Lifted Index

The Lifted Index (LI) is the oldest trick in the forecaster's book: take an imaginary bubble of air from near the ground, lift it to about 5.5 km (the 500 hPa level), and compare its temperature with the air already up there.

>Reading the sign

• Positive LI — the lifted bubble arrives colder than its surroundings. Cold air sinks, so the bubble gives up. Stable — storms struggle.
• Around 0 to −2 — roughly a tie. Storms possible.
• −2 to −6 — the bubble arrives clearly warmer and keeps accelerating. Unstable — storms likely if something sets them off.
• Below −6 — extremely unstable. The atmosphere is a loaded spring.

Notice the flip: with LI, more negative = more dangerous, the opposite of CAPE. The two measure the same idea differently — CAPE adds up the energy of the whole journey, LI takes one snapshot at 5.5 km. When both agree the sky is unstable, trust them.

>Why it survives

LI is over 70 years old and modern models compute far fancier numbers — but one glance at a single value still tells a forecaster whether today is worth watching. In the Tornado Outlook it works as the Fuel meter's second opinion: a strongly negative LI can upgrade marginal CAPE, because it means the energy is arranged for deep, tall storms.