Thunderstorms, which usually occur when there are cumulonimbus (Cb) clouds in the sky, can be a natural hazard if they are severe. Most previous studies have focused on the physical and dynamic structure of thunderstorms, with little focus on exploring the trends of thunderstorm occurrence. In this paper, the time series of days with thunderstorms were studied at 30 synoptic stations in Iran during the past five decades (1961 to 2010). The non-parametric Mann-Kendall statistical method was the main process employed as it is very useful in hydro-meteorological studies. The results of this study showed that the trend of days with thunderstorms in Iran was positive in most parts of the country for all months of the year. The largest area with positive and significant trends occurred in April and May, especially in the northwestern and northern parts of Iran. Based on the annual time series, more than 90% of the country’s area has positive (significant or non-significant) trends. In addition, no significant negative trends were observed in days with thunderstorms for any months of the year in the study region. This research supports the hypothesis that climate change can have significant effects on extreme atmospheric phenomena. Additional research is necessary to explore the climatic and physical causes of the trends presented in this study.

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