عنوان مقاله [English]
In order to identify the pressure patterns causing the cold weather over the northwestern Iran, daily lowest temperatures of six stations of the study area during January- March months of the 1986-2003 period were obtained from Meteorological Organization of Islamic Republic of Iran (IRIMO) in the quality controlled format. According to the defined threshold, the very cold days were extracted and their pressure distributions were classified to four patterns using the Spss Principal Component Analysis and Clustering programs. These four pressure patterns are: western anticyclones, Oral Low pressure, Ararat High pressure, and Zonal pattern. These patterns showed that in 70 percent of the days a deep 500 hPa trough located over Iran brings very cold air to the study area beneath its rear part. And at the surface a strong anticyclone develops. On the 30 percent of the times the surface radiational cooling is the major cause of the cold weather.