During the early Spring/Summer and early Autumn months one of the main features of the UK weather are showers and thunderstorms. In order to assist in the forecasting of thunderstorms, meteorologists will use LI or Lifted Index as part of their toolkit in order to ascertain how unstable the atmosphere is at lower levels.
The Lifted Index is a measurement for telling us how unstable the atmosphere is from the surface to the 500mb level.
We calculate this by taking the surface temperature at any given time and then calculating what the temperature of this parcel of air should be if it was lifted to the 500mb level and cooled adiabatically using the moist adiabatic lapse rate i.e 6c per km.
So, if the surface temperature was 18c and the 500mb level was at 4km above sea level then the 500mb temperature would be expected to be -6c (18c - 4x6c), we call this the parcel temperature. We then take a look at what the forecast temperature or actual temperature is at the 500mb level, which we call the environment temperature and the difference between the two is the Lifted Index.
So, the Lifted Index is the environment temperature-parcel temperature
Once we have this figure we can forecast how thermodynamically unstable the atmosphere is at the lower levels. We use this as a guide :-
6 or More, Very Stable Conditions
Between 1 and 6 : Stable Conditions, Thunderstorms Not Likely
Between 0 and -2 : Slightly Unstable, Thunderstorms Possible, With Lifting Mechanism
Between -2 and -6 : Unstable, Thunderstorms possible
Less Than -6: Very Unstable, Severe Thunderstorms possible
The Lifted Index though is not the holy grail of forecasting thunderstorms and you should only use it as one of the tools in your toolbox for forecasting thunderstorms. Things like thin warm layers, cloud cover which affects surface heating can affect the CAP markedly and can bust thunderstorm forecasts if you use this index alone.
But, when thunderstorms are forecast for your area then calculate the Lifted Index to see how likely they are to develop based on the atmospheric instability at lower levels.