CAPE is the fuel gauge for thunderstorms.
Modern forecast apps expose 40+ parameters — CAPE, sea temperature, wind shear, dew point. CAPE (Convective Available Potential Energy, in J/kg) tells you how violently air wants to rise. Drag the scene, then change the surface conditions and watch the parcel go.
Surface conditions
A parcel gets a nudge
Sun-heated ground warms a bubble of air. Warm air is less dense than its surroundings, so it starts rising — cooling ~9.8 °C per km while it stays dry.
Condensation pays the bill
At the LCL the parcel is cooled to its dew point. Water vapor condenses into cloud, releasing latent heat, so the parcel now cools only ~6 °C per km — staying warmer than the air around it.
CAPE is the running total
Integrate that warmth advantage over the whole climb and you get CAPE in joules per kilogram — literally the kinetic energy available to accelerate the updraft.
Reading the numbers like a forecaster
CAPE alone doesn't make a storm — you also need a trigger and, for severe storms, wind shear. But it sets the ceiling on how strong convection can get. The theoretical max updraft is w = √(2 × CAPE); real storms reach roughly half of that due to mixing and water loading.
| CAPE (J/kg) | Instability | Typical outcome |
|---|---|---|
| 0–300 | Weak / stable | Flat clouds, no thunderstorms |
| 300–1,000 | Marginal | Garden-variety showers, isolated storms |
| 1,000–2,500 | Moderate–strong | Multicell storms, hail possible |
| 2,500–4,000 | Very strong | Supercells with large hail, damaging wind |
| 4,000+ | Extreme | Violent storms; May 3, 1999 Oklahoma outbreak ran ~5,900 J/kg |
Why moisture matters as much as heat
Notice in the simulator that raising the dew point boosts CAPE faster than raising temperature. Moisture lowers the cloud base and supplies latent heat for the whole climb — which is why Gulf-of-Mexico moisture, not just heat, drives U.S. tornado season, and why sea-surface temperature is itself a forecast parameter worth watching.