27 July 1999 6Z - 28 July 1999 6Z -- GOES Sounder Lifted Index
Lifted Index Shows Instability Over The Great Plains

To interpret how the environment affects thunderstorm potential and severity, meteorologists have invented several stability indices that characterize the atmosphere in a single number. Perhaps the most common stability index is the lifted index.

The Lifted Index starts with an air parcel from the surface, lifting and cooling it dry adiabatically to saturation, and then lifting and cooling it moist adiabatically to 500 mb. To compute the lifted index, the temperature of the parcel at 500 mb is subtracted from the environment’s temperature at 500 mb, as measured by a radiosonde or satellite. If the observed 500-mb temperature is colder than the lifted air parcel, then the parcel is unstable and will be able to keep on rising and form a cumulonimbus cloud. The lifted index is negative in these cases.

For this reason, negative values of the lifted index can be related to thunder-storm severity. A lifted index of between 0 and 3 (degrees Celsius) indicates that the air is marginally unstable and unlikely to lead to severe thunderstorms. Values between 3 and 6 indicate moderately unstable conditions. Values between 6 and 9 are found in very unstable regions. Lifted index values less than 9 reflect extreme instability. The chances of a severe thunderstorm are best when the lifted index is less than or equal to 6. This is because air rising in these situations is much warmer than its surroundings and can accelerate rapidly and create tall, violent thunderstorms. However, this index does not tell the probability of occurrence of a thunderstorm; other indices predict this.

An animation of the lifted index on 27-28 July 1999 is shown below. Note the convection forming over the center of instability over the Dakotas.

(this 25-image Java animation sequence may take a minute or two to load...)

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