Definition

Skew-T procedure to determine CAPE

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

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 ValueStability
0Stable
0-1000Marginally Unstable
1000-2500Moderately Unstable
2500-3500Very Unstable
3500 or greaterExtremely Unstable

CAPE may also be related to updraft velocity via the relation

Wmax = sqrt(2*CAPE)

So for a CAPE of 2500 J/kg, the maximum updraft velocity 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.

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