On recent variations of the annual thunderstorm activity in the region of the Alps

Summary Observations of days with thunderstorm at twelve stations in the Alpine and Pre-Alpine region have been used to look for trends and periodicities in the thunderstorm activity in this area. The annual number of days with thunderstorms was represented by negative binomial distributions, then by means of a non-parametric trend test, parts of the series were found in which the numbers of days with thunderstorm were increasing, decreasing or uniform at the various stations. A variance spectrum analysis showed the existence of long-periodic waves and of short-periodic waves superposed on the background red noise or white noise spectrum. Partitioning the series of observations yielded that these periodicities are not stable.