Spatial and temporal analysis of Iran extreme cold day's trend

In order to have spatial and temporal analysis of Iran extreme cold days'' trend، the average of daily temperature data from 663 synoptic and climatologic stations of Iran during the period of 1961/21/03 to 2004/19/03 were used. Then، temperature data on 15*15 kilometer pixels were interpolated using Kriging method for Iran. The extreme cold days were identified by Normalized Temperature Difference (NTD) index. According to the index، the days with NTD -2 were selected as extreme cold days. Then، a trend analysis was applied to the extreme cold days for each month of year. The results showed the negative trend extend of the extreme cold days is further than the positive trend extend. Positive trend located on the high mountains in Shahre Kord، Sannadaj، Zanjan to Ardabil and a thin band on Alborz mountains range. While a negative trend located on central-south of Iran. In January and July extreme cold occurrence is more frequent and least respectively.