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This paper used the relevant data to study the frequency of thunderstorm occurrence in Tabriz as a sample of North West Iran’s climate. Where required, the methods such as Spearman rank correlation, linear and sixth order polynomial trend, as well as the cluster analysis according to the average linkage (within group) has also been applied utilizing the block distance standard for the monthly classification of the thunderstorm occurrence. The obtained results showed that the month of May, during various monthly time scales, and the spring season in seasonal intervals, undergo the majority of thunderstorm occurrences. The methodological statistics show that, secondary climatic features like hails, heavy showers, floods, and lightening are dominant climatic features in Tabriz during the thunderstorm occurrence. The above-mentioned climatic hazards are part of the characteristics of Tabriz climate and all of North West Iran, which bring about many economic, social and environmental damages, particularly for the farmers and animals farms. The probability distribution of thunderstorm occurrence in Tabriz indicates the irreversibility of their occurrence chiefly in short time return periods. Therefore, thunderstorms and their resulting secondary features are regarded to be a permanent hazard in Tabriz.

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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.

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Introduction
Thunderstorms are one of the most important atmospheric hazards that cause human casualties every year, destroying large amounts of agricultural products , construction and infrastructure. Due to the combination of Thunderstorms with rainstorms and its effective role in causing sudden floods, this phenomenon has always been the main focuse of researchers in agriculture and financial issues. Considering this matter, we have studied climatic conditions of Khuzestan province in order to investigate this phenomenon and its causes. It is necessary to determine days of the thunderstorm and its sequence in different months, seasons and years, so experts can identify the atmospheric patterns causing this occurrence. Also they can provide necessary strategies and preconditions to inform farmers, gardeners and officials. Therefore, the purpose of this researh is studing the Thunderstorms in Khuzestan Province using synoptic, mann - kendall and Sen indexes during the period of 2003-2013.
 
 
 
 
 
 
 
 
 
Methodology
In order to investigate the climatic phenomenon of Thunderstorm in Khuzestan province, information and statistics of 12 synoptic stations during the 10 year statistical period (2003-2013) are investigated. The statistics and data are collected and tested in order to extract a similar statistical period. The data are analyzed in Excel and thunderstorms dispersion map is developed in Khuzestan. The data of thunderstorms in Khuzestan province are investigated and spatial-temporal analysis of this climatic phenomenon is studied in the study area. during the statistical period changes in Thunderstorms days are studied based on non-parametric Mann-Kendall and Sen index. also geographical dispersion maps of trend type (incremental, constant and decreasing) associated with Thunderstorm are prepared in seasonal and annual scales, then synoptic maps are prepared and evaluated.
 
Results and Discussion
In the spring data series, there is an increasing and decreasing trend in study stations. By analyzing summer data, it is found that there is significant decreasing trend in the most stations except Dezful, Izeh and Masjed Sulayman stations using the Man-Kendall test. The analysis of autumn and winter data also shows an increasing trend in all stations. In this series of data, the Mann-Kendall test and age index test at 95 and 99 confidence levels are significant. In the spring, the highest significant trend is observed in Dezful, Izeh, Ahvaz, Bostan and Omidieh stations. In the summer, the southern, central and western parts of the region have a significant decreasing trend. In the autumn, there is a significant increase in the whole region. In the winter, the southern and southeastern parts of the region have a steady trend. The spatial distribution of the annual trend of Thunderstorm days in Khuzestan province indicates a significant increase in most of stations. At Abadan, Bandar Mahshahr and Hendijan stations there is a significant decrease, and there is a constant trend in Behbahan and Ramhormoz stations. According to storm occurrence maps and analyzing of the maps, it can be seen that the occurrence of thunderstorms in Khuzestan province is generally frontier and is due to hot air flood from the Persian Gulf. in spite of the conditions for the tornado incident there has not been a twist in the region, due to the salinity of the Persian Gulf and a its small extent to provide enough moisture for the occurrence of this phenomenon.
 
 
 
 
 
 
 
 
 
 
 
 
 
Conclusion
In the spring data series, there is an increasing and decreasing trend in study stations. By analyzing of summer data, it was determined that using Mann-Kendall test, a significant decrease is observed in most stations except Dezful, Izeh and Masjed Soleiman stations. This trend is not observed with the age index test. The analysis of autumn and winter data also shows an increasing trend in all stations. According to the results of Mann-Kendall test on an annual scale, all of the stations have a significant incremental trend. At Abadan, Bandar Mahshahr and Hendijan stations there is a significant decrease, and there is a constant trend in Behbahan and Ramhormoz stations. Investigation of atmospheric patterns is shown that the cause of Thunderstorm phenomenon in the region is the result of strengthening and expansion of low pressure systems in the East of the Red Sea and Saudi Arabia. Therefore, cold airflow from the middle levels with high temperature changes in the upper and lower layers of the atmosphere causes instability and extreme climbing of the air, formation of cumulonimbus clouds, heavy falls of rainfall, lightning, hail and ... in selected periods in Khuzestan province.