CAPE — storm fuel

CAPE stands for Convective Available Potential Energy. It's the outlook's Fuel meter — and the single most important number in storm forecasting.

>The hot-air-balloon idea

Warm air is lighter than cold air, so it rises — like a hot-air balloon. CAPE measures how much push a bubble of warm, moist air gets on its whole journey upward. The units are joules per kilogram (J/kg): energy per kilogram of air. The more energy, the faster the updraft, and the taller and angrier the storm that grows on top of it.

>Reading the number

• 0–300 J/kg — stable air. Updrafts fizzle out. No storms.
• 300–800 J/kg — a little fuel; weak storms at best.
• 800–1500 J/kg — real storm fuel. This is a loaded sky.
• over 1500 J/kg — explosive. Storms can grow violent fast.

(In the US Great Plains, tornado days sometimes reach 3000–5000 J/kg. European tornadoes usually work with less — which is why the other ingredients matter so much here.)

>Where fuel comes from

CAPE builds when the ground heats up during the day and when warm, humid air flows in. That's why storm days often look beautiful in the morning: the sun is the stove, and it's busy cooking the afternoon's fuel. It's also why the outlook's watch-for line often says "watch for daytime heating" — a quiet breakfast sky can be a loaded one by four o'clock.

Fuel alone makes storms, not tornadoes. For those, the wind itself has to be arranged just right — that's the Engine and Spin meters.