Abstract: The characteristics of haze days and the climatic background were analyzed by using daily observations of haze, precipitation, mean wind speed and maximum wind speed of 664 meteorological stations from 1961 to 2012. The results show that haze days are significantly more in eastern China than in western China. The annual haze days are 5-30 d in most central-eastern China, more than 30 d in some areas, but less than 5 d in western China. Haze days are mainly concentrated in winter half year, the most in winter, followed by autumn and spring, and the least in summer. Haze days are the most in December, nearly 20% of annual haze days. The haze days in winter half year averaged in central-eastern China have a significant increasing trend of 1.7 d per decade during 1961-2012. There were great increases in haze days in the 1960s, 1970s and the beginning of the 21st century. There was a significant abrupt change of haze days in the early 1970s and the beginning of the 21st century. Haze days of winter half year increased in South China, the middle-lower reaches of the Yangtze River and North China, but decreased in Northeast China, eastern Northwest China and eastern Southwest China from 1961 to 2012. The consecutive haze process is becoming more and more. The longer the process, the more significant the process’s increasing trend. Unfavourable climate conditions exacerbated the occurrence of haze. The correlation coefficient is mainly negative in central-eastern China between haze days and precipitation days in winter half year, and the precipitation days showed decreasing trends in most China, with the rate of around -4 d per decade in central-eastern China, which reduces the sedimentation capacity of atmospheric pollutants. During 1961-2012, the correlation coefficient is mainly negative in central-eastern China between haze days and mean wind speed and strong wind days, but positive between haze days and breeze days in winter half year. The mean wind speed and strong wind days reduced, while the breeze days increased in most China, which leads to the reduction of pollutants diffusion capacity. As a result, haze occurs more easily.

Key words: haze, winter half year, precipitation days, wind speed