关键词: 冻结始日, 冻结终日, 年际变化

Abstract: By using the upper limit and lower limit of frozen soil in 58 meteorological stations from 1981 to 1999 in Qinghai Province and Tibet Region, the special distributing and inter-annual variation of the beginning date and the ending date of the soil freezing is analyzed;When soil begins to freeze in fall, the early, the late and the mean beginning dates occur from north to south gradationally. Soil freezes early in the northeastern Qinghai Province and in the source areas of the Yellow River. Soil freezes late in the northern slopes of the Himalayas, the southeastern and the northern Tibet Region. When soil concludes to freeze in the spring, the early, the late and the mean ending date is rather consistent, early in the south and north of the plateau and late in the Middle Plateau. Soil thaws early in the northern slopes of the Himalayas, the southeastern Tibet Region, the Qaidam Basin and the northeastern Qinghai Province. Soil thaws late in the northern Tibetan Region and the source areas of the Yellow River;The difference between the last and the earliest beginning dates of soil freezing are 20 d to 90 d in different stations. The maximum difference, more than 50 d, appears in the source areas of the Yellow River, the southeastern and the northern Tibetan Region. The difference between the last and the earliest ending dates of soil freezing are 30 d to 90 d in different stations. The maximum difference concentrates in Qaidam Basin;The series of the beginning date of soil freezing varies in a period of 3～4 a, while that of ending date of soil freezing varies in a period of about 7 a. Thus the frozen days varies in a period of 2～6 a. The cumulative depth and the maximum depth of frozen soil varies in a period of 3～8 a, but the variation is slight, which cannot pass the significance test of 5%;The beginning date of soil freezing has a trend to occur latter and the ending date of soil freezing has a trend to occur earlier, showing permafrost is degenerating in the plateau. In the 1980s, the soil frozen early and thawed late, resulting in a longer frozen duration. But in the 1990s, the soil frozen late and thawed early, resulting in a shorter frozen duration;In 1981 and 1982 soil frozen early and thawed late; 1983 and 1990 soil frozen late and thawed late; 1993 and 1999 soil frozen late and thawed early.

Key words: beginning date of soil freezing, ending date of soil freezing, inter-annual variation