A rising air parcel cools at the dry adiabatic lapse rate (~9.8 °C/km) until it reaches saturation — the lifting condensation level (LCL) — where its water vapour condenses into cloud droplets. Above the LCL it cools more slowly (moist adiabat) and keeps rising while it stays warmer than the surrounding air.
LCL height ≈ 125 · (T_surface − T_dew) [metres]
T_parcel(z) = T_surface − Γ_d·z (below LCL, Γ_d ≈ 9.8 °C/km)
T_parcel(z) = T_LCL − Γ_m·(z − z_LCL) (above LCL, Γ_m ≈ 5 °C/km)
buoyant while T_parcel(z) > T_env(z) = T_surface − Γ_env·z
- Surface temperature — how warm the rising thermals start; higher values mean stronger, higher-reaching updrafts.
- Surface dew point — sets how much moisture is available; closer to surface temperature means a lower cloud base.
- Environmental lapse rate — how fast the surrounding air cools with height; steeper (closer to 9.8) makes the atmosphere more unstable and lets clouds grow taller.
- Thermal buoyancy — scales how fast parcels accelerate upward while still warmer than their surroundings.
This is exactly the physics forecasters use to predict cumulus cloud base height and whether a hot humid afternoon will spawn thunderstorms — a shallow gap between surface temperature and dew point means low, flat clouds, while a steep environmental lapse rate lets towering cumulonimbus develop.