Characteristics of anomalous first/last frost day in Aletai region from 1961 to 2008

Frost index was defined by the minimum air temperature less than or equal 0 ℃ in this paper. Based on the daily minimum air temperature data from 7 weather stations in Aletai regions during 1961 to 2008, the characteristics of first/last fronts day and frost-free season were analyzed by modern climatology diagnosis method. The results indicate that the first frost day delays and the last frost day is ahead, and frost-free season extends in the recent 48 years in Aletai regions. Thus, growth seasons of crops are also prolonged obviously, especially in Habahei region. Frequency of the exceptional first frost day is 2-6-time and the most occurs in Fuyun, while that of the exceptional last frost day is 1-4-time. The differences between the earliest and latest first frost day and between the earliest and latest late frost day are more than 30 days. Furthermore, only in Habahe region the frost day occurs an abrupt change.