Thelifted index (LI) is thetemperature difference between the environment Te(p) and anair parcel liftedadiabatically Tp(p) at a given pressure height in thetroposphere (lowest layer where most weather occurs) of theatmosphere, usually 500hPa (mb). The temperature is measured in Celsius. When the value is positive, the atmosphere (at the respective height) is stable and when the value is negative, the atmosphere isunstable.
LI can be computed using computer algorithms but can also be determined graphically. To do this, generally, theparcel is lifted from the portion of theplanetary boundary layer (PBL) that lies below the morninginversion. The air here should be about 60 to 65%RH, which is then lifted along the dry adiabat (see alsoadiabatic process) to thelifting condensation level (LCL), which is the intersection of that curve with the averagemixing 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).
LI is generally scaled as follows:
The lifted index can be used inthunderstorm forecasting, however,convective available potential energy (CAPE) is considered by most as a superior measurement of instability and is preferred by many meteorologists for convection forecasting.[1] However, LI is easier and faster to determine without using a computer, as determining CAPE requires integration from one level to another.