Abstract:Based on the daily temperature data of Urumqi Meteorological Station from 1 January 1951 to 31 December 2015 and using daily minimum temperature and temperature drop range as the index, database of cooling process in Urumqi City ever the past 65 years is established. The cooling process is divided into 5 grades, i.e. Ⅰ (weak), Ⅱ (medium), Ⅲ (relatively strong), Ⅳ (strong), Ⅴ (cold wave). Some climate characteristics of cooling process are analyzed, such as frequency, continuous days, temperature drop range in different periods, minimum temperature, and anomaly of minimum temperature. The results show that: (1) From 1951 to 2015, there were 5834 cooling processes， annually 89.8 processes in average, of which 78.1% is of Grade Ⅰ (weak) cooling process. The seasonal distribution of the frequency of cooling process is uniform, but the largest number of Grade Ⅳ (strong) and Ⅴ (cold wave) cooling processes is in spring. Annual mean frequency of cooling process in the seven decades is not so different. Between the years with more and less cooling processes, the largest process frequency difference is in June-August, and the smallest difference is in April and September. With the decadal increases, the average annual frequency increased in Grade Ⅰ (weak) cooling process, and decreased in Grade Ⅴ (cold wave) process. (2) The continuous days of cooling process is 1.89 days in average and 49.0 % of the process lasts one day. The highest frequency of the continuous days changes from one day to three days as the grade changes from I (weak) to Ⅴ (cold wave). The continuous days of Grade Ⅳ (strong) and Ⅴ (cold wave) cooling processes are relatively more from late autumn to winter and less in spring. (3) The drop range of temperature of process is -4.4℃ in average and the largest drop in temperature is in autumn and the smallest drop in summer. The largest drop ranges of Grade Ⅳ (strong) and Ⅴ (cold wave) processes are in June and December. The maximum drop range of temperature in 24 h, 48 h and 72 h is -3.1℃,-5.5℃ and -7.4℃ in average, respectively. For 24 h, the largest temperature drop range happens in Spring and the smallest in winter. For 48 h, the largest drop is in spring and the smallest in summer. For 72 h, the largest drop is in winter and the smallest in summer. (4) The minimum temperature of the cooling process is 0.3℃ in average, the lowest is in winter and the hightest is in summer with distinct seasonal characteristics. The maximum daily temperature anomaly is -1.9℃ in average. As the grade changes from Ⅰ (weak) to Ⅴ (cold wave), the temperature anomaly gets much lower. The maximum daily temperature anomaly of Grade Ⅴ (cold wave) is -8.5℃. (5) During the months with more cooling processes in Urumqi, the 500 hPa high level is mainly controlled by zonal westerlies in Xinjiang Region, the short wave trough and east ridge with the stable westerlies affecting Xinjiang. In the abnormally less cooling process months, the 500 hPa high level is controlled by northwest airflow in Xinjiang Region, in the high latitudes of cold air from the north to the south channel, which is more favorable for the strong cold air to enter Xinjiang.