فهرست مطالب bohlool alijani

The ambient air temperature in the troposphere usually decreases with increased altitude (per 1000 meters increases the height of 5 to 6 degrees Celsius), but sometimes with increased air temperature, which is called temperature or inversion inversion. Temperature inversion occurs when a layer of hot air is above cold air adjacent to the Earth. In this case, air stability is created and instead of increasing temperature height to a few hundred meters above Earth with increased height. We will increase temperature. The importance of temperature inversion phenomenon is doubled when examining the effects of temperature inversion phenomenon. The phenomenon of temperature inversion is important because it causes fumation. This phenomenon occurs when the sun's radiation is unstable in the vicinity of the surface for a short time after sunrise, then combined with the scattered material in the nightly layer, causing the scattered material to return to the surface. . As a result, the concentration of pollutants increases sharply and the phenomenon of fiomia is created.

In this study, the daily data of the radio atmosphere above the Birjand Synoptic Station (Table 1) for 00 Greenwich (3.5 local) over the last 11 years (2010 to 2020) to 11 km high from Earth from Vioming University He got. Indicators used include thermodynamic indices such as SI, LI, KI and TT and potential temperature. Also of other data used in this study, radiosvand transmitted information including inversion height from ground to meter (ZBASE), inversion height from ground to meter (ZTOP), base temperature in base and inversion layer to degree Selicius (TBASE), the temperature difference between the base and the top of the inversion layer to the grade of silicius (DTINV) is the height difference between the base and the apex of inversion to meter (DZINV) and the pressure in the base (PBSE) and the inversion layer (PTOP) from Relationships (1 and 2) are calculated First calculated using the relationship (1) the potential of the ceiling and the floor of the inversion layer of the relationship (1)Θ: Potential temperature to Kelvin grade T: Temperature to Kelvin P: Air pressure to hectopaskal After calculating the potential temperature of the ceiling and floor of the inversion layer using the relationship (1), we have calculated the intensity of temperature inversion using relationship (2) on a monthly, seasonal and annual time scale. Relationship (2):Δθ Difference of the temperature potential and the base of inversion to the grade of Kelvin Δz thick layer of inversion to meter Z station height to hectometry

The results showed that the average annual temperature inversion phenomenon at Birjand Station was about 90 cases per year, as it may not occur in different heights in some days, about 11.1 % of the radiation temperature inversion, front. A 12.4 %, and the other 76.5 % is related to temperature inversion of subsidence. Due to the air session underneath, the share of subsidence inversions is more than other types of inversion. The results showed that the highest average inversion layer in Birjand was formed in 2010 and 2015 at 9 ° C. The highest annual temperature of the inversion is related to the inversion of subsidence, which is due to the subsidence of the air subcutaneous air subcutaneous in the upper layers of the atmosphere and the high temperature on the ground. In terms of annual altitude, the highest height of the inversion layer occurred in 2019 with 4490 meters. In terms of thickness of the inversion, the inversion of the type of subsidence with 207 and the radiation with 145 meters form the thickest and the thinnest layer of inversion. Results of the average inversion layer pressure in Birjand showed that 2014 and 2015 were formed with about 870 miles and 2019 with 592 milligrams. Among the types of inversion, the most severe inversion of the front was 0.044 % and then subsidence with 0.030 %. In terms of the intensity of severe inversion with 0.7 % and poor inversions with 0.92 % were the lowest and the highest in Birjand. In fact, the inversions of the city of Birjand are poor because of their physiographical and geographical properties.

The correlation results also showed that there was a direct and significant relationship between the intensity of inversion and the inversion layer temperature at 99 %. That is, the higher the temperature of the inversion layer, the greater the inversion of inversion and vice versa. But there is a significant relationship between the inversion intensity and the height of the layer at 95 % probability level. This reverse relationship indicates that whenever the inversion layer occurred at the lower altitude, the inversion of inversion has also increased; But the relationship between the thickness and intensity of the inversion layer showed that with the increase in the thickness of the inversion layer, the inversion of inversion in Birjand also increased as the layer temperature was higher as the inversion was more severe. There is also a direct and significant relationship between the intensity and pressure of the inversion layer at 95 % so that with increased pressure, the inversion will increase. In general, the city of Birjand is under the tranquility of the tropical climate because of its specific location, which is also on the roads of 120 -day Sistan, so it will be weak if inversion occurs.

In this research, the influence of the polar jet streams on the daily rainfall of synoptic stations in Iran in the cold season (autumn and winter) during a period of 30 years (1988-2017) has been studied. Based on the threshold of rainfall exceeding 30 mm as heavy rainfall and based on the 95th percentile, 972 days were identified and clustered as days of heavy rainfall in 31 synoptic stations of Iran, and the representative days of each cluster were selected. In the following, in order to find behavioral patterns and investigate the interaction of the polar jet stream on the intensification of the processes of frontal formation and the occurrence of heavy precipitation, the data of the zonal and meridian wind component, divergence, geopotential height, vertical velocity of the atmosphere (omega), vorticity, relative humidity, specific humidity, temperature and average sea level pressure were studied. The results of these analyzes showed 6 circulation patterns in the region, each of these patterns, due to the dominance and establishment of numerous troughs and ridges in the atmosphere, on the path of the polar front jet stream transfer to the middle latitudes of the earth and also the fluctuations of its seasonal average position have been influential. The results showed that the meridian mode of the jet stream axis has caused the creation and production of cores for the rate of occurrence of more rainfall in the western region of Iran.

Abstract In this research, the seasonal behavior of polar jet streem has been identified based on the cold season (autumn and winter) on Iran. General analysis of jet streem process based on what is mentioned in the methods section was done in order to find the behavior patterns of jet streem in GRADS software environment . The results of the analysis indicate the existence of two main medium nuclei, one above Europe and a wider nucleus over North Africa and Saudi Arabia. The prevalence of nuclei in Europe has been higher than in North Africa. In autumn, the incidence of North African nuclei was about 30% lower than in Europe, and in winter the frequency of both nuclei was similar. The core of the polar jet streem in the two seasons, is located approximately 35 degrees north latitude. According to the information obtained from the average speed maps in autumn and winter, it seems that the speed change has been more than before.

1- Introduction One of the main elements of the general circulation in mid latitudes are the fast and narrow flow maxmia called jet streams whoes speed is usually more than 30 mters per second (Magata, 1950; Geer et al, 1996). They are one of the dominant features of upper level weather maps but changing through time, space and layers of atmosphere. The jetstreams cause vertical motion underneath through which produce stability and unstability over the earth surface; it should be mentioned that they are much known for their instability production (Magata, 1950). Jet cores are one of the main components of the general circulation and their location and displacement are controlled by the elements of the circulation such as the Arctic Oscillation (Strong and Davis, 2008). According to these researches the location of the jet streams are very important in climate events. Therefore, this research tried to identify and present the location and speed of jet streams in Middle East. Because a comprehensive study as this scale has not been carried out so far. 2- Materials and methods In order to study the jet streams, the six hourly (00, 06, 12 and 18 GMT) speed of U and V components of the winds at the 700, 600, 500, 400, and 300 hPa levels during 1965-2014 period were obtained for the window of 0 to 120E and 0 to 80N (Figure 1) from www.esrl.noaa.gov. In total 20 time series were produced from the combination of these hour and level scales for each pixel with the size of 2.5 by 2.5 degrees. In each time series the wind maxima of 30 m/s and higher were extracted. At the final stage, the mean monthly speed and monthly frequency of jet streams of all pixels were mapped for the study area. 3-Results The frequency and speed of jet cores were mapped and are described here in monthly, seasonal and annual scales. The annual frequency of jet cores are mapped in Figure 2. According to this figure, the highest speed maxima in 300 hPa level are located over the North Africa at 40 percent of times, depicting the main track of the jet cores. Considering the fact that jet cores enter the study region only in the cold period of year, this temporal frequency of 40 percent is not a low value. The location of speed maxima is the same in 500 hPa level but with a lower value of about 20 percent of the time. This low value is reasonable for this level. As we know, the synoptic systems of this level control most of the time the weather and climate of the surface and these surface systems are not very frequent during a normal year. In this level, the bifurcation of westerlies is obvious. The southern branch is very influencial in the Middle East. This bifurcation indicates the presence of a blocking high in the region, which most of the times prevent the entering of jet cores and hence an unstable conditions over the region. There are no speed maxima in the region in 700 hpa level. Since in the annual scale only the 500 and 300 hPa levels showed the jet cores, in the seasonal scale we look only for these levels. The frequency of jet cores in winter is more than the other seasons and show two separate belts. The jet core affecting the climate of Iran is passing over the Persian Gulf which extends from north of Africa towards China. Jet cores pass through this belt all of the winter season. However, their frequency decreases toward the north over Iran and from there increases toward the other maxima over the latituds 50N to 60. The jet frequencies have decreased in spring reaching to about 52 percent over the area (Figure 3B). This rate of decrease indidates the suden and rapid change from winter to the spring conditions over Iran. Its northward shift is also obvious over Iran. In this season, the jet maxima travel in a ridge like track over Iran. It indicates an unstable weather over the west of Iran while dominating the stable conditions over the east of the country. 4- Conclusion Jet streams are very important instability factors in the atmosphere. Their spatial location and speeds control the pressure systems tracks and surface climate. For this reason, this study tried to understand their speed and spatial variations in the Middle East. Their speed and frequencies were studied at the pressure levels of 700 hPa to 300 hPa levels in the monthly, seasonal and annual scales. The result showed that in all levels and scales two speed maxima tracks were established over the latitude belts of 20N – 30N and 50N – 60N which is confirmed by findings of Li et al (2004); Zhang et al ( 2006); Li and Wettstein( 2012) and Pena-Ortiz et al( 2013). The most important finding of the research is the coincidence of speed maxima with frequency maxima of jet cores in these latitude belts. During the warm period, both the frequency and speed of the jet cores decreased over the Middle East. The jet cores were absent at the levels lower than 500 hPa, and in 300 hPa all winds experienced speeds higher than jet threshold which is in accordance with Geer et al (1996). In brief we can conclude that through the fall season jet cores move southward to establish over the Middle, East including, Iran during winter and through the spring season they began backward movement to the northern latitudes so that in May no ject core could be found in the area.This research demonstrated the importance of the wind patterns of 500 hPa on the climate and weather conditions of the Middle East as well as Iran. Keywords: Jet cores, Middle East, climatology of jet streams, upper level jets, polar front and subtropical jets.

The main of the factors limiter the happen the sultry comfort conditions in the coastal zone, which is under the control of the temperature and humidity. The research designed as the country's southern coast within the province of Bandar Abbas, (Because of little distance to the coast) is selected. The climate data used in this study included based on The air temperature, relative humidity percentages, and the wind speed in knots whose hourly Statistics hourly data parameters of the are three synoptic stations in the Persian Gulf coasts the range, of Meteorological Organization in the 30-year period (1985 -2014) of Bandar Abbas stations. This index is of the formula often used to calculate heat index. After calculation of the thermal index chart every month the average both daily and hourly resolution from the index values were provided in the station.  Then, the final tables were obtained which, incidentally, were not specified for the sincere period for every hour of the day, different levels of probability (the number of occurrences with the probability of 50 to 70, 70 to 90, and above 90) was determined. Synoptic analysis method was used for synoptic analysis. In this study, using data related to components, sea level pressure, Geopotential heights, temperature, wind direction, wind speed and relative humidity, sea level pressure maps, Geopotential elevation, thickness map, Relative Humidity Distribution Map and vorticity Map (vorticity) were mapped and analyzed. The results showed that the establishment of a very strong heat Low pressure on the Persian Gulf and the high pressure of the deserts of Saudi Arabia and Central Iran caused an Extend the pressure and the transfer of hot and burning air to the Persian Gulf and, eventually, the occurrence of severe evaporation of the sea, has provided.

Identifying the climatic indices and study of teleconnection is an important method in association with the purpose of synoptic climatology science. What are the most important indicators for Iran's climate? For this aim, digital data of daily height for important atmospheric levels from the National Centers for environmental prediction and Atmospheric research for a period of 63 years (1948-2010) and daily temperature and precipitation data for 43 synoptic stations was received in 30years (since 1977-2008). from meteorological organization of Iran. By using the analysis method of main component the important atmospheric indices in the geographical limit of northern latitude of 10 to 70 degrees and 10 to 80 degrees of geographical east longitude for the cold half of year (fall and winter) were identified and through correlation method, the manner and importance of each one of these indices in Iran's climate were determined. The results showed that there are seven climatic indexes in the under study geographic area and the most important of them for Iran's climate from beginning of cold season up to the middle of March include the indices of Central Asia ,North Siberia, Western Europe, Anatolia and the Western Mediterranean respectively. Indicators have the greatest impacts on the temperature of Iran’s climate and their precipitation effect is zonal. Most of the precipitation relationship is between the Caspian Sea coastal area and the index of Central Asia and Scandinavia - Central Siberia respectively in autumn and winter and whole of Iran with western Mediterranean. Thus, according to tobler’s principal in geography, the centers and the most important indicators of Iran's climate are near Iran and their importance is more than the climatic indices located in far away distances.

Variability and changes in precipitation amounts, including extreme precipitation and droughts, trigger relevant societal and environmental impacts. Atmospheric circulation strongly determines precipitation variability over southern half of Iran. The influence of prevalent synoptic situations on the distribution of precipitation amounts needs to be understood, as well as their frequency changes to appraise the impact of atmospheric circulation on intra- and inter-annual (decadal) fluctuations of precipitation amounts. On the other hand, the atmospheric moisture budget plays important role in the hydrology of this region. The aridity in the southwest and south of Iran is due to subsidence scale caused by Hadley circulation and a location far from oceanic sources of moisture. However the moisture and other properties of atmospheric quantities are transferred by local circulation from another source to the given region. Many investigations have been carried out on this subject. The consideration of the precipitation and its source region of moisture flux over south and southwest of Iran, during 1970-74 has been shown that 23 percent of the total low pressure system which passed over this area are from the southwest of the Red Sea (Faraji, 1982). In the investigation of moisture flux over Iran, which is carried out by Alijani (1995) noted that the Mediterranean Sea is the main source of moisture for precipitation over Iran. Evans, Smith & Oglesby (2004) have shown that the Persian Golf and the Caspian Sea are the main sources of moisture for precipitation over the Alborz and the Zagros mountains. They have shown that the subtropical jet current brings the warm and moist air from the South part of Saudi Arabia and the Aden golf to the Middle East in the middle and higher troposphere (Dayan &Abramski, 1983). The Red sea inverted trough at the surface with regard to its amplitude, is the Major source that transfer warm and moist air from the Aden golf to the East and North of the Arabian peninsula and also southwest and south of Iran (Alpert, 2001). The low pressure system which is formed at southwest of the Red sea is associated with high potential acceptability of moisture and warm air due to its characteristics of dynamical and thermo dynamical pressure for heavy rainfall over the study area. Rurerde (2006) using specific humidity map and its extension from the Caspian Sea to the North part of Iran derived a value of moisture quantities from each source region of moisture to the precipitation over Iran. The principal objective of this research was to understand the characteristics of moisture flux from each region of moisture source and their contribution to the rainfall during the above period.
Study Area : The study area for this study is southern half of Iran, which lies approximately between 25N and 35N in latitude and between 44_E and 64_E in longitude. The study area of this research contains some provinces such as: Ilam, Khozestan, Lorestan, CharmahalBakhtiyari, Kohkiloye and Boyerahmad, Bushehr, Esfehan, Yazd, Kerman, Hormozgan, Fars, and Khorasan Jonobi. Based on the Koppen climate classification, most parts of this area are categorized as generally having arid (BW) and semi-arid (BS) climates. The important mountain of this area is the Zagros, which play an important role in nonuniform spatial and temporal distribution of precipitation.
Material and Examination of precipitation properties needs long and high quality records of data. In the present study, the time series of precipitation data at 183 stations for the period 1974-2004 were collected from the Islamic Republic of Iran meteorological organization (IRIMO) and were analyzed. Data homogeneity is assessed by IRIMO previously. The Empirical orthogonal function (EOF) was applied to detect and describe spatial and temporal change in the precipitation. In addition, the principal component (PC) was applied to detect dry and wet years. In addition, to identify the spatial and temporal variations of the precipitation, we used the NCEP-NCAR reanalysis monthly mean values of the temperature, Geopotential height and pressure in 1000, 850, 500 and 300 hPa levels. These data have been widely used by many researchers over the last few years in tropical climate research. Finally, we calculated 30 years anomaly for these levels in dry and wet years.
The maps of the wettest months have shown the synoptic situation over study and its adjacent area during the above period. These figures have show the high pressure over the Oman and the Arabian seas, deep trough over the East of the Mediterranean and the west of the Red sea. In this case, eastern Saudi Arabia received more moisture and latent heat fluxes than the other parts of the area. Two moisture source regions as observed from the divergent flux, which are located over the Aden golf and Red sea, supply moisture to study region. Southern half of Iran receives precipitation during this study, mainly from the west of the Indian Ocean (the Arabian Sea, the Oman Sea, The Aden golf), the Red sea and the some extent from the Mediterranean. The divergent moisture and latent heat flux during wet cases were observed over north part of the Arabian Sea, the Aden golf, the central part of the Red sea and convergence was observed over South, central of the Arabian Peninsula, southern half of Iran. These processes are indicative of the upward motion of moist air, which in turn may release latent heat due to condensation. This available heat energy may be the source of low-level latent heat instability. The large amount of moisture and latent heat flux over Saudi Arabia in is transported, by rotational wind from the Red sea and the Mediterranean as mentioned above. Conversely, in dry years due to weak trough of East Mediterranean, systems move from latitudes upper than 40ºN and the cold air does not transfer from the southeast Europe to the Northeast of Africa resulting development of Sudan low. Overall, decreasing transfer of cold air will lead to decreasing of moisture and latent heat flux from the Indian Ocean to the southern half of Iran.
Atmospheric circulation strongly modulates precipitation patterns. Precipitation is one of the most important atmospheric variables in the global hydrological cycle and plays a key role in the Earth’s energy balance.The investigation of synoptic maps in 1000, 850 and 500 hPa levels in four systems shows that in wet years due to deep trough of East Mediterranean and West of the Red Sea, the cold air transfers from the southeast Europe to the Northeast of Africa resulting development of Sudan low. This low transfer to the center and north of the Arabian Peninsula, then move to the Southern half of Iran. The development of this low is due to release of sensible heat from the Arabian Desert and latent heat flux which transfer from the Indian Ocean, to this area. This low affects the southern half of Iran with significant value of perceptible water and low- level latent instability. It is seen that the transfer of moisture flux in the low-layer from the Indian Ocean plays an important role in precipitation over study region. Conversely, in dry years due to weak trough of the East Mesiterranean, systems move from latitudes upper than 40ºN and the cold air does not transfer from the southeast Europe to the Northeast of Africa resulting development of Sudan low. Overall, decreasing transfer of cold air will lead to decreasing of moisture and latent heat flux from the Indian Ocean to the southern half of Iran. The findings of this study confirmed the results of the previous studies (Alijani, 1995; Farajzadeh, 2007;Milind, 2006; Mofidi&Zarin, 2005).

In this research, for determining the synoptic patterns of dust storm occurrence in northwest and northeast of Iran; four types of data were used including: Hourly data of dust phenomena and horizontal visibility for 30 meteorological stations in northwest and northeast of Iran. Six-hourly global data analysis from NCEP/NCAR reanalysis, including air temperature, geo-potential height, U-wind and V-wind components, relative humidity, soil moisture and omega from 1000hPa to 500hPa, were used for the preparation of maps and identify the synoptic patterns by using GRADS software In order to identify the source of dust generation, tracing and simulating the path of dust particles, HYSPLIT model Lagrangian approach of backward trajectory was used. And to detect dust, the unreal images of METEOSAT-9 second generation for EUMETSAT satellite were used. Synoptic studies have shown that low and high pressure, and the vertical motion of air are the main causes of dust storms in Iran. The circulation of the atmosphere during dust storm, shows that a low pressure cell has been stretched from Pakistan to the south of Iran and from there to the deserts in Iraq, Syria.This condition causes the formation of cyclonic circulation in the East of Syria and west of Iraq on 31 July. Wind speed Increasing, cyclonic circulation, dry soil and lack of coverage, provided the conditions for removing the soil particles. Due to the extended ground surface's low pressure and weakening of the stable conditions of low level of troposphere, the approaching of trough has led to the beginning of a dust storm in Iraq deserts on jul31 and this unstable condition providing enough power to carry the soil particles away from its origin. Besides that, the stable location of ridge on Iran made anticyclone circulation in the wind blowing and it cause that the dust storm cycle the northern part of Iran and enter from northeast in to the country.

Precipitation is the most variable climatic elements. These changes occur in the area in both the spatial and time dimension in the form of climatic condition of the area. The aim of this study is the review of the spatial correlation pattern of changes within a decade of precipitation over the past half-century in Iran. For this purpose، the daily precipitation data was extracted by using 664 synoptic and climatic stations during the time period of 1961-2011 and was used as the data base (Asfazari data). For obtaining the changes information of a decade''s precipitation in Iran، the methods of ground statistics such as spatial autocorrelation of global Moran index، local Anselin index of Moran and Hotspots and also planning facilities in Matlab، Surfer and GIS environments have been used. The results obtained of this study showed that the changes of precipitation during one decade in Iran has a high cluster pattern. However، based on local Moran index and hot spots، precipitation at coastal areas and some parts at the west and south west of Iran (mainly Zagros mountains) had a positive spatial autocorrelation (precipitation clusters with high value) and parts of central areas and also parts of the south east of the country (mainly Zabol) had a negative autocorrelation (precipitation clusters with low value). In other areas of Iran، precipitation has had no significant pattern or spatial autocorrelation. For the purpose of reviewing the changes of precipitation cycles، spectrum analysis method has been used. The results obtained from the said method indicates the existence of short and medium term cycles in annual precipitation of the country.

Iran is a country with arid and semiarid climates, so its average annual rainfall is about one third of the average annual rainfall in the world. Besides, the eastern and south-eastern regions of Iran are one part of this climate whose main features is the high intensity precipitation. In the present study the daily precipitation data of 54 climatic stations in the eastern and south-eastern regions of Iran with more than 10 statistics years are provided. The data were provided in standardized Matlab software 2 matrix with the arrangement of 20824٭54 (row*column) in which the rows represent the number of days while the columns represent the statistics of rainfall stations. Using recent matrix data, a cluster analysis was done by Ward linkage method and Euclidean distance. At the end, 14 one-day rainfall events with magnitude greater than 70 mm were­ detected as a representative of heavy and pervasive rainfall events. The six-hour data includes geopotential height, sea level pressure, temperature, specific humidity, u wind and v wind, between 0 to 80 degree of northern latitudes and 0 to 120 degree of eastern longitude were used to recognize the dynamic, thermodynamic and synoptic factors affecting the occurrence of rainfalls. Then, the maps of spatial patterns have been drawn with the resolution in cell size of 2. 5٭2. 5 degree. The findings showed that torrential rainfall events over eastern and south-eastern parts of Iran are made of one or several different circulation patterns, while the most of the torrential rainfall events mostly provide confrontation of polar low-pressure with Sudan-Saudi low pressure which make the conditions of extreme pressure in northwest-southeast direction.

Sever frosts during autumn, winter and spring seasons cause damages in the agriculture, industry and transport parts of Iran every year. These frosts are including radiation and frontal types. The location of Iran in mid-latitudes and the spreading of westerlies over much of Iran especially during cold half-year provide the happening and strengthening of frontal frosts. The cA air-masses from north (Arctic basin), cP and mP ones respectively from northeast (Siberian basin) and (Europe and Black Sea basin) causes frosts in Iran. Our study area within 25° - 40° north latitude and 44° - 63° east longitude is at the risk of cold air attacks during cold half-year. Thus we selected 50 air stations from throughout Iran in the southwest of Asia. In first step, we determined 75 frost waves based on mean daily temperatures including the zero and under zero degrees Celsius during recent decade from 1994 to 2003 years. In second step, we could determine 31 cold air waves based on the daily distribution of low temperatures and so their beginning, peak and end (based on date), their severity (based on air temperature) and duration (based on day). We measured the depth of every trough during the peak day of every cold air waves for statistical analysis including plot and regression. Results showed the severe frosts have happened during winter except the frost wave of 1994 that happened during autumn. We found February as the most frosty month (based on the frequency of frosty days) and January as the coldest one. While it was accepted that spring frosts cause more damages, instead it our results showed that autumn ones has more frequency. It was determined that the location of western troughs at most in Iran is 10 degrees lower than middle location in southern hemisphere and frosts severity had significant relationship with troughs depth (58 %). Synoptic patterns showed that all axes were located over Eastern Iran and they are oriented northeastern-southwestern during peak days. This orientation facilitates very cold air flows from high latitudes toward southern areas. Migrant troughs from higher latitudes caused sever frosts and migrant troughs from low latitudes cause widespread ones. The irregular occurrence of continued and low temperatures over vast areas as frost waves is an expected characteristic for Iran climate. We were losing resources and products in North, Central and sometimes South of Iran because of these cold air waves. Although the safety of the frosts over Southern Iran is better than other parts but the water and soil resources of southern areas for agriculture are so limited. It means, we have many fertile plains and permanent rivers over Western and Northern Iran and so frost risk can not take up agriculture activities. It looks; reducing damage to growers in these damage-prone areas require changes in cropping patterns, usage of resistant and late seed and varieties as basic strategies.

Global temperature has increased about 0. 74°C over the last century (IPCC، 2007، 30). In recent studies، the potential of increase in heat waves، heavy precipitation، cold winter، summer storms and drought event due to climate change noticed (Zhang،2005،11). On the other hand، it is believed that climate change will affect most aspects of weather and climate، especially precipitation and temperature extreme events (Radinovi and Curi، 2009، 200; Lehner et al.، 2006، 293). Therefore، the socio-economic effects of extremes (Ryoo et al.، 2004، 145)، require more attention to such studies. Assessment of the temperature extremes changes is done in many region of the world in the last century (Mudelsee et al. 2003; New et al. 2001; Moberg and Jones 2005; Klein Tank and K¨onnen 2003; Alexander et al. 2006). The socio - economic effects of extremes in arid and semi arid regions like Iran due to very vulnerable and fragile climates are more and their sudden changes may be followed by the devastating events. Spatial and temporal variability of climate in Iran is one of its inherent characteristics and devastative socio-economic effects of climatic disasters such as floods and droughts have been high in recent years (Nazemosadat and Cordery 2000، 59; Barlow et al. 2002، 697; Nazemosadat and Ghasemi 2004، 4016). Studies such as Rasouli (2004)، about spatial analysis of cold winds in the Southwestern of Iran and Kaviani and his colleagues (2004)، the effective temperatures in the country، show the variability and instability of climate in Iran. Consequences of global warming in Iran include increased frequency of extreme events، especially in cold and heat waves، long time severe droughts and torrential rain (Rahimzadeh et al. 2009، 342). Numerous studies in recent years، has been investigated the average temperature and precipitation variability in Iran (Alijani 1997; Jahadi Toroghi 2000; Rasooli 2002; Rahimzadeh and Asgari 2003; Rahimzadeh and Asgari 2005; Pedram et al. 2005; Asadi and Heydari 2011). Furthermore، Rahimzadeh and her colleagues (2009) were considered the temperature and precipitation extreme variability in Iran and Taghavi، and Mohammadi (2007)، stated that the frequency of warm and cold events، respectively، has been associated with decreases and increases. Moreover، kari (2010)، has confirmed significant changes in heat waves and cold periods in Tehran. The purpose of this study provides a more detailed analysis about the spatial and temporal distribution of the temperature extremes in Iran.