Convective available potential energy

In meteorology, convective available potential energy (commonly abbreviated as CAPE) is a measure of the capacity of the atmosphere to support the vertical movement of air that can lead to cloud formation and storms. As air rises in an atmosphere, it expands and cools. CAPE exists when a given mass of air (called an air parcel) can ascend and remain warmer than the surrounding air. The warm parcel is less dense than the surrounding air and accelerates upward. While the rate of cooling for an ascending parcel of dry air would quickly cool that parcel below the surrounding air temperature in most cases, water vapor within a parcel of moist air releases heat if it condenses. This slows the air parcel's rate of cooling and may keep the parcel warmer than the surrounding air across a particular depth of the atmosphere. The continued ascent of relatively warm and moist air can stimulate the formation of cumulus or cumulonimbus clouds and fuel thunderstorms.

More technically, CAPE is the integrated amount of work that the upward (positive) buoyancy force would perform on a given mass of air if it rose vertically through the entire atmosphere. The computation of CAPE for a given atmospheric environment depends on the initial characteristics ascribed to the hypothetical air parcel, giving rise to specific versions of CAPE like surface-based CAPE (SBCAPE) or most-unstable CAPE (MUCAPE). The presence of nonzero CAPE in an atmospheric sounding is an indicator of convective instability, a necessary condition for the development of cumulus and cumulonimbus clouds with attendant severe weather hazards. Some atmospheric conditions (such as in humid environments with air that cools rapidly with height) support large values of CAPE that can promote strong and sustained upward air movement, resulting in a more conducive environment for thunderstorms.

CAPE is typically expressed in units of Joules per kilogram (J/kg). Values of CAPE in environments conducive to severe weather are often in the thousands of J/kg. Due to the relationship between CAPE and the vertical speeds in the updrafts of storms, the magnitude of CAPE in a given environment can be used as a rough measure for the potential intensity of storms in that setting. Larger values of CAPE can support stronger thunderstorms, but the presence of CAPE alone is not sufficient for storm development.

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