Introduction
According to geographical location of Iran and relatively high elevation of most of its areas above sea-level, occurrence of frosts has many negative effects on different sections, such as agricultural, transportation and energy. Frosts are generally related to displacement and movement of cold anticyclones from northern latitudes to Iran. Wide range of synoptic characteristics is formed during occurrence of this phenomenon. Some of these characteristics include short term polar outbreaks, cold surges, cold waves and temperature extremes on each of which many climatologists and meteorologists investigated.
In this paper, monthly and seasonal circulation patterns of pervasive frosts in Iran are investigated by atmospheric fields’ maps for years with minimum and maximum frequency of pervasive frost days. Monthly and seasonal analyses are performed by comparing months and seasons during which frequency of days with pervasive frosts was greater (lower) than mean frequency.
Materials and Methods
In this research, data of daily minimum temperatures from 663 synoptic and climatology stations in Iran were used as surface environment data. These data were gathered as raw data from Iran Meteorological Organization. With these raw data, one database of daily minimum temperatures from these stations was prepared for periods October to April of 1962 to 2004.
When database of minimum temperatures in Iran were prepared, daily minimum isothermal maps for period 1 January 1962 to 31 December 2004, 9116 days, were interpolated by Kiriging Method.
Upper air variables which were used in this research include: geopotential height in terms of meter, sea level pressure in terms of Pa, zonal wind component in terms of m/s, meridional wind component in terms of m/s, temperature in terms of Kelvin, specific moisture in terms of gr/kgr. All of these data were gathered from website www.esrl.noaa.gov.
Then, frosts (means those days with temperatures less than or equalto0°C) are classified into three categories: partial frosts, semi-pervasive frosts and pervasive frosts. According to below defined criteria, each day is placed in one of above three categories:

Partial frosts: are those frosts that percentage of area with frost is less than or equal to 25 percent of total area of Iran.
Semi-pervasive frosts: those frosts that area with frost includes 25 to 65 percent of total area of Iran.
Pervasive frosts: those frosts which cover about or more than 65 percent of total area of Iran.

In next step, extreme frequencies of days with pervasive frosts, whose standard deviation is a unit less  or greater  than long term mean of period 1962-2004 were selected. Note that in this research total days with pervasive and semi-pervasive frosts are known as pervasive frosts. Then, determining two frosts categories, synoptic maps of various meteorological variables which correspond to defined conditions for each category were prepared.
Discussion and Result
Analysis of sea level pressure circulation patterns showed that spatial arrangement of Siberian high pressure and its interaction with two subpolar low (Icelandic low) and Sudan monsoon low systems are the main pressure factors in occurrence of maximum and minimum days with pervasive frosts in Iran. At periods with maximum days of pervasive frosts, it was observed that intensification and meridional development of subpolar low (Icelandic low) cause developed western tongue of Siberian high to shift towards lower latitudes. This shift leads to establishment of a local high pressure over Mediterranean Sea, whose anticyclonic circulation causes precipitation of cold air over western half of Iran as well as recession of meridional tongue of Sudan monsoon low to lower latitudes. Consequently, moisture and temperature are reduced over western half of Iran. Wind vector at 850 HPa level suggests an anticyclonic circulation over Mediterranean Sea.
But at periods during which minimum frequencies of days with pervasive frost occurred opposite conditions were observed. By weakening of subpolar low and consequently reduction in its zonal and meridional spatial development, meridional development of Siberian high pressure toward north of its central core increases and development of its western tongue is reduced. Then, a local low is established over Mediterranean Sea. Under these conditions, it is observed that Sudan monsoon low tongue penetrates more to higher latitudes through Red Sea. Therefore, penetration of this tongue and its combination with low over Mediterranean Sea lead to an increase in temperature and moisture over south, west and northwest of Iran. This situation is followed by a balance in cold climatic conditions over western half of Iran, which is in agreement with cyclonic circulation of wind vector at 850 Hpa level.
Conclusion
The results showed that at periods with minimum and maximum frequencies of days with pervasive frosts, intensification and meridional development of sub- polar low (Icelandic Low) causes western tongue of Siberian high pressure to shift towards lower latitudes. This shift causes a local high pressure to be established over Mediterranean Sea and anticyclonic circulation of this high pressure results in precipitation of cold air over western half of Iran as well as shift of meridional tongue of Sudan Monsoon Low to lower latitudes, which consequently reduces temperature and moisture over western half of Iran.
But at periods during which minimum frequencies of days with pervasive frost occurred opposite conditions were observed. Thus by weakening of subpolar low pressure and consequently a reduction in its zonal and meridional spatial development, meridional development of Siberian high pressure toward north of its central core increases and development of its western tongue is reduced. Therefore, a local low pressure is established over Mediterranean Sea. Under these conditions, Sudan Monsoon Low’s tongue will penetrate more toward higher latitudes through Red Sea. Thus, penetration of this tongue and its combination with low pressure over Mediterranean Sea causes temperature and moisture to decrease in south, west and northwest of Iran and this leads to a balance in cold climatic condition over western half of Iran.