Lifted Index Calculator
Find the Lifted Index (LI) from a 500 mb environmental temperature and a lifted-parcel temperature, and see the resulting stability classification.
⛈️ What is the Lifted Index (LI)?
The Lifted Index (LI) is a widely used measure of atmospheric instability, computed as the environmental temperature at 500 mb minus the temperature a surface air parcel would have if it were lifted, dry-adiabatically to its lifting condensation level (LCL) and then moist-adiabatically the rest of the way, up to that same 500 mb level. A negative LI means the lifted parcel arrives at 500 mb warmer than its surroundings, so it remains positively buoyant and keeps rising, the hallmark of an unstable atmosphere favorable for thunderstorm development.
Meteorologists rely on the Lifted Index heavily during severe weather forecasting, as a fast, single-number instability check that can be read straight off a sounding without integrating a full CAPE profile, to compare instability at a fixed reference level (500 mb) across many stations or times consistently, and alongside CAPE, CIN, wind shear, and moisture as one of the standard ingredients used to assess overall severe thunderstorm and tornado potential each forecast cycle.
A common misconception is that Lifted Index and CAPE always agree. They usually point the same direction but can diverge, since LI captures instability concentrated specifically at 500 mb while CAPE integrates buoyant energy through the entire depth of the troposphere above the level of free convection. A sounding with strong low-level instability but a stable mid-troposphere can show high CAPE with a comparatively mild LI, or vice versa.
This calculator applies the standard LI definition directly: subtract the lifted-parcel temperature from the environmental temperature at 500 mb, then classify the result using the stability bands meteorologists use operationally, stable, marginally unstable, unstable, or very unstable.
📐 Formula
Per the AMS Glossary of Meteorology and NOAA Storm Prediction Center convention, LI is negative when the atmosphere is unstable at 500 mb, since the parcel arrives warmer than the environment.
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Stable Atmosphere
T500,env = -5°C, T500,parcel = -9°C
Example 2 — Unstable, Showers/Thunderstorms Likely
T500,env = -10°C, T500,parcel = -6°C
Example 3 — Very Unstable, Severe Potential
T500,env = -12°C, T500,parcel = -3°C
❓ Frequently Asked Questions
🔗 Related Calculators
What is the Lifted Index (LI)?
The Lifted Index is a measure of atmospheric instability equal to the environmental temperature at 500 mb minus the temperature a surface air parcel would have if lifted to 500 mb. A negative LI means the lifted parcel is warmer than its surroundings, indicating instability.
How do you calculate the Lifted Index?
LI = T500_env - T500_parcel, where T500_env is the measured or sounding-derived environmental temperature at 500 mb and T500_parcel is the temperature the parcel would have after being lifted dry-adiabatically to its LCL, then moist-adiabatically up to 500 mb.
What does a negative Lifted Index mean?
A negative LI means the lifted parcel is warmer, and therefore less dense, than the surrounding environment at 500 mb, so it is positively buoyant and tends to keep rising. More negative values indicate greater instability and higher severe weather potential.
What LI value indicates severe thunderstorm potential?
Using standard NWS/SPC thresholds, LI below -6 indicates very unstable conditions with the potential for severe thunderstorms. LI between -2 and -6 is considered unstable, with showers and thunderstorms likely.
Is a positive Lifted Index always stable?
A positive LI generally indicates a stable atmosphere at that level, meaning a lifted parcel would be cooler than its environment and resist further ascent. However, other factors like low-level moisture, lift mechanisms, and CAPE can still support convection even when LI is mildly positive.
How is the lifted-parcel temperature at 500 mb normally found?
It is found by lifting a representative surface parcel dry-adiabatically from the surface to its lifting condensation level (LCL), then continuing the lift moist-adiabatically from the LCL up to 500 mb, tracking the parcel's temperature the entire way. This calculator accepts that resulting value directly as an input.
How does Lifted Index differ from CAPE?
LI is a single-level snapshot of instability at 500 mb, quick to compute and easy to interpret, while CAPE integrates buoyant energy through the full depth of the troposphere above the level of free convection. They usually agree but can diverge, especially when instability is concentrated at levels other than 500 mb.
Can Lifted Index be used alone to forecast severe weather?
No. LI is one piece of the picture. Forecasters combine it with CAPE, CIN, wind shear, low-level moisture, and lift mechanisms to assess full severe weather potential, since a single stability index at one level cannot capture the whole atmospheric column.
What is considered a marginally unstable Lifted Index?
An LI between 0 and -2 is typically classified as marginally unstable. This range suggests only weak instability, so any thunderstorms that do form are usually less intense than on days with more strongly negative LI values.
Why is 500 mb used for the Lifted Index rather than another level?
500 mb (roughly 5.5 km / 18,000 ft altitude) is near the mid-troposphere, a level where the temperature difference between a rising parcel and its environment strongly influences whether deep, sustained convection can develop, making it a practical single-level proxy for overall instability.
What units does the Lifted Index calculator use?
Both input temperatures are entered in degrees Celsius, and the resulting Lifted Index is reported in degrees Celsius as well, since it is simply the difference between two Celsius temperature values.