Short Primer on CAPE and CIN

On a skew-T (see below), CAPE, which stands for Convective Available Potential Energy, is the area between the temperature sounding and a positively buoyant air parcel's moist adiabat between the Level of Free Convection (LFC) and the Equilibrium Level (EL). CAPE, represented by the positive area on a skew-T, is a measure of the degree of instability. More specifically, CAPE is a proxy for the amount of kinetic energy that an air parcel can gain from temperature differences between the parcel and the surrounding air. Recall from Meteo 101 that air parcels warmer than their environment will tend to accelerate upward (this acceleration will increase as the temperature difference between the parcel and its environment increases). For the record, units of CAPE are expressed in Joules per kilogram.

CAPE equals the positive area between the environmental sounding and the moist adiabat along which a rising parcel follows from the Level of Free Convection to the Equilibrium Level.

The Level of Free Convection (LFC) is simply the altitude at which a parcel becomes warmer than its environment after it is lifted dry adiabatically to its LCL and then moist adiabatically thereafter. The Equilibrium Level is the altitude above the LFC where the temperature of a positively buoyant parcel again equals the temperature of its environment.

In general, you should rank values of CAPE between 0 and 1000 Joules per kilogram as small. CAPE values between 1000 and 2500 Joules per kilogram typically qualify as moderate. When you see CAPE values between 2500 Joules per kilogram, think large. Values greater than 4000 Joules per kilogram are extreme. Please note that the presence of CAPE in no way guarantees that thunderstorms will erupt.

That's because there may be too much Convective Inhibition, which acts to suppress the release of CAPE (and the onset of thunderstorms). On a Skew-T (see below), CIN, which is short for Convective INhibition, is the area between the temperature sounding and the adiabat / moist adiabat followed by a lifted parcel on its way to its LFC. CIN, represented by the negative area, is a proxy for the amount of energy needed to lift a parcel to its LFC.

A sounding showing the spatial relationship between the LFC and negative area. Also known as Convective Inhibition (CIN), this area is directly proportional to the amount of energy required to lift an air parcel to its LFC.

In general, you can rank CIN values between 0 and minus 25 Joules per kilogram as weak inhibition. CIN values between minus 25 and minus 50 Joules per kilogram typically qualify as moderate. When you see CIN values of minus 50 Joules per kilogram ... minus 100 Joules per kilogram ..., think large inhibition.

Don't get nervous. I'll fill in more details about CAPE and CIN as the course unfolds.