Lifting Condensation Level (LCL) -- the level at which the lofting air parcel becomes saturated due to expansional cooling. Procedurally, the elevation at which the mixing ratio line extended from the surface dew point intersects the dry adiabat extended from the surface temperature. Above this level, the air parcel, if lofted, must cool at the wet adiabatic rate.
Level of Free Convection (LFC) -- the elevation above which a lofting air parcel becomes warmer than the air surrounding it at the same elevation. As long as the air parcel is warmer than its surroundings, it will accelerate upwards, and that acceleration will be directly proportional to how warm the air parcel is relative to its surroundings (usually shaded in red).
Convective Available Potential Area (CAPE) -- a direct measure of how buoyant (unstable) the lofting air parcel will be above the LFC. It really is a measure of how much warmer the air parcel is than its surroundings for every level from the LFC to the spot at which the air parcel's ascent curve recrosses the environmental lapse rate (sounding) for the day (normally shaded in red).
Convective Inhibition Energy (CIN) -- a direct measure of how resistant an air parcel will be to lofting and is related to how cold the air parcel is relative to its surroundings. CIN has the same units as CAPE, but is negative. If the atmosphere is conditionallly unstable, some mechanism must be present to force lift the air parcel through the area of CIN (normally shaded in blue).
Equivalent Potential Temperature (qe) – The temperature at 1000 mb (K) an air parcel would have if it were wet adiabatically lifted from the LCL along a moist adiabat until all the water vapor condenses out. Procedurally, the equivalent potential temperature is obtained by lifting an air parcel from its LCL along a moist adiabat to the point at which the moist adiabat becomes tangent to a dry adiabat (isentrope) and then bringing the air parcel to 1000 mb dry adiabatically and recording its temperature (K).
Conditional Instability – Defined in a number of ways based upon a “condition” being met: (a) the situation in which a rising air parcel must be force lofted through an initially stable layer, but if lifted moist adiabatically to an elevation (LFC) above which it is warmer than the surrounding air; (b) a situation in which the actual environmental lapse rate lies between the dry adiabatic and moist adiabatic rates; (c) a situation in which the equivalent potential temperature decreases with height. For all three conditions the saturation plays a role.
Absolute Instability – Defined in a number of ways: (a) the situation in which the LFC is at the base of a layer so that any lofting of an air parcel at the bottom of the layer brings it immediately to a situation in which it is warmer than the air around it at the same elevation; (b) the potential temperature decreases with height.
Absolute Stability– Defined in a number of ways: (a) the situation in which there is no LFC at any level in the layer so that any lofting of an air parcel at the bottom of the layer brings it immediately to a situation in which it is colder than the air around it at the same elevation; (b) the potential temperature and/or the equivalent potential temperature increases with height.