شناخت پهنه¬های همدید فشار مؤثر بر اقلیم ایران در نیمۀ سرد سال

Introduction
Air pressure, also known as the atmospheric pressure, is the magnitude of force exerted by the atmosphere on a certain extent of surface area.  The average of atmospheric pressure is about 1013 hpa at the sea level. The air pressure is considered as mean, maximum and minimum sea level pressure. The sea level pressure is often investigated as the first step in the study of meteorological events. Lots of researches have been conducted about sea level pressure, map patterns of the pressure in various regions and their linkage to some of indices or different climatic elements. A number of these investigations will be mentioned as the following.
Jones and Simmonds (1993) analyzed the spatial and temporal anomaly of sea level pressure and the center of cyclogenesis in the northern hemisphere. Their findings indicated the significant difference between cyclogenesis centers and the maximum anomaly of the sea level pressure in high latitudes. The highest anomaly of sea level pressure has also been indicated to be in latitudes from 30 N to 40 N and cyclogenesis centers have been seen around 5 to 7 degrees in the north of the region.
Knaff(1997) studied the anomaly effects of sea level pressure on tropical cyclones in Atlantic ocean. The results showed that tropical cyclones of Atlantic Ocean are often developed in the condition of extreme negative anomaly of the sea level pressure and the existence of a deep trough in the upper layer of the troposphere. When the anomaly of sea level pressure is high, the mid layers are drier and Adiabatic cooling in the mid layers of atmosphere is enhanced subsequently. The deep trough of upper layer develops severe baro-clinicity leading to the formation of tropical cyclones.
Yi Yu and Tae Kim (2011) examined the relationship between oscillation of extra tropical sea level pressure and ENSO position in the center and east of the Pacific Ocean. The results showed that the oscillation of sea level pressure in extra tropical region of Pacific Ocean has a great role in ENSO excitement and its movement.  
Mohammadi (1388) investigated the sea level pressure of this kind of precipitation in order to synoptically analyze Iran's extreme rainfalls. Three major sea level pressure patterns have been identified to have a role in the occurrence of the rainfalls. In the first pattern Arabian low pressure/Siberian high pressure dominates over Iran and 28 percent of the heavy and widespread rainfall is resulted from this pattern. In the second pattern, Siberian high pressure in northeastern of Africa and Arabian low pressure account for 53 percent of heavy and widespread rainfall. In the third pattern western Siberian pressure- Iraqi low pressure are the dominant patterns accounting for 19 percent of heavy and widespread rainfall in Iran. So it can be concluded that Arabian/Iraqi low pressure are the main factor in providing indispensible condition at the ground  for the occurrence of super heavy rainfall in the country.
 
Methodology
In the present paper synoptic patterns of sea level pressure have been studied in latitudes from 0 to 80 E and in the longitudes form 0 to 60 N. The 6 hourly pressure data of sea have been applied for fall and winter seasons for the period from 1948 to 2010. The spatial resolution of the data was on a 1°x1° lat/lon grid. Therefore, two individual databases are developed for both fall and winter seasons. A cluster analysis by the method of Ward  is applied on the data of each season.
In Cartesian coordinates, if  and  are the vectors, so their distances are calculated as followed.
 
The position of a point in a Euclidean n-space is a Euclidean vector. So, p and q are Euclidean vectors, starting from the original space, and their tips indicate two goals. Ward merging method was used to the linkage of observations.
 
Results and Discussion
Appling a cluster analysis on 6 hourly sea level pressure data in both winter and fall in the period from 1948 to 2010 have indicated that there are seven major pressure regions in the study area. With regard to the location of the grids chosen to be the representative of each region a name was attributed to them. The area and position of major sea level pressure regions are also depicted.
Conclusion
The investigations indicated that there are seven major sea level pressure regions for each fall and winter seasons. It can be supposed that the major pressure regions identified in this paper are indeed the major pressure systems (sometimes with different titles) that climatologists often refer to them. To better understanding of theses major pressure regions some of their statistical attributes are presented.