Lifted index: Galway developed it, back in 1956.
Lifted Index. Temperature difference between the environmental and parcel temperatures at the 500 mb level.
LI= 500 mb envir. Temp - 500 mb parcel Temp
LI = T500 - TP500
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Source: NWS A Comprehensive Glossary of Weather Terms for Storm Spotters
Lifted Index (or LI) - A common measure of atmospheric instability. Its value is obtained by computing the temperature that air near the ground would have if it were lifted to some higher level (around 18,000 feet, usually) and comparing that temperature to the actual temperature at that level. Negative values indicate instability - the more negative, the more unstable the air is, and the stronger the updrafts are likely to be with any developing thunderstorms. However there are no "magic numbers" or threshold LI values below which severe weather becomes imminent.
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Source: NWS Springfield
The LI is a measure of potential instability from the surface to 500 mb. Lift a parcel with an average mixing ratio and dry adiabat in the lowest 100 mb of the sounding. It is very similar to the Showalter Index, but better considers available low level moisture below 850mb.
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Source: NWS Louisville
Lifted Index (LI)
The LI is a commonly utilized measure of stability which measures the difference between a lifted parcel's temperature at 500 mb and the environmental temperature at 500 mb. It incorporates moisture and lapse rate (static stability) into one number, which is less vulnerable to observations at individual pressure levels. However, LI values do depend on the level from which a parcel is lifted, and rally cannot account for details in th environmental temperature curve above the LCL and below 500 mb. LI was originally intended to utilize average moisture and temperature properties within the planetary boundary layer.
LI = T(500 mb envir) - T(500 mb parcel)
in degrees C, where T (500 mb envir) represents the 500 mb environmental temperature and T (500 mb parcel) is the rising air parcel's 500 mb temperature.
LI over 0: Stable but weak convection possible for LI = 1-3 if strong lifting is present.
LI = 0 to -3: Marginally unstable.
LI = -3 to -6: Moderately unstable.
LI = -6 to -9: Very unstable.
LI below -9: Extremely unstable.
These LI values are based on lifted parcels using the average lowest 50 to 100 mb moisture and temperature values (i.e., the boundary layer). Variations exist on how LI values are calculated,
Surfaced-based LI: Surface-based LIs can be calculated hourly, and assume a parcel is lifted from the surface using surface-based moisture and temperature values, as well as assigned environmental temperatures at 500 mb. This method is valid for a well-mixed nearly dry adiabatic afternoon boundary layer where surface characteristics are similar to those in the lowest 50 to 100 mb layer. However, these values would not be representative of the ambient elevated instability if a nocturnal inversion or shallow cool air to the north of a frontal boundary is present. In these cases, more instability resides above the surface, and parcels may be lifted to form thunderstorms from the top of the inversion.
Best LI: The Best LI represents the lowest (most unstable) LI computed from a series of levels from the surface to about 850 mb. This index is most useful during cases when shallow cool air exists north of a frontal boundary resulting in surface conditions and boundary layer-based LI values that are relatively stable. However, the airmass at the top of the inversion, from which lifting may occur, is potentially unstable. An example of this would be elevated ("overrunning") convection (possibly a nocturnal MCS).
Mike
