CAPE
Thunderstorms Convective Available Potential Energy
CAPE describes energy potentially available to a rising air parcel through buoyancy. High values can support strong thunderstorm updraughts, but are not a probability of thunderstorms.
Why warm air may keep rising
A lifted parcel expands and cools. If it remains warmer and usually less dense than its surroundings, it gains buoyancy. CAPE, Convective Available Potential Energy, adds up this potential buoyant energy through the relevant layer. Its unit is joules per kilogram (J/kg).
Energy available, thunderstorm not yet booked
A value of 1,000 J/kg describes an energy supply. It means neither 1,000 flashes nor a 100 per cent storm probability. If a restraining layer persists or the air lacks a trigger to rise, the sky can remain quiet despite high CAPE.
What else matters?
- Lift: for example from a front or converging winds.
- CIN: inhibition that a parcel must first overcome.
- Wind shear: changes of wind with height, which influence storm organisation.
Compare like with like
Calculations may start with a surface parcel, an averaged lower layer or the most unstable layer. These produce different types of CAPE. A higher value on another chart therefore need not indicate a weather change. Read CAPE alongside other evidence.