METEOROLOGIST JEFF HABY
The Lifted Index is a thermodynamic parameter that compares the theoretical 500 mb parcel temperature
(temperature of a parcel of air lifted from the lower
PBL to the 500 mb level) to the actual (environmental)
temperature at 500 millibars. The Lifted Index is plotted with the relative humidity
on model progs and is available at:
Regions with an unstable LI are hatched and partially hide the RH colors. Contours of LI values
are also drawn in white.
A lifted Index value that is positive represents stability of the troposphere with respect to boundary
layer convection. A negative LI indicates
instability. A LI of 0 is neutral. LI should only be used
when forecasting in the warm season or in the warm
sector of a mid-latitude cyclone. Parcels of air will NOT rise from the lower PBL on the colder side of a
frontal boundary or
within dense PBL polar air. The LI is best used when the troposphere has the potential to produce boundary layer
based thunderstorms. The more negative the LI, the more potential acceleration an air parcel has if
lifted to the
Level of Free Convection (LFC). LI values from -1 to -3 are unstable, -4 to -6 are very unstable,
-7 or less are extremely unstable.
CAPE and LI are related in that as a general rule: when the LI decreases,
The LI value says nothing about if storms will occur. It gives a forecaster a general idea of
convective forcing if thunderstorms do develop. Why is LI plotted with
RH? Because unstable LI values combined
with a high RH indicates the troposphere is near saturation and has instability. A
"trigger mechanism" such
as a front will be able to produce thunderstorms and
heavy rain in this high RH low LI environment. Again, LI is
not of much use in the winter because the PBL tends to be
dry (low dewpoints) and cold (stable).
"dynamic forcing without thermodynamic forcing" and
"isentropic lifting" do not mesh well with using the LI.
The LI can be very stable but the troposphere produces precipitation when the three terms in parenthesis