Parameters & indices#instability#sounding

Lifted Index (LI)

Also: LI

At a glance

A one-number instability check: the temperature difference between a lifted parcel and the environment at 500 hPa. Negative = unstable.

Deep dive

LI = T(env, 500 hPa) − T(parcel, 500 hPa), where the parcel is lifted from the surface. A parcel warmer than its environment at 500 hPa is buoyant → negative LI → instability.

LI 0 to −2 — slightly unstable; showers.
−2 to −4 — unstable; thunder likely.
−4 to −6 — very unstable; severe weather possible.
< −6 — extremely unstable.

LI predates CAPE and is coarser — CAPE is the continuous, vertically integrated version. Forecasters still report LI because it's a quick mental shortcut.