This is the 2D companion to the 3D "Cloud Formation" laboratory. A rising parcel of surface air is tracked thermodynamically rather than just drifted upward on a curve: below cloud base it cools at the true dry adiabatic lapse rate, and above cloud base it cools at the true moist adiabatic lapse rate, which is always slower because condensing water vapour releases latent heat into the parcel.
Dew point (Bolton/Tetens):
gamma = ln(RH/100) + 17.625 T / (243.04 + T)
Td = 243.04 gamma / (17.625 - gamma)
Cloud base (LCL), dry-lapse / dew-point-lapse convergence:
z_LCL = 125 * (T - Td) [metres]
Dry adiabatic lapse rate: Gamma_d = 9.8 degC/km (below z_LCL)
Moist adiabatic lapse rate: Gamma_m(T,P) = g(1 + L r_s /(Rd T))
/ (cp + L^2 r_s eps /(Rd T^2))
(above z_LCL, always < Gamma_d)
- Cloud base is the real Lifting Condensation Level: the altitude where the parcel's temperature, falling at 9.8°C/km, reaches its dew point. Drier air needs a bigger gap between T and Td, so cloud base rises — deserts get high, thin cloud; humid air gets low cloud.
- Cloud top is the level where the parcel, now cooling at the slower moist rate, cools back down to the temperature of a fixed-lapse-rate (6.5°C/km) environment column started from the same surface temperature. Warm, humid air releases more latent heat and cools more slowly once condensed, so it stays buoyant far above cloud base — this is the real reason humid summer afternoons build tall cumulonimbus while cool, dry air only manages a shallow stratus deck.
- Droplet vertical speed above cloud base follows the real buoyancy acceleration a = g·(Tparcel − Tenv)/Tenv (in kelvin), not a scripted rise — particles slow and spread out once they reach the equilibrium level where that difference goes to zero.
Real-world relevance: pilots and glider pilots use the LCL to predict cloud base for VFR flight and thermalling, and the moist-vs-dry lapse rate difference is exactly what forecasters mean by "instability" when predicting thunderstorms.