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 thunder storm 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.
Showalter Index
Showalter index is calculated using the environmental temperature and the dew point at 850 hPa and environmental temperature at 500 hPa. The index is derived from an air parcel which is lifted dry-adiabatically from 850 hPa level to the lifted condensation level (LCL). From the LCL it rises up to 500 hPa along the moist adiabat. The resulting
Tparcel is subtracted from the actual 500 hPa temperature,
Tenv
Showalter Index = Tenv - Tparcel
The Showalter index which was presented in this case study shows large negative values. The larger the negative values, the larger the instability and the chance on thunderstorms. Threshold values for thunderstorms vary, but generally values lower than +3 already indicate a possibility for thunderstorms, values lower than -3 depict even a further unstable situation and the possibility for heavy thunderstorms is inevitable.
| Showalter-index |
TS probablity |
| >3 |
Thunderstorms unlikely |
| 0 to 3 |
Thunderstorms possible |
| -3 to 0 |
High potential of heavy thunderstorms |
| <-3 |
Extremely unstable |
Boyden Index
The Boyden index has been developed to allow better diagnosis of frontal thunderstorms. No humidity information is included in the calculation. The index is calculated using the following formula:
Boyden Index = Z700-1000 - T700 - 200
in which
Z700-1000 is the thickness of the layer between 700 and 1000 hPa in decametres,
T700 the temperature (in degrees Celsius) at 700 hPa. Scaling factor 200 is used to bring the absolute values of the Boyden index close to a range from 90 to 100. Since the index does not contain surface temperature information, there is little variation between continental and marine air masses. The diurnal variation is also small.
The higher the value of the Boyden index, the greater the risk of thunderstorm. The threshold value is approximately 95.
K Index
The K-index was composed for forecasting air mass thunderstorms, or thunderstorms with no dynamic triggering mechanism. The index is computed by first taking the 850 hPa temperature minus the 500 hPa temperature. Then secondly dew point temperature at 850 hPa is added to this difference. Larger values of this dew point indicate low level moisture present and increase the chance of convection. In a final step the 700 hPa dew point depression is substracted for the moisture input at mid levels. A small dew point depression at 700 hPa indicates a possibility for deep convection. If there is no significant moisture at 700 hPa there is a greater chance that entrainment of dry air would occur, given a parcel were lifted from beneath the 700 hPa level. If entrainment of dry air occurs, the parcel will become less buoyant (Bluestein, 1993).
K Index = (T850 - T500) + Td850 - (T700 - Td700)
The thunderstorm potential then reads itself according to the following scale:
| K-index |
TS probablity |
| <20 |
Thunderstorms unlikely |
| 20 to 25 |
Isolated thunderstorms |
| 26 to 30 |
Widely scattered thunderstorms |
| 31 to 35 |
Numerous thunderstorms |
| Above 35 |
Thunderstorms very likely |
Lifted index
A fourth common measure of atmospheric instability is the lifted index (LI). Its value is obtained by computing the temperature that air near the ground would have if it were lifted dry-adiabatically to some higher level (usually 500 hPa) and to compare that temperature to the actual temperature at that level.
Negative values indicate instability - the more negative, the more unstable the air is, and the stronger the updrafts are likely to be with any developing thunderstorms.
The lifted index should be used cautiously as it only shows instability at one point in the atmopshere, not the measure of instability across the whole atmospheric region (CAPE). LI values of less than -6 usually indicate that severe storms are possible. LI values can occasionally reach -12 and lower.
References