Sources: National Weather Service and Wikipedia

CAPE or Convective Available Potential Energy is the amount of fuel available to a developing thunderstorm. More specifically, it describes the instabilily of the atmosphere and provides an approximation of updraft strength within a thunderstorm. A higher value of CAPE means the atmosphere is more unstable and would therefore produce a stronger updraft.

CAPE is calculated by determing the area between the environmental temperature trace and the trajectory of an air parcel that is forced upwards on an atmospheric sounding. The image below shows an atmospheric sounding. The red line is the measured temperature of the surrounding air as you climb through the depth of the atmosphere. The yellow line represents the trajectory of an air parcel that has been forced to rise from the surface. As long as the yellow line (parcel) remains to the right of the red line (environment), the parcel is warmer than its surroundings and will continue to rise. The amount of CAPE in the sounding is therefore the area contained between the red and yellow lines. An atmospheric sounding such as the one below gives forecasters a quick visual way to assess how much fuel is available within the atmosphere.

CAPE is expressed in joules per kilogram (J/kg) and can range from zero to over 5000. In general, CAPE values of less than 1000J/kg represent weak instability, 1000 to 2500J/kg moderate instability, 2500-4000J/kg strong instabilty, and greater than 4000J/kg extreme instability. This of course is a sliding scale that is dependent on location and time of year. For example, a CAPE value of 500J/kg is considered quite weak in the month of May in central Illinois, but would be much more impressive in January.

More technically, CAPE is the integrated amount of work that the upward (positive) buoyancy force would perform on a given mass of air (called an air parcel) if it rose vertically through the entire atmosphere. Positive CAPE will cause the air parcel to rise, while negative CAPE will cause the air parcel to sink. Nonzero CAPE is an indicator of atmospheric instability in any given atmospheric sounding, a necessary condition for the development of cumulus and cumulonimbus clouds with attendant severe weather hazards.

Pivotal Weather produces a map of surface-based CAPE. Mouseover on the map to see CAPE values. Clicking any point on the map will show "sounding" details.