Presets
Playback
Parameters
Model checks
The science
Parcel theory
A small parcel of air rises fast enough that it exchanges no heat with its surroundings, so it cools adiabatically. Dry air cools at Γd = g / cp = 9.8 K per km. When the parcel reaches saturation (the lifting condensation level, LCL, about 125 m per kelvin of temperature minus dew point) water vapour condenses and releases latent heat, so the cooling slows to the saturated rate Γs = g (1 + Lv rs / Rd T) / (cp + Lv2 rs ε / Rd T2), typically 3 to 6 K per km. The parcel feels the buoyant acceleration B = g (Tp - Te) / Te relative to the environment.
CAPE, CIN and the updraft
Convective available potential energy is the area of positive buoyancy, CAPE = ∫ B dz from the level of free convection (LFC) to the equilibrium level (EL); convective inhibition (CIN) is the negative area below the LFC that a trigger such as a front or a hill must overcome. Since w dw/dz = B, the updraft speed obeys w2 = w02 + 2 ∫ B dz, so the parcel-theory maximum is w = √(2 CAPE), for example 2 000 J/kg gives 63 m/s. Entrainment of cooler air, which grows with wind shear, is modelled as dT/dz = -Γ - ε (Tp - Te).
Honest simplifications: a single undiluted-or-uniformly-entraining parcel; the environment is a straight-line temperature profile with an isothermal stratosphere (the sliders change its lapse rate); no virtual-temperature correction and no condensate loading, so CAPE and w are upper bounds (real storms reach roughly half of √(2 CAPE)); the moisture is not mixed with the environment; the shear only enters through entrainment and the tilt of the drawing. The scene is a sketch of a storm, not a forecast.
What to try
- Load Summer thunderstorm, then Capped humid day: the same air has CIN and stalls until you raise the trigger lift.
- Drag the lapse rate towards 9.8 K/km (dry adiabat) and watch CAPE climb in the third graph.
- Choose Dry desert plume: the cloud base is above 4 km because the dew point is far below the air temperature.
- Raise the entrainment slider and see the updraft weaken.