Convective available potential energy

In meteorology, convective available potential energy (CAPE) is the amount of energy a parcel of air would have if lifted a certain distance vertically through the atmosphere. CAPE is measured in joules per kilogram of air (J/kg). Any value greater than 0 J/kg indicates instability and the possibility of thunderstorms.

Thunderstorms form when air parcels are lifted vertically. The atmosphere is warm at the surface, but turns substantially cooler with height. When the airparcel cools more slowly than the surrounding atmosphere as it is lifted, it remains warmer and relatively less dense. The parcel continues to rise through the atmosphere until it reaches an area of air less dense (warmer) than itself.

CAPE for a given area can be measured in a thermodynamic or sounding diagram e.g. the skew-T thermodynamic diagram.

An extreme day for severe weather sees CAPE values over 5,000 J/kg. Two hours before the Oklahoma Tornado Outbreak occurred on May 3, 1999, the CAPE value sounding at Oklahoma City was at 5,885 J/kg. A few hours later, an F5 tornado ripped through the southern suburbs of the city.

Severe weather and tornadoes can develop in an area of relatively low CAPE values. The surprise severe weather event that occurred in Illinois on April 20, 2004 is a good example.

See also

*Convective inhibition
*Thunderstorm
*List of tornadoes and tornado outbreaks
*Weather

External links

*Gives basic definition of CAPE
*CAPE for Instability Descriptors
*NWS Forth Worth Glossary
*[1] a short but signigficant description
*[2] a class (Macromedia Flashplayer necessary)