Characteristics of Frost Changes from 1961 to 2007 over China

Based on the daily minimum temperature data at 577 meteorological stations over China from 1961 to 2007, the first frost date, the last frost date and the frost-free days are calculated by using the common climatic index on frost day. The results show that the standard deviation of the first frost date, the last frost date and the frost-free days is less in the north than in the south of China, which indicates that they are more stabile in the north than in the south. The inter-annual variation of the last frost date is larger than those of the first frost date, and the inter-annual variation of the frost-free days is larger than those of the last frostdate in most areas of China. It can be seen from the linear change trend that the last frost date is 2.0 d/10a ahead of time, the first frost date has 1.3 d/10a delay and the frost-free days prolong 3.4 d/10a over the past 47 years. From the climate trend it can be found that the range of the ahead last frost date is more than that of the delayed first frost date, which implies that the main cause of the prolonged frost-flee days is the ahead of last frost day. The first-frost date obviously delays since 1990s. The last frost date is obviously ahead of time and the frost-free days obviously prolong since 1980s. The ahead of last frost date is earlier than the first frost date delay in China from the inter-annual variation.