Lifted index
Thunderstorms LI
The lifted index compares a theoretically lifted air parcel with its surroundings around five to six kilometres high. A negative value indicates potential buoyancy.
Giving an air parcel a trial lift
To calculate the lifted index (LI), a selected, usually near-surface parcel is raised to the 500 hPa level. Its temperature and moisture change according to defined physical assumptions. Its final temperature is then compared with the environment.
Why a negative value can mean more buoyancy
The calculation is environmental temperature minus parcel temperature. If the surroundings are −15 °C and the parcel is −11 °C, the LI is −4 K. The parcel would be four degrees warmer and could continue rising. For a temperature difference, one kelvin has the same size as one Celsius degree.
What a single-level comparison misses
CAPE considers an entire layer. The lifted index compares temperatures at one pressure level. A stable layer lower down can prevent ascent, and that barrier is not fully described by the LI value.
The choice of starting parcel also changes the result. No LI value alone reliably fixes a storm’s location, timing or strength. A negative sign is a clue for analysis, not an alarm button.