Convective Available Potential Energy, or CAPE for short, is a value that represents how much buoyant energy it takes to lift an air parcel vertically. CAPE is measured in joules per kilogram (J/kg).
Thunderstorms form when the sun heats the land and a warm air parcel lifts vertically. The atmosphere is warm at the surface, but turns substantially cooler as height increases. CAPE just measures how much work it takes for the air parcel to rise. The higher the CAPE, the greater the potential for severe weather.
CAPE for a given area is measured on a sounding diagram called a skew-T thermodynamic diagram .
Severe weather forms when an air parcel gains in height extremely quickly. CAPE values over 2,000 joules per kilogram is sufficient enough for severe weather to occur.
An extreme day for severe weather sees CAPE values over 5,000 J/kg. Two hours before the Oklahoma Tornado Outbreak occurred, the CAPE value sounding at Oklahoma City was at 5,885 J/kg. A few hours later, an F5 tornado ripped through the northern suburbs of the city.
However, when estimating thunderstorm probability, one should have a look at the lifted index . 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.
The range of CAPE values below provide a measure of instability that would be expected for a parcel:
- Stable: 0-1000 J/kg
- Unstable: 1001-2500 J/kg
- Very Unstable: 2501-3500 J/kg
- Extremely Unstable: 3501+ J/kg[1]
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