Measures of convective instability
There are many different kinds of measures of convective instability. The most common, and you will come across in virtually an in-depth forecast or storm chaser discussions, is CAPE = Convective Available Potential Energy. Chances are you have heard storm chasers and enthusiasts talk about this “CAPE this, CAPE that…”. CAPE is fuel for convection – and thunderstorms.
CAPE is the energy a parcel of air has for upward motion. The higher the CAPE, the faster and higher the air parcel can rise. CAPE depends on the temperature and amount of moisture (humidity) in the air near the ground and the vertical temperature gradient. The hotter and more humid the air on the ground is, the higher the CAPE. Also, the faster the temperature drops with height, the higher the CAPE.
There are several variations of CAPE you are likely to come across: SBCAPE, MUCAPE, MLCAPE. We will cover this, and other measures of convective instability in detail in one of future tutorials.
Fun fact(s): under the right conditions the rising air goes up really fast! While most thunderstorms form in moderately unstable conditions (CAPE up to ~1000 J/kg), in some cases there is much more energy available. This is called extreme instability and in such cases CAPE values can reach 6000 J/kg and more. When thunderstorms form in this type of environment, the upward speed of rising air is extreme, sometimes over 150 m/s! This is enough to keep hailstones 15+ cm in diameter airborne!