The Lifted Index (LI) is defined as a rising parcel's temperature when it reaches the 500 millibars level (at about 5,500m or 18,000 feet asl), subtracted from the actual temperature of the environmental air at 500 mbar. If the Lifted Index is a large negative number, then the parcel will be much warmer than its surroundings, and will continue to rise. Thunderstorms are fueled by strong rising air, thus the Lifted Index is a good measurement of the atmosphere's potential to produce severe thunderstorms.

The Lifted Index (LI)
RANGE IN K	COLOR	AMOUNT OF INSTABILITY	THUNDERSTORM PROBABILITY
more than 11	BLUE	Extremely stable conditions	Thunderstorms unlikely
8 to 11	LIGHT BLUE	Very stable conditions	Thunderstorms unlikely
4 to 7	GREEN	Stable conditions	Thunderstorms unlikely
0 to 3	LIGHT GREEN	Mostly stable conditions	Thunderstorm unlikely
-3 to -1	YELLOW	Slightly unstable	Thunderstorms possible
-5 to -4	ORANGE	Unstable	Thunderstorms probable
-7 to -6	RED	Highly unstable	Severe thunderstorms possible
less than -7	VIOLET	Extremely unstable	Violent thunderstorms, tornadoes possible