Convective Available Potential Energy, or CAPE, is the total potential energy a lifted air parcel could gain from positive buoyancy between the level of free convection and the equilibrium level, expressed in joules per kilogram of air. The AMS Glossary defines it as the potential energy of a parcel due to positive buoyancy, and it is visualized as the positive area between the parcel path and the environmental temperature curve on a Skew-T.
The physics
Between the LFC and the equilibrium level, a lifted parcel is warmer, and therefore less dense, than the environment around it. Positive buoyancy in this layer accelerates the parcel upward. CAPE is the integral of that buoyancy over height across the layer, so it captures both how strongly buoyant the parcel becomes and how deep the buoyant layer is.
Expressed as joules per kilogram, CAPE is energy per unit mass. Parcel theory says an undiluted parcel starting from rest at the LFC could in principle reach a vertical velocity equal to the square root of twice CAPE. A CAPE of 2500 J/kg gives a theoretical upper bound around 70 m/s. In practice, entrainment and water loading knock observed maximum updrafts to roughly half of that in strong storms, and much less in weaker ones.
Choosing a parcel
The CAPE value depends on which parcel is lifted. Surface-based CAPE, or SBCAPE, uses the actual near-surface temperature and dewpoint and is most representative when the boundary layer is fully mixed and the storm can feed directly off the ground. Mixed-layer CAPE, or MLCAPE, averages the lowest 100 hPa and better represents an afternoon well-mixed boundary layer that has been stirring for hours. Most-unstable CAPE, or MUCAPE, searches for the parcel with the largest buoyancy in the low levels and is the appropriate choice for elevated storms above a shallow cool layer or nocturnal convection feeding on a residual moist layer aloft.
SPC guidance on parcel selection emphasizes that no single parcel is right for every case, and cross-checking SBCAPE, MLCAPE, and MUCAPE together is standard practice. A profile where all three are comparable describes a fully mixed warm sector. A profile where MUCAPE greatly exceeds SBCAPE points to an elevated most-unstable layer that is decoupled from the surface.
How forecasters use it
CAPE is checked on mesoanalysis maps and in point soundings before drawing conclusions about the day's storm strength. Rough guides in NWS training use ranges such as 1000 to 2500 J/kg for moderate instability and above 2500 J/kg for strong instability, but those are conventions rather than switches and depend heavily on which parcel is being used.
The vertical distribution of CAPE often matters more than the total. 0 to 3 km CAPE describes low-level buoyancy that helps stretch near-ground rotation into a tornado. CAPE concentrated in the mid-levels favors tall, wide updrafts and larger hail. Long, thin CAPE profiles produce a different storm character than short, wide ones, even if the totals are equal.
Where CAPE misleads
CAPE describes potential, not certainty. A profile with 4000 J/kg of SBCAPE under a strong capping inversion can produce nothing all afternoon if the cap holds. A profile with 800 J/kg over strong low-level shear and a low LCL can support significant tornadoes when a boundary supplies the lift. The value alone does not decide the day.
CAPE is also an idealization. Entrainment of dry environmental air along the updraft edges, precipitation loading, and forced ascent from below all reduce the fraction of CAPE that becomes real kinetic energy in the updraft. Observed vertical velocities routinely fall well short of pure parcel theory, and the shortfall grows in drier environments and weaker forcing. Treat CAPE as an upper bound on what the environment can support, and cross-check it with the shape of the sounding, storm mode, and expected forcing before leaning on the number.