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The temporal and spatial distribution characteristics and variation of heavy fog days in China are analyzed by using the data of visibility and relative humidity of 541 ground stations over China from 1961 to 2005. The results show that there is obviously more heavy fog days in the winter half year than in the summer half year, the most in November and the least June. The spatial distribution shows that the annual mean fog days are much more in the eastern China, and obviously less in the most parts of Inner Mongolia and the western China, with the annual mean number of fog days less than 1 day mostly. Heavy fogs occur frequently, and there was obvious increasing and fluctuating trends in annual fog days with a period of about 1.5 years in the middle and lower reaches of the Yangtze River and some parts of the Huanghuai region. The years of 1982, 1987, 1989 to 2000, and 2002 were morethannormal years, and 1967 was a lessthannormal year.