Study of annual maximum and minimum temperatures Changes in Iran

In this investigation، using Mann – Kendall graphical-statistical test، which is one of the proposed methods of World Meteorological Organization (WMO) for time series analysis، the change trend of long – term mean of annual minimum and maximum temperatures in Iran will be studied. To evaluate this trend and access a suitable pattern in this situation، average of annual minimum and maximum temperatures from 44 synoptic stations for a 40 year period (1969-2008) has been collected from the Meteorological Organization of Iran. Results of the study show that most area of Iran has witnessed both types of changes: maximum temperatures average and minimum temperatures average. However، these changes have not been of the same form and amount in most area of Iran. 28 stations which were in west، northwest، southwest، northeast and centre of Iran have had increasing in annual maximum temperatures average، all of which have been significant in. From among residue stations only two stations of Chabahar and Torbate Heydarieh have had decreasing changes and in other stations no changes were observed. These occurred changes in all stations were all from abrupt changes type. From the viewpoint of time، the most centralization of start time of changes is seen in the third decade (1989-1998). From among 33 stations that have had increasing in annual minimum temperatures average، 19 stations have possessed changes from trend kind and remainder of stations had abrupt changes. These increasing changes have been seen in most area of Iran except small parts of west، southwest، south and one station in east of Iran. Two stations of Fassa and Shahre Kurd have had decreasing trend in annual minimum temperature average and in remainder stations no trend or changes were observed. From the viewpoint of time، the most centralization of start time of changes is seen in the third decade (1989-1998). The arithmetic mean of Mann-Kendall test statistics was found to be 0. 24 for maximum temperatures and 0. 4 for minimum temperatures، which indicate double changes in minimum temperatures compared with maximum temperatures average.