Definition
A measure of atmospheric stability calculated by taking a parcel of air near the surface, lifting it dry-adiabatically to its lifting condensation level and then moist-adiabatically to the 500-millibar pressure level (around 18,000 feet), and subtracting that parcel's temperature from the actual environmental temperature at 500 millibars. The result is expressed in degrees Celsius. Negative values indicate an unstable atmosphere prone to thunderstorm development; positive values indicate stable conditions.
Plain English
A number that tells you how likely the atmosphere is to produce thunderstorms. It is found by imagining a bubble of surface air being pushed up to about 18,000 feet and comparing how warm that bubble would be to the actual air at that height. If the bubble would be warmer than its surroundings (a negative number), the air is unstable and storms are likely.
Context Anchor
Seen in weather briefings, stability charts, and forecasts that discuss thunderstorm potential.
Derivation
From 'lifted' (the parcel of air is lifted upward in the calculation) and 'index' (a single number used as an indicator). The name describes the method: lift a parcel, then index the result.
Why Pilots Care
Negative values signal possible convective activity that can produce turbulence, icing, and reduced visibility.
Grounding Statement
If a lifted sample of air is warmer than the air around it, it tends to keep rising; if it is cooler, it tends to sink back down.
Intuition Check
“Lifted” does not mean the height of cloud bases or how high the weather is. In Lifted Index, it means an imagined sample of air is raised to a standard pressure level so its temperature can be compared with the air around it.
Example Sentence 1
The forecast showed a Lifted Index of -6 over the destination, so the pilot delayed the flight until the afternoon storms passed.
Example Sentence 2
With a positive lifted index the air felt stable and the pilot expected smooth conditions.