The Convectively Available Potential Energy (CAPE) map - updated every 6 hours - shows the modelled convectively available
potential energy. CAPE represents the amount of buoyant energy (J/kg) available to accelerate a parcel vertically, or the amount of work
a parcel does on the environment. The higher the CAPE value, the more energy available to foster storm growth. The
potential energy can be converted to kinetic energy reflected in upward motion.
It should be remembered that CAPE represents potential energy, and will only be used should a parcel be lifted to the level of free convection.
When values are above 3500 j/kg and storms do develop, they may build rapidly and quickly become severe.
Often these storms are referred to as "explosive storms" by chasers and professionals. In a high CAPE environment
storms that develop can usually be seen by the human eye as rising rapidly.
Higher CAPE typically involves stronger storms with a higher chance of large hail and other severe weather. Note that
CAPE is usually of lesser importance than the vertical shear environment for tornadoes. The probability of large hail increases
with CAPE, given at least moderate shear(values around 500-1000 J/kg are sufficient).
CAPE is very sensitive to small differences in the moisture and temperature profiles. While the maps indicate
1000 J/kg CAPE at some location, a
skew-T thermodynamic diagram at that location may indicate 500-1500 J/kg.
(Source:
The Lightning Wizard)
Table 1: Characteristic values for CAPE
| CAPE value |
Convective potential |
| 0 |
Stable |
| 0-1000 |
Marginally Unstable |
| 1000-2500 |
Moderately Unstable |
| 2500-3500 |
Very Unstable |
| 3500 + |
Extremely Unstable |