Drag on the vapor-pressure curve — moves an explorer marker along the curve independent of the sliders, so you can read e_s(T) at any temperature.