Synoptic patterns of atmosphere middle level producing late spring freezes over Kermanshah province

The aim of this research is recognizing and Classifying Synoptic patterns of the level 500 hp of late spring freezes over Kermanshah province during 1365-1384. After determining freeze temporal-local threshold, 27 days freezing days spread over the region were extracted. Then, level 500 HP height data located in a scope of 15-60 northern latitude and 15-65 eastern longitude was formulated as an S-mode matrix for freezing days. After that, the synoptic patterns were classified using statistical method of Component Analysis and Ward’s clustering method. Finally, the composite map of high a low level of each cluster was prepared and analyzed synoptically. The findings show that at the time of the most severe late spring freezes, mainly five synoptic patterns prevailed in to two main groups: Short waves trough, high atmosphere Cut-Off Low system synoptic. So that, downwards moves of the rear side of the above-mentioned direct cold and relatively dry air masses of high latitude from North–West of Iran towards Kermashah provinces and determine temperature descending and bring about freeze