FAA Flight Instructor Airplane (FIA)Weather and Weather ServicesHard
A meteorologist calculates a Lifted Index (LI) value of -6 for an air mass over an airport during the afternoon. How should this value be interpreted regarding atmospheric stability?
- AThe atmosphere is strongly unstable, favoring vigorous thunderstorm development
- BThe value cannot be used to assess stability without surface temperature data
- CThe value indicates a neutral atmosphere with no vertical motion
- DThe atmosphere is strongly stable, and thunderstorms are unlikely
Show answer & explanationAnswer & explanation
Correct answer: A. The atmosphere is strongly unstable, favoring vigorous thunderstorm development
The Lifted Index compares the temperature a parcel would have if lifted to 500mb against the actual environmental temperature at that level. A negative LI means the lifted parcel is warmer (less dense) than its surroundings, indicating instability; more negative values (like -6) indicate stronger instability and greater potential for severe thunderstorms.
Why the other options are wrong
- B. LI is already a calculated stability index and does not require additional surface data to interpret.
- C. A neutral atmosphere would have an LI near zero, not -6.
- D. A negative LI indicates instability, not stability.
Lifted Index (LI)
The Lifted Index is a stability index comparing a lifted parcel's temperature at 500mb to the environmental temperature; negative values indicate instability, with more negative values indicating greater instability.
- LI near 0: neutral stability
- LI slightly negative (0 to -3): marginal instability
- LI strongly negative (below -6): severe thunderstorm potential
Memory trick: Lower the LI, higher the sky-high instability