Changes in Temporal trend of Thunderstorms in Iran

In this study، trends of temporal variations in thunderstorm hazard in Iran were investigated using the monthly data for the frequency of occurrence of thunderstorm-day during the period from 1960 to 2010 in 25 synoptic stations in Iran and methods of Ward hierarchical cluster analysis using squared Euclidean distance and non-parametric Mann-Kendall test. Three climate zones، with similar trends for the frequency of occurrence of thunderstorms، were identified using the method of cluster analysis which include the zone of Northwest and North-central، the zone of western half and Northeast، and the zone of Southern half، Caspian Sea coast، and part of the Central and Eastern Iran. The results of investigating temporal distribution of thunderstorms in Iran show that the highest frequency of the hazard occurs in May، April، June، and October. The most activity of thunderstorms in seasonal intervals is concentrated in spring and fall seasons. In addition، the maximum frequency of occurrence of thunderstorms in monthly، seasonal and annual intervals in the Northwest and West are much more than elsewhere in Iran. The slope of variation trends of thunderstorm occurrence in Iran show that variations trend in all of the monthly، seasonal and annual intervals is increasing and statistically significant according to the Mann-Kendall test.