Short Primer on CAPE and CIN
Right off the bat, you'll read references to CAPE (Convective Available Potential Energy) in Convective Outlooks, Weather Watches, and Mesoscale Discussions routinely issued by the Storm Prediction Center.
On a skew-T (check out the example below), CAPE, which stands for Convective Available Potential Energy, is simply the area between the temperature sounding (data obtained from radiosonde measurements) and the local moist adiabat that a lifted air parcel follows between the Level of Free Convection (LFC) and the Equilibrium Level (EL).
That's a mouthful! For the record, the Level of Free Convection (LFC) is simply the altitude at which a parcel becomes warmer than its environment after it's lifted dry adiabatically to its Lifted Condensation Level (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 (often near the tropopause).
From the standpoint of stability, an air parcel reaching the LFC becomes positively buoyant. When parcels are positively buoyant through a deep layer of the troposphere, thunderstorms typically result. So CAPE, represented below by the positive (green) area on a skew-T gives forecasters a proxy for the potential for strong updrafts. 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 (the parcel is warmer than its surroundings above the LFC, and the larger this temperature difference, the greater the upward acceleration of air parcels). For the record, the units of CAPE are expressed in Joules per kilogram.
In general, you should rank values of CAPE between 0 and 1000 Joules per kilogram as relatively 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 high CAPE in no way guarantees that thunderstorms will erupt. Unless air parcels can get to their LFC, CAPE can go ballistically high ... yet thunderstorms might never materialize.
When CAPE is high and thunderstorms fail to materialize, Convective Inhibition (CIN) might be too large for lift to overcome CIN (relatively large CIN + weak lift = no thunderstorms). In other words, air parcels can't reach the LFC, the release of CAPE is suppressed, and thunderstorms aren't initiated. On a skew-T (see below), convective inhibition (CIN) is the area between the temperature sounding and the adiabat / moist adiabat followed by a lifted parcel headed toward the LFC. Another way to think about convective inhibition, which is represented by the negative (red) area on the skew-T below, is that CIN is a proxy for the amount of energy needed to lift a 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.
Revisit the skew-T above. ln a nutshell, there is a temperature inversion near 850 mb that is responsible for a large chunk of the CIN you observe. Do you see it? Often, low-level convergence and solar heating are not strong enough to overcome this CIN in just a single lift. Rather, upward motion and its associated cooling gradually "nibble away" and erode the temperature inversion in the lower troposphere over time. The erosion of this temperature inversion then paves the way for air parcels to eventually reach their LFC's later in the day (with the assistance of low-level convergence, solar heating, etc.).
Here endeth the primer.