Lifted index on:
[Wikipedia]
[Google]
[Amazon]
The lifted index (LI) is the
Lifted Index
(
A look at LI
Convective indices
(NWS St. Louis)
AMS Glossary stability indices, including LI
* {{Meteorological variables Severe weather and convection Atmospheric thermodynamics
The lifted index (LI) is the temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied o ...
difference between the environment Te(p) and an air parcel In fluid dynamics, within the framework of continuum mechanics, a fluid parcel is a very small amount of fluid, identifiable throughout its dynamic history while moving with the fluid flow. As it moves, the mass of a fluid parcel remains constant, ...
lifted adiabatically
Adiabatic (from ''Gr.'' ἀ ''negative'' + διάβασις ''passage; transference'') refers to any process that occurs without heat transfer. This concept is used in many areas of physics and engineering. Notable examples are listed below.
A ...
Tp(p) at a given pressure height in the troposphere
The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. Fro ...
(lowest layer where most weather occurs) of the atmosphere
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A ...
, usually 500 hPa ( mb). The temperature is measured in Celsius. When the value is positive, the atmosphere (at the respective height) is stable and when the value is negative, the atmosphere is unstable.
Determining LI
LI can be computed using computer algorithms but can also be determined graphically. To do this, generally, the parcel is lifted from the portion of the planetary boundary layer (PBL) that lies below the morninginversion
Inversion or inversions may refer to:
Arts
* , a French gay magazine (1924/1925)
* ''Inversion'' (artwork), a 2005 temporary sculpture in Houston, Texas
* Inversion (music), a term with various meanings in music theory and musical set theory
* ...
. The air here should be about 60 to 65% RH, which is then lifted along the dry adiabat (see also adiabatic process
In thermodynamics, an adiabatic process (Greek: ''adiábatos'', "impassable") is a type of thermodynamic process that occurs without transferring heat or mass between the thermodynamic system and its environment. Unlike an isothermal process, ...
) to the lifting condensation level (LCL), which is the intersection of that curve with the average mixing ratio
In chemistry and physics, the dimensionless mixing ratio is the abundance of one component of a mixture relative to that of all other components. The term can refer either to mole ratio (see concentration) or mass ratio (see stoichiometry).
In ...
in the boundary layer. Once the LCL is found, the parcel is lifted along the moist adiabat to 500 mb. It is then that one finds LI = Te(p) - Tp(p).
LI is generally scaled as follows:
* LI 6 or Greater, Very Stable Conditions
* LI Between 1 and 6 : Stable Conditions, Thunderstorms Not Likely
* LI Between 0 and -2 : Slightly Unstable, Thunderstorms Possible, With Lifting Mechanism (i.e., cold front, daytime heating, ...)
* LI Between -2 and -6 : Unstable, Thunderstorms Likely, Some Severe With Lifting Mechanism
* LI Less Than -6: Very Unstable, Severe Thunderstorms Likely With Lifting Mechanism
Significance to thunderstorms
The lifted index can be used inthunderstorm
A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorms are some ...
forecasting, however, convective available potential energy (CAPE) is considered by most as a superior measurement of instability and is preferred by many meteorologists for convection forecasting. However, LI is easier and faster to determine without using a computer, as determining CAPE requires integration from one level to another.
See also
* Atmospheric convection *Atmospheric thermodynamics Atmospheric thermodynamics is the study of heat-to- work transformations (and their reverse) that take place in the earth's atmosphere and manifest as weather or climate. Atmospheric thermodynamics use the laws of classical thermodynamics, to des ...
* Convective instability
* Lapse rate
References
External links
Lifted Index
(
University of Illinois
The University of Illinois Urbana-Champaign (U of I, Illinois, University of Illinois, or UIUC) is a public land-grant research university in Illinois in the twin cities of Champaign and Urbana. It is the flagship institution of the Uni ...
)
A look at LI
Convective indices
(NWS St. Louis)
AMS Glossary stability indices, including LI
* {{Meteorological variables Severe weather and convection Atmospheric thermodynamics