thumb|upright=1.2|On [[thermodynamic diagrams such as the skew-T log-P, CAPE is proportional to the area of the region separating the temperature of a moist air parcel as it ascends (labeled as the "moist adiabat" in this example) and the surrounding air temperature (labeled as "T").|alt=Idealized diagram plotted on axes of temperature and pressure with an area corresponding to CAPE labeled]]

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.

Calculation

The change in density of a hypothetical parcel of air as it rises relative to the density of the surrounding air determines where in the atmosphere it can continue rising by buoyancy. The density of an air parcel is based on its temperature, pressure, and water vapor content, provided that its chemical makeup is otherwise constant. CAPE is a measure of the maximum kinetic energy per unit mass that an air parcel can acquire by remaining less dense than its surroundings. This requires an approximation of the change in the parcel's density as it rises. For calculations of CAPE, the hypothetical air parcel is assumed to initially rise and cool at the dry adiabatic lapse rate: the rate at which air cools as it expands without any release of latent heat. Once the parcel cools to the point of saturation, it is then assumed to cool at the moist adiabatic lapse rate: the rate at which air cools adjusted for the release of latent heat from water vapor condensing within the saturated air parcel. For typical daytime atmospheric conditions, accounting for moisture produces larger and more accurate estimates of CAPE. The parcel begins with temperature and moisture characteristics of its surroundings but then deviates from those conditions as it rises. MUCAPE may be a more appropriate measure of the buoyant energy available to a thunderstorm with inflow originating well above the surface. While CAPE quantifies instability in the context of air moving directly upward, slantwise CAPE (SCAPE) can be computed in situations where buoyant ascent can be realized if parcels move in some combination of both the horizontal and vertical. thermodynamically representing a reversible moist adiabatic process. This calculation is better suited for humid tropical environments such as within tropical cyclones.

The numerical methodology underlying CAPE can also be performed for portions of the atmosphere where an air parcel would be denser and cooler than its surroundings. These areas have negative buoyancy, resulting in the force of buoyancy acting downwards. Integrating within these areas, typically between the surface and the LFC, results in a negative value also known as convective inhibition (CIN). Additional upward forces are required for an air parcel to rise against negative buoyancy, with CIN providing a measure of the work required to overcome negative buoyancy and reach a freely buoyant height. A similar quantity is downdraft CAPE (DCAPE), which integrates the negative buoyancy potentially imparted on an initially saturated parcel as it descends from some arbitrary height to the ground. This measure is used to quantify the potential for downbursts. Parcel trajectories that take large and extended excursions away from the environmental air temperature thus indicate large amounts of CAPE. In these areas, the 95th percentile of CAPE annually as analyzed between 1979 and 2019 was over  J/kg. The highest CAPE values over land are found over the Congo Basin with a 95th percentile value around  J/kg.

Two notable days for severe weather exhibited CAPE values over 5 kJ/kg. Two hours before the 1999 Oklahoma tornado outbreak occurred on May 3, 1999, the CAPE value sounding at Oklahoma City was at 5.89 kJ/kg. A few hours later, an F5 tornado ripped through the southern suburbs of the city. Also on May 4, 2007, CAPE values of 5.5 kJ/kg were reached and an EF5 tornado tore through Greensburg, Kansas. On these days, it was apparent that conditions were ripe for tornadoes and CAPE wasn't a crucial factor. However, extreme CAPE, by modulating the updraft (and downdraft), can allow for exceptional events, such as the deadly F5 tornadoes that hit Plainfield, Illinois on August 28, 1990, and Jarrell, Texas on May 27, 1997, on days which weren't readily apparent as conducive to large tornadoes. CAPE was estimated to exceed 8 kJ/kg in the environment of the Plainfield storm and was around 7 kJ/kg for the Jarrell storm.

Severe weather and tornadoes can develop in an area of low CAPE values. The surprise severe weather event that occurred in Illinois and Indiana on April 20, 2004, is a good example. Importantly in that case, was that although overall CAPE was weak, there was strong CAPE in the lowest levels of the troposphere which enabled an outbreak of minisupercells producing large, long-track, intense tornadoes.

See also

  • Atmospheric thermodynamics
  • Lifted index
  • Maximum potential intensity

References

Further reading

  • Barry, R.G. and Chorley, R.J. Atmosphere, weather and climate (7th ed) Routledge 1998 p. 80-81
  • Map of current global CAPE