studying the trend of changes in the frequency of days with thunderstorms in iran using nonparametric methods

thunderstorms are climatical phenomena, which sometimes can cause irreparable damage to the ecology. in this study to identify any trends in the frequency of days with thunderstorms in iran, long-term statistics of 33 synoptic stations, from the beginning until 2005 was used. at first by applying nonparametric man-kendall and sen’s estimator methods on four seasonal time series, and one annual time series for each station, their significance is assessed. also, for recognizing the starting years of this trend, man-kendall step by step test plot was employed. results are showing that the maximum spatial frequency of days with thunderstorms across iran is a function of time. evaluation processes using both tests indicate that all significant trends in all series except the annual series of bam stations are increasing. so due to these two test similarity, the trend of thunderstorms in all five time series could be considered increasing which maximum and minimum of trends belong to annual and wintery time series respectively so the trends annual time series of 58% of the stations were significant. also the step by test revealed that in annual time series most of the significant trends have been started from decades of 70 and 80.