Stability of the Atmosphere
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, as discussed below.
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).
Source: NWS
Other UKAWC Stability Indices:
Ag Weather Center, Department of Biosystems &
Agricultural Engineering, University of Kentucky
