2. Parameters

Stability Assessment » Convective Available Potential Energy (CAPE) Definition

Definition: Convective Available Potential Energy (CAPE)

The convective available potential energy (CAPE) is represented by the area on a skew-T enclosed by the environmental temperature profile and the saturation adiabat running from the LFC to the EL. This area, depicted in the diagram below, indicates the amount of buoyant energy available as the parcel is accelerated upward. CAPE is measured in units of joules per kilogram (J/kg).

Depiction of CAPE on a skew-T

The larger the positive area, the higher the CAPE value and instability, and the greater the potential for strong and perhaps severe convection. This table offers a general correlation between CAPE and atmospheric stability, however CAPE climatologies vary widely.

CAPE Value Stability
0 Stable
0-1000 Marginally Unstable
1000-2500 Moderately Unstable
2500-3500 Very Unstable
3500 or greater Extremely Unstable

CAPE may also be related to updraft velocity via the relation:

    Wmax (in m/s) = sqrt(2 * CAPE)

Hence, for a CAPE of 2500 J/kg, the maximum updraft velocity, Wmax, would be about 71 m/s! In reality, water loading, entrainment, and other factors can reduce Wmax by as much as a factor of 2.