What does a lifted index indicate?

A lifted index is a meteorological parameter used to assess air stability, specifically in relation to convection and potential thunderstorm formation. The index measures the difference in temperature between a parcel of air lifted from the surface and the temperature of the environment at a specific altitude, usually around 500 hPa (about 18,000 feet).

When the lifted index is high and positive, it indicates that the air is relatively stable, meaning that a parcel of air will be cooler than the surrounding environment as it rises. This stability discourages vertical motion, thus reducing the likelihood of convection and severe weather. Conversely, a negative lifted index suggests an unstable atmosphere, which can lead to vigorous convection and the development of thunderstorms.

Understanding the significance of the lifted index helps pilots and meteorologists make informed decisions regarding flight safety and weather conditions. The other options do not accurately represent the purpose of the lifted index, as it does not relate to visibility, ground temperature variations, or humidity levels.