Potential instabil means that only in the case of an upward motion a destabilization of the atmosphere and convection occur. There exist many indices to forecast the intensity of convection. In this case study we only use the KO-index. In the KO-Index the potential equivalent temperatures (Θa) at 1000, 850, 700 and 500 hPa are considered.
The KO-Index is defined as:
Θa is the temperature after condensation of all water vapour without changing the pressure at one particular level. The air package gets all condensation energy and is moved afterwards to 1000 hPa. During stable situations Θa is increasing with height. For a potential unstable air mass we get a decrease. The formular indicates that negative values are related to a decrease of Θa between lower and middle altitudes.
The following thresholds are being used:
- KO > 6: no thunderstorms
- 6 > KO > 2: thunderstorms possible
- KO < 2: thunderstorms likely