Physical Background
Stability indices are a group of indices that have been developed to illustrate the potential for convection within an air mass based upon a radiosonde or model derived sounding. In most cases stability indices are derived from the temperature and humidity data of the sounding at certain fixed levels. Some stability indices even use information from all observed levels of the sounding (such as CAPE). Other indices even contain a contribution of wind information.
All indices, however, eventually give only one numerical value (which may be dimensionless). This value represents the potential for convection at a certain fixed location. The value of the stability index is compared to some statistical threshold value differentiating e.g. non-thunderstorm and thunderstorm cases.
For air mass classification purposes index values are plotted and analysed on a weather map, preferably with a suitable satellite underlay image. This latter method is particularly effective for model derived stability indices, since model derived soundings can be calculated at each model grid point for each model time step.
In meteorology, the use of convection indices varies from country to country, reflecting different methods, but also the different climatologies. A full list of indices used would contain tens of different indices and would go beyond the scope of this physical background. For this reason only a small selection of stability indices are presented which are commonly used in EUMeTrain.
Allthough the above described stability indices give a good indication where thunderstorms may occur an external trigger (such as a WV dark stripe) is still needed to start the convection.
Especially in situation with a strong capped inversion the stability indices may depict a highly unstable situation but due to the absence of an external trigger convective development is hampered. Such situations which is also reffered as a "loaded gun" is indicative for the summerseason and also for the case of 30 July 2005 over Croatia. This case is thoroughly described in the following section.