عنوان مقاله [English]
The present study aims to investigate the changes in atmospheric conditions causing a hazardous climatic phenomenon known as hailstorm. These changes are classified and related to thermal and dynamic changes. In order to study the dynamics and synoptic of hail phenomenon in Mashhad, daily hail rainfall data were used during the 1980-2010 period. Then, one heavy rainfall specimen was identified and selected during the statistical period. The results of the hail analysis indicate that the highest frequency of hail occurrences were from 9 to 13 (UTC) in Mashhad city. Moreover, the highest frequency was from March to May months. In atmospheric predictions, there are different indicators for assessing the instability of atmosphere which lead to hail occurrence. The parameters and indicators under investigation in this study for 28 July 2009 include SHOW, TT, K, BI, RI, JEFF, Boyden, DCI, KO and SOI. According to the results of this research, when hail occurred in Mashhad, instability indices increased meaningfully, In addition, all instability indicators showed severe instable conditions (thunderstorm) during hailstone occurrence. Analyzing the weather charts indicated the changes of climatic parameters on the days of hailstorm and the days before them. Charts showed that the temperature and air pressure decreased while the relative humidity and dew point temperature increased on the days of hail in comparison with the earlier day.