عنوان مقاله [English]
Occurrence of frosts can cause a lot of damages to economic, cultural, and political sectors. In order to avoid financial losses, it is necessary to identify the characteristics of this phenomenon. This research aims at the synoptic patterns at the level of 500 hPa and the sea level in a freezing period of 30 years. To determine the synoptic patterns of the frosts days, the days with a temperature of 0 degrees centigrade or below zero reported from at least five stations were selected. Then, the geopotential data of the 500hpa level and the data of the sea level pressure were extracted at 10°-100°E and 20°-80°N in a matrix of 312 by 925 based on the widespread frost days from NCEP/NCAR daily data. In the following, the frost phenomenon was analyzed by the synoptic method. In this regard, Principle Components Analysis (PCA) and Clustering analysis (CA) with Euclidian distance and Ward linkage were used too. The results showed that a high-level system building up in the north of the Caspian sea and the east north of Europe (polar vortex) in the original pattern of 500hpa causes cold air with anticyclonic circulation from the east and brings about frosts. Also, the influence of a low-height front from the Caspian Sea with a northern Siberian focus leads to cold air with cyclonic circulation from the west of the region.