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