Analyses of Unusual First and Last Frost During the Last 40 Years in the Changjiang and Huaihe River Valley

According to the frost index defined by the soil surface minimum temperature ≤0℃, the criterion for the unusual frost was quantitatively given and the climatic features of the unusual first and last frost during the last 40 years were studied. The results show that there distinct geographical differences in the unusual first and last frost days, and 2~3 year and guasi－12 year cycles are universal. Roughly, two fluctuation patterns are discovered. One is the opposite tendency between the first and last frost days and the other is the same fluctuation tendency between them. The former is obvious in the Huaihe River Valley and the latter is generally in the Changjiang River Valley. When the departures of the first or last frost come into being the opposite tendency or the same one and their departures to the average reach the largest, the frost periods are defined as unusual active one.