A parcel of air is stable at A and B and becomes unstable at C indicating convective instability from C onwards.
The state of the atmosphere in which there is a high relative humidity at low levels and a low relative humidity at upper levels, such that if a low layer of air is lifted bodily, the ELR (environmental lapse rate) value through the layer will gradually increase until it attains an unstable value. ELR is the lapse rate in the surrounding air within which a volume of air is being lifted. DALR, or dry adiabatic lapse rate, and SALR, or saturated adiabatic lapse rate, are lapse rates of dry and saturated air masses, respectively, when they are lifted adiabatically. The values of DALR and SALR are 3°C and 1.5°C/1000 ft, respectively. This is often seen in cold fronts, where convective instability is the major cause of instability. Also known as potential instability