Piling and melting of snow is a dynamic process of solid precipitation which dramatically relates with meteorological and hydrological processes has important effect on the radiation balance,energy exchange and water circulation of the Earth.Accurate information on piling and melting of snow is essential for the study of hydrology and climatology,particularly in the Northern China.As one of the most ecologically fragile region in environmental system of the Earth,arid and semi-aid region of northern China characterize special environmental conditions where water circulation and its changes play a predominant rule in sustainability of the region.Snow is a major source of water in the high altitude region of China and the snow covers there constitute of the preliminary water supplies to the large rivers of China.Snow cover direct measurements are difficult to carry out owning to too heavy logistic needs and economic burdens in maintenance.Therefore,it is almost impossible to obtain any characteristics of long-term,inter-annual and annual variability of snow covers using only the limited data of field measurements.Passive microwave remote sensing has the advantage to penetrate the cloud,and its all-weather capability for snow cover monitoring makes it superior to the optical sensor and near-infrared remote sensing techniques.By means of passive microwave remote sensing techniques,not only snow cover,but also snow depth and snow water equivalent (SWE) could be obtained.With a 7-channel 4-frequency radiometer,SSM/I has been utilized for retrieval of various snow parameters.The daily snow cover datasets derived from SMMR (from 1978 to 1987) and SSM/I (from 1987 to 2005) for north hemisphere,such as the ten-day periods and yearly mean,the maximum,the minimum of snow depth and the snow-cover days…etc,have been collected and analyzed with special attentions being paid on snow covers in northern China.Meanwhile,the daily data of precipitation and temperature of 412 meteorological stations and snow depth measurements from 4 meteorological stations in northern China were collected from 1960 to 2005.