The lifted index (LI) is the temperature difference between an air parcel lifted adiabatically Tp(p) and the temperature of the environment Te(p) at a given pressure height in the troposphere (lowest layer where most weather occurs) of the atmosphere, usually 500 hPa (mb). When the value is positive, the atmosphere (at the respective height) is stable and when the value is negative, the atmosphere is unstable.

LI can be computed using computer algorithms but can also be determined graphically To do this, generally, the parcel is lifted from the portion of the planetary boundary layer (PBL) that lies below the morning inversion. The air here should be about 60 to 65% RH, which is then lifted along the dry adiabat (see also adiabatic process) to the lifting condensation level (LCL), which is the intersection of that curve with the average mixing ratio in the boundary layer. Once the LCL is found, the parcel is lifted along the moist adiabat to 500 mb. It is then that one finds LI = Te(p) - Tp(p).