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.
Determining LI 
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).
LI is generally scaled as follows:
- LI 6 or Greater, Very Stable Conditions
- LI Between 1 and 6 : Stable Conditions, Thunderstorms Not Likely
- LI Between 0 and -2 : Slightly Unstable, Thunderstorms Possible, With Lifting Mechanism (i.e., cold front, daytime heating, ...)
- LI Between -2 and -6 : Unstable, Thunderstorms Likely, Some Severe With Lifting Mechanism
- LI Less Than -6: Very Unstable, Severe Thunderstorms Likely With Lifting Mechanism
Significance to thunderstorms 
The lifted index can be used in thunderstorm 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. However, LI is easier and faster to determine without using a computer, as determining CAPE requires integration from one level to another.
See also 
- Lifted Index (University of Illinois)
- A look at LI
- Convective indices (NWS St. Louis)
- AMS Glossary stability indices, including LI
- Doswell, III, Charles A.; D.M. Schultz (2006). "On the Use of Indices and Parameters in Forecasting Severe Storms". Electronic Journal of Severe Storms Meteorology 1 (3).