The KO-index is an index for the potential instability of stratification of the atmosphere. A stratification is potential unstable, in case of decreasing pseudopotential or equivalent-potential temperature with height.
The reason for the destabilization of the atmosphere is the release of latent energy by condensation in the lower parts of the uplifted layer.
The KO-Index is derived by the mean differences of the equivalent-potential temperature between 500/700 hPa and 850/1000 hPa and is used as an indicator for thunderstorm early warning.
The following thresholds are being used:
Negative values of -6 to -10 in combination with initial uplifting processes indicate a very high potential for intense convective development.
In the image, low values are found in the forefield of the cold front where the convective development will take place. Over western Germany values of up to -9 indicate an extremely unstable stratification of the atmosphere. Therefore, severe thunderstorms must be expected in adjacent areas.