Gradual changes of climate in the west of Iran based on frequency of air masses presence in winter

Significant repetition of daily dominance of definite air mass in each region that requires good condition of meteorological conditions creates a definite climate. In this research, winter air masses were studied in 5 synoptic stations in the west of Iran including Kermanshah, Ilam, Khoram Abad, Nozhe and Sannadaj in period 1961-2004. 3 kinds of winter air mass being identified by Spatial Synoptic Classification of Islamic Republic of Iran (SSCIRI), annual and monthly distribution and daily share of each of them in each month were evaluated in terms of historical and climatic changes. The west of Iran in a brief look and from temperature aspect is divided into the stations including Kermanshah, Nozheh of Hamedan and Sanandaj as being cold and Khoram Abad and Ilam stations inclined to southwest and are hot. The calculations of monthly and annual evaluation of air masses frequency based on Kermanshah station was done as relative representation of the west of Iran. Totally and according to the studies, it is concluded that winter in Kermanshah station and for west of Iran had hot condition and winter period with its historical characteristics in which coldness and snowfall are its main components, had less occurred and it is getting short and spring condition is more and more visible namely in March. Keywords: Iran; Climate change; air masses; presence frequency; trend; Kermanshah; SSC