National Weather Service United States Department of Commerce

What is CAPE?

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. 

To better understand this, consider a parcel of air that has been forced upward by some mechanism such as an approaching frontal boundary.  Since warm air is less dense (lighter) than cold air, the parcel will continue rising on its own as long as its temperature remains warmer than its surroundings.  This is the same concept as a hot air balloon, which uses a heat source to maintain a temperture within the balloon (air parcel) that is warmer than its surroundings.  As long as this condition is met, the balloon will continue rising.  Once the air temperature within the balloon cools and becomes equal to the air outside the balloon, it will stop rising.  

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.  CAPE is just one of many tools forecaster use to determine atmospheric instability and the potential for severe weather.  It should not be considered a magic number, but instead is best used in conjunction with other important parameters and analysis to provide the most complete and accurate severe weather forecast.