The temporal and spatial distributions of hails are studied based on the data of the number of monthly hail days in early summer(from 1961 to 2001) at 194 stations in Northwest China,and the climate characteristics of the circulation patterns in typical more/less-than-normal hail years are discussed.The results indicate that hail events occurs more frequently in such areas as the Pamirs Plateau in Xinjiang,the area along the Qilian Mountain and the most part of Qinghai Province south of the Qilian Mountain(except for the Chaidamu basin),and the high-elevation areas of the Southern Gansu plateau.There is more obvious interdecadal variation in hailing days over the area with the greater number of hailing days,and vice versa.The numbers of hailing days exhibited an increasing trend from the 1960s to the middle 1970s,with its peak in 1977 and the years of high-frequency hail events concentrated around the 1970s.From the 1980s,the numbers of hailing days exhibited a decreasing trend with the minimum in 2001.In addition,there are significant differences in the polar vortex,height fields over the Tibetan Plateau,trough over India and Burma,circulation patterns over Europe,and 500 hPa circulation patterns and OLR in the Northern Hemisphere in more/less-than-normal hail years.